Patent Publication Number: US-10334722-B2

Title: Tamper-respondent assemblies

Description:
BACKGROUND 
     Many activities require secure electronic communications. To facilitate secure electronic communications, an encryption/decryption system may be implemented on an electronic assembly or printed circuit board assembly that is included in equipment connected to a communications network. Such an electronic assembly is an enticing target for malefactors since it may contain codes or keys to decrypt intercepted messages, or to encode fraudulent messages. To prevent this, an electronic assembly may be mounted in an enclosure, which is then wrapped in a security sensor and encapsulated with polyurethane resin. A security sensor may be, in one or more embodiments, a web or sheet of insulating material with circuit elements, such as closely-spaced, conductive lines fabricated on it. The circuit elements are disrupted if the sensor is torn, and the tear can be sensed in order to generate an alarm signal. The alarm signal may be conveyed to a monitor circuit in order to reveal an attack on the integrity of the assembly. The alarm signal may also trigger an erasure of encryption/decryption keys stored within the electronic assembly. 
     BRIEF SUMMARY 
     Provided herein, in one or more aspects, is an enhanced tamper-respondent assembly which includes, for instance: an electronic assembly, an electronic enclosure, and a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor. The electronic assembly includes at least one electronic component to be protected, and the electronic enclosure surrounds, at least in part, the electronic assembly. The electronic enclosure includes an inner surface. The tamper-respondent sensor of the tamper-respondent electronic circuit structure covers, at least in part, the inner surface of the electronic enclosure. The tamper-respondent sensor includes at least one flexible layer having opposite first and second surfaces, and circuit lines forming at least one resistive network, the circuit lines disposed on at least one of the first surface or the second surface of the at least one flexible layer. The tamper-respondent electronic circuit structure further includes a multilayer circuit board and an embedded tamper-respondent sensor disposed within the multilayer circuit board. The at least one electronic component is associated with the multilayer circuit board. The electronic enclosure seals to the multilayer circuit board, and the tamper-respondent sensor covering, at least in part, the inner surface of the electronic enclosure and the embedded tamper-respondent sensor within the multilayer circuit board facilitate defining a secure volume about the at least one electronic component. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a partial cut-away of one embodiment of a tamper-proof electronic package which may incorporate a tamper-respondent electronic circuit structure, in accordance with one or more aspects of the present invention; 
         FIG. 2  is a cross-sectional elevational view of one embodiment of a prior art, tamper-proof electronic package comprising an electronic circuit; 
         FIG. 3A  depicts one embodiment of a tamper-respondent sensor comprising one or more flexible layers and circuit lines forming at least one tamper-detect network, in accordance with one or more aspects of the present invention; 
         FIG. 3B  is a cross-sectional elevational view of another embodiment of a tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 3C  is a cross-sectional elevational view of another embodiment of a tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 3D  is a cross-sectional elevational view of a further embodiment of a tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 3E  depicts a cross-sectional elevational view of another embodiment of a tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 4A  is a partial depiction of one embodiment of a tamper-respondent sensor comprising a corrugated layer of flexible dielectric with circuit lines, in accordance with one or more aspects of the present invention; 
         FIG. 4B  depicts an alternate embodiment of a tamper-respondent sensor comprising multiple corrugated layers of flexible dielectric with circuit lines, in accordance with one or more aspects of the present invention; 
         FIG. 5A  depicts one embodiment of a tamper-respondent sensor comprising a flattened, folded layer with circuit lines, in accordance with one or more aspects of the present invention; 
         FIG. 5B  is a partial plan view of the flattened, folded layer with circuit lines of  FIG. 5A , in accordance with one or more aspects of the present invention; 
         FIG. 5C  is a partial cross-sectional elevational view of a tamper-respondent sensor comprising a flattened, folded layer with circuit lines, and at least one other layer overlying one or both sides of the flattened, folded layer, in accordance with one or more aspects of the present invention; 
         FIG. 5D  depicts another embodiment of the tamper-respondent sensor of  FIG. 5C , further comprising a breakable layer for enhanced tamper-detection capabilities, in accordance with one or more aspects of the present invention; 
         FIG. 5E  is a partial cross-sectional elevational view of a tamper-respondent sensor comprising a flattened, folded layer with circuit lines, and at least one other layer overlying the lower surface of the flattened, folded layer, in accordance with one or more aspects of the present invention; 
         FIG. 5F  is a partial cross-sectional elevational view of a further embodiment of a tamper-respondent sensor comprising a flattened, folded layer with circuit lines sandwiched between two other layers overlying opposite sides of the flattened, folded layer, in accordance with one or more aspects of the present invention; 
         FIG. 5G  depicts a partial cross-sectional elevational view of another embodiment of a tamper-respondent sensor which comprises multiple flattened, folded layers with circuit lines separated by at least one other layer in a stack of layers, in accordance with one or more aspects of the present invention; 
         FIG. 5H  depicts a partial cross-sectional elevational view of a further embodiment of a tamper-respondent sensor comprising a stack of layers with multiple flattened, folded layers with circuit lines, and multiple other layers, for instance, multiple other flexible layers, disposed above and/or below the flattened, folded layers with circuit lines, in accordance with one or more aspects of the present invention; 
         FIG. 6A  is a cross-sectional elevational view of a tamper-respondent assembly comprising an electronic enclosure and a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor, where the tamper-respondent sensor comprises a flattened, folded layer with circuit lines that wraps around the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 6B  is a cross-sectional elevational view of a tamper-respondent assembly comprising an electronic enclosure and a tamper-respondent electronic circuit structure comprising multiple tamper-respondent sensors, where the tamper-respondent sensors comprise multiple discrete flattened, folded layers with circuit lines, wherein one flattened, folded layer along the edge or side of the enclosure wraps around and doubles over the flattened, folded layers with circuit lines located above and below the enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 6C  is an upper (or lower) plan view of one embodiment of the tamper-respondent assembly of  FIG. 6B , in accordance with one or more aspects of the present invention; 
         FIG. 6D  is a cross-sectional elevational view of a further embodiment of a tamper-respondent assembly comprising an electronic enclosure and a tamper-respondent electronic circuit structure comprising multiple tamper-respondent sensors, where the tamper-respondent sensors comprise multiple flattened, folded layers with circuit lines, and one flattened, folded layer wraps around the edge of the electronic enclosure, and the other flattened, folded layers located above and below the electronic enclosure wrap over the flattened, folded layer positioned around the edge of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 6E  is a plan view of one embodiment of an upper (or lower) tamper-respondent sensor for use in a tamper-respondent assembly such as depicted in  FIG. 6D , wherein the upper (or lower) tamper-respondent sensor is depicted by way of example only as a flattened, folded layer, in accordance with one or more aspects of the present invention; 
         FIG. 6F  is a plan view of a further embodiment of an upper (or lower) tamper-respondent sensor for a tamper-respondent assembly such as depicted in  FIG. 6D , wherein the upper (or lower) tamper-respondent sensor is depicted by way of example only as a flattened, folded layer, in accordance with one or more aspects of the present invention; 
         FIG. 6G  is a cross-sectional elevational view of a further embodiment of a tamper-respondent assembly comprising an electronic enclosure and a tamper-respondent electronic circuit structure comprising multiple tamper-respondent sensors, where the tamper-respondent sensors comprise two flattened, folded layers with circuit lines surrounding the electronic enclosure and overlapping along the edge or side thereof, in accordance with one or more aspects of the present invention; 
         FIG. 7A  is a plan view of one embodiment of a first tamper-respondent sensor to be interweaved with a similarly constructed, second tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 7B  is a plan view of one embodiment of a tamper-respondent electronic circuit structure comprising two discrete tamper-respondent sensors, such as depicted in  FIG. 7A , interweaved in a multi-sensor interweaved layer, in accordance with one or more aspects of the present invention; 
         FIG. 7C  depicts, by way of further example, a stack of multi-sensor interweaved layers which may be employed, for instance, in association with an electronic enclosure within a tamper-respondent assembly to define a secure volume, in accordance with one or more aspects of the present invention; 
         FIG. 8A  is a cross-sectional elevational view of one embodiment of a tamper-respondent assembly, or tamper-proof electronic package, which includes (in part) a tamper-respondent sensor embedded within a multilayer circuit board, in accordance with one or more aspects of the present invention; 
         FIG. 8B  is a top plan view of the multilayer circuit board of  FIG. 8A , depicting one embodiment of the secure volume where defined, in part, within the multilayer circuit board, in accordance with one or more aspects of the present invention; 
         FIG. 9  is a partial cross-sectional elevational view of a tamper-respondent assembly comprising (in part) a multilayer circuit board and embedded tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 10  depicts one embodiment of a process of fabricating a multilayer circuit board with an embedded tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 11  is a partial cross-sectional elevational view of a tamper-respondent assembly comprising an electronic enclosure and associated tamper-respondent sensor, and a multilayer circuit board with an embedded tamper-respondent sensor therein, in accordance with one or more aspects of the present invention; 
         FIG. 12  depicts one embodiment of a process for affixing a tamper-respondent sensor to an inside surface of an electronic enclosure, such as for use with a tamper-respondent assembly described herein with reference to  FIGS. 8A-11 , in accordance with one or more aspects of the present invention; 
         FIG. 13A  depicts an underside, isometric view of one embodiment of an electronic enclosure such as depicted in  FIGS. 8A, 11 &amp; 12 , and illustrating placement of an inner-sidewall tamper-respondent sensor over an inner sidewall surface of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 13B  depicts the structure of  FIG. 13A , with an inner main surface tamper-respondent sensor provided over an inner main surface of the electronic enclosure, and with the inner main surface tamper-respondent sensor shown overlapping, at least in part, the inner-sidewall tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 13C  is an enlarged, corner depiction of the electronic enclosure and tamper-respondent sensors of  FIG. 13B , illustrating the inner main surface tamper-respondent sensor overlying the inner-sidewall tamper-respondent sensor at an inner corner of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 14A  depicts an underside, isometric view of one embodiment of an electronic enclosure, or electronic assembly enclosure, such as depicted in  FIGS. 13A-13C , in accordance with one or more aspects of the present invention; 
         FIG. 14B  is an enlarged view of the inner-sidewall corner of  FIG. 14A , illustrating region  14 B thereof, in accordance with one or more aspects of the present invention; 
         FIG. 15A  depicts an underside, perspective view of one embodiment of a tamper-respondent assembly comprising an electronic enclosure such as depicted in  FIGS. 14A-14B , in accordance with one or more aspects of the present invention; 
         FIG. 15B  depicts an exploded view of the tamper-respondent assembly of  FIG. 15A , in accordance with one or more aspects of the present invention; 
         FIG. 16A  is an isometric view of one embodiment of an inner-sidewall tamper-respondent sensor for covering an inner sidewall surface of electronic enclosure such as depicted in  FIG. 14A , in accordance with one or more aspects of the present invention; 
         FIG. 16B  depicts an underside, isometric view of the electronic enclosure and inner-sidewall tamper-respondent sensor of  FIGS. 15A &amp; 15B , with the inner-sidewall tamper-respondent sensor shown positioned over the inner sidewall surface of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 16C  is an enlarged depiction of the tamper-respondent assembly of  FIG. 16B , illustrating region  16 C thereof, in accordance with one or more aspects of the present invention; 
         FIG. 17A  is an enlarged depiction of the inner main surface tamper-respondent sensor embodiment illustrated in  FIGS. 15A &amp; 15B , in accordance with one or more aspects of the present invention; 
         FIG. 17B  depicts the inner main surface tamper-respondent sensor of  FIG. 17A , with the corner tabs shown raised for positioning, as illustrated in  FIGS. 15A &amp; 15B , in accordance with one or more aspects of the present invention; 
         FIG. 17C  depicts the tamper-respondent assembly of  FIGS. 15A &amp; 15B , with the inner main surface tamper-respondent sensor positioned therein, and with the security elements(s) removed, in accordance with one or more aspects of the present invention; 
         FIG. 18  is a perspective view of the security elements(s) illustrated in  FIGS. 15A &amp; 15B  for the tamper-respondent assembly depicted, in accordance with one or more aspects of the present invention; and 
         FIG. 19A  is a partial cross-sectional elevational view of another embodiment of a tamper-respondent assembly comprising a structure with a rigid surface and a tamper-respondent sensor secured to the rigid surface of the structure, in accordance with one or more aspects of the present invention; 
         FIG. 19B  is a cross-sectional elevational view of another embodiment of a tamper-respondent assembly comprising a tamper-respondent sensor secured to an electronic enclosure, and to a rigid surface of another structure, such as a thermal spreader, in accordance with one or more aspects of the present invention; 
         FIG. 20A  is a partial isometric view of a further embodiment of a tamper-respondent sensor for a tamper-respondent assembly, in accordance with one or more aspects of the present invention; 
         FIG. 20B  is a partial isometric view of another embodiment of a tamper-respondent sensor for a tamper-respondent assembly, in accordance with one or more aspects of the present invention; 
         FIG. 21A  is a cross-sectional elevational view of another embodiment of a tamper-respondent assembly comprising first and second tamper-respondent sensors secured to an electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 21B  is an isometric view of one embodiment of an inner main surface tamper-respondent sensor with one or more conductive traces in one or more bond regions thereof, in accordance with one or more aspects of the present invention; and 
         FIG. 21C  is a cross-sectional elevational view of the tamper-respondent assembly of  FIG. 21A , illustrating an attempted line of attack through the electronic enclosure and adhesive encountering the one or more conductive traces, in accordance with one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention and certain features, advantages, and details thereof, are explained more fully below with reference to the non-limiting example(s) illustrated in the accompanying drawings. Descriptions of well-known materials, fabrication tools, processing techniques, etc., are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific example(s), while indicating aspects of the invention, are given by way of illustration only, and are not by way of limitation. Various substitutions, modifications, additions, and/or arrangements, within the spirit and/or scope of the underlying inventive concepts will be apparent to those skilled in the art for this disclosure. Note further that reference is made below to the drawings, which are not drawn to scale for ease of understanding, wherein the same reference numbers used throughout different figures designate the same or similar components. Also, note that numerous inventive aspects and features are disclosed herein, and unless otherwise inconsistent, each disclosed aspect or feature is combinable with any other disclosed aspect or feature as desired for a particular application, for establishing a secure volume about an electronic component or electronic assembly to be protected. 
     Reference is first made to  FIG. 1  of the drawings, which illustrates one embodiment of an electronic assembly package  100  configured as a tamper-proof electronic assembly package for purposes of discussion. In the depicted embodiment, an electronic assembly enclosure  110  is provided containing, for instance, an electronic assembly, which in one embodiment may include a plurality of electronic components, such as an encryption and/or decryption module and associated memory. The encryption and/or decryption module may comprise security-sensitive information with, for instance, access to the information stored in the module requiring use of a variable key, and with the nature of the key being stored in the associated memory within the enclosure. 
     In one or more implementations, a tamper-proof electronic package such as depicted is configured or arranged to detect attempts to tamper-with or penetrate into electronic assembly enclosure  110 . Accordingly, electronic assembly enclosure  110  also includes, for instance, a monitor circuit which, if tampering is detected, activates an erase circuit to erase information stored within the associated memory, as well as the encryption and/or decryption module within the communications card. These components may be mounted on, and interconnected by, a multi-layer circuit board, such as a printed circuit board or other multi-layer substrate, and be internally or externally powered via a power supply provided within the electronic assembly enclosure. 
     In the embodiment illustrated, and as one example only, electronic assembly enclosure  110  may be surrounded by a tamper-respondent sensor  120 , an encapsulant  130 , and an outer, thermally conductive enclosure  140 . In one or more implementations, tamper-respondent sensor  120  may include a tamper-respondent laminate that is folded around electronic assembly enclosure  110 , and encapsulant  130  may be provided in the form of a molding. Tamper-respondent sensor  120  may include various detection layers, which are monitored through, for instance, a ribbon cable by the enclosure monitor, against sudden violent attempts to penetrate enclosure  110  and damage the enclosure monitor or erase circuit, before information can be erased from the encryption module. The tamper-respondent sensor may be, for example, any such article commercially available or described in various publications and issued patents, or any enhanced article such as disclosed herein. 
     By way of example, tamper-respondent sensor  120  may be formed as a tamper-respondent laminate comprising a number of separate layers with, for instance, an outermost lamination-respondent layer including a matrix of, for example, diagonally-extending or sinusoidally-extending, conductive or semi-conductive lines printed onto a regular, thin insulating film. The matrix of lines forms a number of continuous conductors which would be broken if attempts are made to penetrate the film. The lines may be formed, for instance, by printing carbon-loaded Polymer Thick Film (PTF) ink onto the film and selectively connecting the lines on each side, by conductive vias, near the edges of the film. Connections between the lines and an enclosure monitor of the communications card may be provided via, for instance, one or more ribbon cables. The ribbon cable itself may be formed of lines of conductive ink printed onto an extension of the film, if desired. Connections between the matrix and the ribbon cable may be made via connectors formed on one edge of the film. As noted, the laminate may be wrapped around the electronic assembly enclosure to define the tamper-respondent sensor  120  surrounding enclosure  110 . 
     In one or more implementations, the various elements of the laminate may be adhered together and wrapped around enclosure  110 , in a similar manner to gift-wrapping a parcel, to define the tamper-respondent sensor shape  120 . The assembly may be placed in a mold which is then filled with, for instance, cold-pour polyurethane, and the polyurethane may be cured and hardened to form an encapsulant  130 . The encapsulant may, in one or more embodiments, completely surround the tamper-respondent sensor  120  and enclosure  110 , and thus form a complete environmental seal, protecting the interior of the enclosure. The hardened polyurethane is resilient and increases robustness of the electronic package in normal use. Outer, thermally conductive enclosure  140  may optionally be provided over encapsulant  130  to, for instance, provide further structural rigidity to the electronic package. 
     Note that, as an enhancement, within a sealed electronic package, such as the tamper-proof electronic package depicted in  FIG. 1  and described above, structures and methods for facilitating heat transfer from one or more electronic components disposed therein outwards through the enclosure and any other layers of the electronic package may be provided. 
       FIG. 2  depicts in detail one embodiment of a typical tamper-proof electronic package  200 . Electronic package  200  is defined by, for instance, a base metal shell  202  and a top metal shell  204 . Outer surfaces of base metal shell  202  and top metal shell  204  may be provided with standoffs  206 , with an electronic assembly  208  resting on standoffs  206  defined in base metal shell  202 . Electronic assembly  208  may include, for instance, a printed circuit board  210  with electronic components  212  that are electrically connected via conductors (not shown) defined within or on printed circuit board  210 . 
     Hollow spacers  213  may be placed below dimples  206  in top metal shell  204 , and rivets  214  provided, extending through openings in dimples  206 , through hollow spacers  213  and through openings in printed circuit board  210  to base metal shell  202  in order to fixedly secure electronic assembly  208  within the enclosure formed by base and top metal shells  202 ,  204 . A security mesh or tamper-respondent sensor  216  is wrapped around the top, base, and four sides of the enclosure formed by base and top metal shells  202 ,  204 . As illustrated, in one or more embodiments, top metal shell  204  may have an opening through which a bus  220  extends. One end of bus  220  may be connected to conductors (not shown) on printed circuit board  210 , and the other end may be connected to conductors (not shown) on a printed circuit board  222 . As bus  220  passes through the opening, the bus extends between an inner edge region  223  of the security mesh  216  and an overlapping, outer edge region  224  of the security mesh  216 . A group of wires  226  connect, in one embodiment, security mesh  216  to conductors on printed circuit board  210 . Circuitry on printed circuit board  210  is responsive to a break or discontinuity in security sensor array  216 , in which case, an alarm signal may be emitted on bus  220 , and also encryption/decryption keys stored within electronic assembly  208  may be erased. 
     In one or more implementations, liquid polyurethane resin may be applied to security mesh  216  and cured. An outer, thermally conductive enclosure  228 , such as a copper enclosure, may be filled with liquid polyurethane resin with the electronic assembly and inner enclosure and security mesh suspended within it. Upon curing the resin, the electronic assembly and inner enclosure and security mesh become embedded in a polyurethane block or encapsulant  230 , as shown. The enclosure  228  is mounted on the printed circuit board  222 , which can be accomplished using, for instance, legs  240  which extend through slots in printed circuit board  222  and terminate in flanges  242 , which are then bent out of alignment with the slots. Bus  220  may be connected, by way of printed circuit board  222  to connectors  244  located along, for instance, one edge of printed circuit board  222 . 
     When considering tamper-proof packaging, the electronic package needs to maintain defined tamper-proof requirements, such as those set forth in the National Institutes of Standards and Technology (NIST) Publication FIPS 140-2, which is a U.S. Government Computer Security Standard, used to accredit cryptographic modules. The NIST FIPS 140-2 defines four levels of security, named Level 1 to Level 4, with Security Level 1 providing the lowest level of security, and Security Level 4 providing the highest level of security. At Security Level 4, physical security mechanisms are provided to establish a complete envelope of protection around the cryptographic module, with the intent of detecting and responding to any unauthorized attempt at physical access. Penetration of the cryptographic module enclosure from any direction has a very high probability of being detected, resulting in the immediate zeroization of all plain text critical security parameters (CSPs). Security Level 4 cryptographic modules are useful for operation in physically unprotected environments. Security Level 4 also protects a cryptographic module against a security compromise due to environmental conditions or fluctuations outside of the module&#39;s normal operating ranges for voltages and temperature. Intentional excursions beyond the normal operating ranges may be used by an attacker to thwart the cryptographic module&#39;s defenses. The cryptographic module is required to either include specialized environmental protection features designed to detect fluctuations and zeroize critical security parameters, or to undergo rigorous environmental failure testing to provide reasonable assurance that the module will not be affected by fluctuations outside of the normal operating range in a manner that can compromise the security of the module. 
     To address the demands of ever-improving anti-intrusion technology, and the higher-performance encryption/decryption functions being provided, enhancements to the tamper-proof, tamper-evident packaging for the electronic assembly at issue are desired. Numerous enhancements are described hereinbelow to, for instance, tamper-respondent assemblies and tamper-respondent sensors. Note that the numerous inventive aspects described herein may be used singly, or in any desired combination. Additionally, in one or more implementations, the enhancements to tamper-proof electronic packaging described herein may be provided to work within defined space limitations for existing packages. For instance, one or more of the concepts described may be configured to work with peripheral component interconnect express (PCIe) size limits, and the limitations resulting from being capsulated in, for instance, an insulating encapsulant. 
     Thus, disclosed hereinbelow with reference to  FIGS. 3A-21C  are various approaches and/or enhancements to creating a secure volume for accommodating one or more electronic components, such as one or more encryption and/or decryption modules and associated components of a communications card or other electronic assembly. 
       FIG. 3A  depicts a portion of one embodiment of a tamper-respondent layer  305  (or laser and pierce-respondent layer) of a tamper-respondent sensor  300  or security sensor, such as discussed herein. In  FIG. 3A , the tamper-respondent layer  305  includes circuit lines or traces  301  provided on one or both opposite sides of a flexible layer  302 , which in one or more embodiments, may be a flexible insulating layer or film.  FIG. 3A  illustrates circuit lines  301  on, for instance, one side of flexible layer  302 , with the traces on the opposite side of the film being, for instance, the same pattern, but (in one or more embodiments) offset to lie directly below spaces  303 , between circuit lines  301 . As described below, the circuit lines on one side of the flexible layer may be of a line width W and have a pitch or line-to-line spacing W s  such that piercing of the layer  305  at any point results in damage to at least one of the circuit lines traces  301 . In one or more implementations, the circuit lines may be electrically connected in-series or parallel to define one or more conductors which may be electrically connected in a network to an enclosure monitor, which monitors the resistance of the lines, as described herein. Detection of an increase, or other change, in resistance, caused by cutting or damaging one of the traces, will cause information within the encryption and/or decryption module to be erased. Providing conductive lines  301  in a pattern, such as a sinusoidal pattern, may advantageously make it more difficult to breach tamper-respondent layer  305  without detection. Note, in this regard, that conductive lines  301  could be provided in any desired pattern. For instance, in an alternate implementation, conductive lines  301  could be provided as parallel, straight conductive lines, if desired, and the pattern or orientation of the pattern may vary between sides of a layer, and/or between layers. 
     As noted, as intrusion technology continues to evolve, anti-intrusion technology needs to continue to improve to stay ahead. In one or more implementations, the above-summarized tamper-respondent sensor  300  of  FIG. 3A  may be disposed over an outer surface of an electronic enclosure, such as an electronic enclosure described above in connection with  FIGS. 1 &amp; 2 . Alternatively, as described further herein, the tamper-respondent sensor may cover or line an inner surface of an electronic enclosure to provide a secure volume about at least one electronic component to be protected. Numerous enhancements to the tamper-respondent sensor itself are described below. 
     In one or more aspects, disclosed herein is a tamper-respondent sensor  300  with circuit lines  301  having reduced line widths W l  of, for instance, 200 μm, or less, such as less than or equal to 100 μm, or even more particularly, in the range of 30-70 μm. This is contrasted with conventional trace widths, which are typically on the order of 350 μm or larger. Commensurate with reducing the circuit line width W l , line-to-line spacing width W s    303  is also reduced to less than or equal to 200 μm, such as less than or equal to 100 μm, or for instance, in a range of 30-70 μm. Advantageously, by reducing the line width W l  and line-to-line spacing W s  of circuit lines  301  within tamper-respondent sensor  300 , the circuit line width and pitch is on the same order of magnitude as the smallest intrusion instruments currently available, and therefore, any intrusion attempt will necessarily remove a sufficient amount of a circuit line(s) to cause resistance to change, and thereby the tamper intrusion to be detected. Note that, by making the circuit line width of the smaller dimensions disclosed herein, any cutting or damage to the smaller-dimensioned circuit line will also be more likely to be detected, that is, due to a greater change in resistance. For instance, if an intrusion attempt cuts a 100 μm width line by 50%, it reduces the remaining available line width for conducting current to 50 μm. This change is more likely to result in a detectable change in resistance, compared with, for instance, a 50% reduction in a more conventional line width of 350 μm to, for instance, 175 μm. The smaller the conductive circuit line width becomes, the more likely that a tampering of that line will be detected. 
     Note also that a variety of materials may advantageously be employed to form the circuit lines. For instance, the circuit lines may be formed of a conductive ink (such as a carbon-loaded conductive ink) printed onto one or both opposite sides of one or more of the flexible layers  302  in a stack of such layers. Alternatively, a metal or metal alloy could be used to form the circuit lines, such as copper, silver, silver carbon, or nickel-phosphorus (NiP), or Omega-Ply®, offered by Omega Technologies, Inc. of Culver City, Calif. (USA), or Ticer™ offered by Ticer Technologies, Chandler, Ariz. (USA). Note that the process employed to form the fine circuit lines or traces on the order described herein is dependent, in part, on the choice of material used for the circuit lines. For instance, if copper circuit lines are being fabricated, then additive processing, such as plating up copper traces, or subtractive processing, such as etching away unwanted copper between trace lines, may be employed. By way of further example, if conductive ink is employed as the circuit line material, fine circuit lines on the order disclosed herein can be achieved by focusing on the rheological properties of the conductive ink formulation. Further, rather than simple pneumatics of pushing conductive ink through an aperture in a stencil with a squeegee, the screen emulsion may be characterized as very thin (for instance, 150 to 200 μm), and a squeegee angle may be used such that the ink is sheared to achieve conductive ink breakaway rather than pumping the conductive ink through the screen apertures. Note that the screen for fine line width printing such as described herein may have the following characteristics in one specific embodiment: a fine polyester thread for both warp and weave on the order of 75 micrometers; a thread count between 250-320 threads per inch; a mesh thickness of, for instance, 150 micrometers; an open area between threads that is at least 1.5× to 2.0× the conductive ink particle size; and to maintain dimensional stability of the print, the screen snap-off is kept to a minimum due the screen strain during squeegee passage. 
     In one or more implementations, circuit lines  301  of tamper-respondent sensor  300  are electrically connected to define one or more resistive networks. Further, the circuit lines may include one or more resistive circuit lines by selecting the line material, line width W l  and line length L l , to provide a desired resistance per line. As one example, a “resistive circuit line” as used herein may comprise a line with 1000 ohms resistance or greater, end-to-end. In one specific example, a circuit line width of 50 μm, with a circuit line thickness of 10 μm may be used, with the line length L l  and material selected to achieve the desired resistance. At the dimensions described, good electrical conductors such as copper or silver may also be employed and still form a resistive network due to the fine dimensions noted. Alternatively, materials such as conductive ink or the above-noted Omega-Ply® or Ticer™ may be used to define resistive circuit lines. 
     In a further aspect, the flexible layer  302  itself may be further reduced in thickness from a typical polyester layer by selecting a crystalline polymer to form the flexible layer or substrate. By way of example, the crystalline polymer could comprise polyvinylidene difluoride (PVDF), or Kapton, or other crystalline polymer material. Advantageously, use of a crystalline polymer as the substrate film may reduce thickness of the flexible layer  302  to, for instance, 2 mils thick from a more conventional amorphous polyester layer of, for instance, 8 mils. A crystalline polymer can be made much thinner, while still maintaining structural integrity of the flexible substrate, which advantageously allows for far more folding, and greater reliability of the sensor after folding. Note that the radius of any fold or curvature of the sensor is necessarily constrained by the thickness of the layers comprising the sensor. Thus, by reducing the flexible layer thickness to, for instance, 2 mils, then in a four tamper-respondent layer stack, the stack thickness can be reduced from, for instance, 30 mils in the case of a typical polyester film, to 10 mils or less with the use of crystalline polymer films. 
     As noted, the circuit lines  301  forming the at least one resistive network may be disposed on either the first side or the second side of the opposite sides of the flexible layer(s)  302  within the tamper-respondent sensor  300 , or on both the first and second sides. One embodiment of this depicted in  FIG. 3B , wherein circuit lines  301  are illustrated on both opposite sides of flexible layer  302 . In this example, circuit lines  301  on the opposite sides of the tamper-respondent sensor  302  may each have line widths W l  less than or equal to 200 μm, and those lines widths may be the same or different. Further, the line-to-line spacing width W s  between adjacent lines of the circuit lines  301  may also be less than or equal to 200 μm, and may also be the same or different. In particular, the circuit lines may be different line widths on the two different sides of the tamper-respondent layer, and the line-to-line spacing widths may also be different. For instance, a first side of the tamper-respondent layer may have circuit line widths and line-to-line spacings of approximately 50 microns, while the second side of the tamper-respondent layer may have circuit lines and line-to-line spacing microns. Intrusion through the sensor is potentially made more difficult by providing such different widths. Circuit lines  301  on the opposite sides of the flexible layer  302  may also be in the same or different patterns, and in the same or different orientations. If in the same pattern, the circuit lines may be offset, as noted above, such that the circuit lines of one side align to spaces between circuit lines on the other side. 
     As illustrated in  FIG. 3C , the tamper-respondent sensor  300  may comprise a stack of tamper-respondent layers  305  secured together via an adhesive  311 , such as a double-sided adhesive film. The process may be repeated to achieve any desired number of tamper-respondent layers, or more particularly, any desired number of layers of circuit lines  301  within the tamper-respondent sensor to achieve a desired anti-intrusion sensor. 
     An alternate tamper-respondent sensor  300 ′ is depicted in  FIG. 3D , where multiple flexible layers  302  with circuit lines are secured together via an adhesive  311 , and by way of example, circuit lines are provided on one or both sides of each flexible layer. In this example, a first flexible layer  302  has first circuit lines  301  and a second flexible layer  302  has second circuit lines  301 ′. In one or more implementations the first circuit lines may have a first line width W l  and the second circuit lines may have a second line width W l , where the first line width of the first circuit lines  301  is different from the second line with the second circuit lines  301 ′. For instance, the first circuit line width may be 50 μm, and the second circuit line width may be 45 μm. Note that any desired combination of circuit line widths may be employed in this example, which assumes that the circuit line widths may be different between at least two of the layers. Additionally, the first circuit lines  301  of the first flexible layer may have first line-to-line spacing width W s  and the second circuit lines  301 ′ of second flexible layer may have a second line-to-line spacing width W s , where the first line-to-line spacing width of the first circuit lines may be different from the second line-to-line spacing width of the second circuit lines. Note that this concept applies as well to circuit lines on only one side of flexible layer  302 , where two or more of the flexible layers in the stack defining the tamper-respondent sensor may have different circuit line widths and/or different line-to-line spacing widths. This concept may be extended to any number of tamper-respondent layers within the tamper-respondent sensor to provide a desired degree of tamper protection. 
     In addition, or alternatively, the first circuit lines  301  of the first flexible layer may be formed of a first material, and the second circuit lines  301 ′ of the second flexible layer may be formed of a second material, where the first material of the first circuit lines  301  may be different from the second material of the second circuit lines  301 ′. For instance, first circuit lines  301  may be formed of conductive ink, and second circuit lines  301 ′ may be formed of a metal, such as copper. By providing tamper-respondent sensor  300 ′ with at least some of the circuit lines formed of a metal material, such as copper, enhanced tamper detection may be obtained. For instance, an intrusion tool passing through one or more layers of circuit lines  301 ′ formed of a metal could generate debris which may be distributed during the intrusion attempt and result in shorting or otherwise damaging one or more other tamper-respondent layers within the tamper-respondent sensor  300 ′. If desired, more than two materials may be employed in more than one layers of circuit lines within the tamper-respondent sensor. 
       FIG. 3E  depicts another embodiment of a tamper-respondent assembly  300 ″, in accordance with one or more aspects of the present invention. In this implementation, multiple tamper respondent layers  305  are secured with another flexible layer  320  in a stack using, for instance, one or more layers of an adhesive film  311 . In one or more implementations, the another flexible layer  320  could comprise a malleable metal film. In the example shown, the malleable metal film is disposed between two tamper-respondent layers  305 , and thus, is disposed between two layers of circuit lines  301  on the different tamper-respondent layers  305 . By way of example, malleable metal film  320  could comprise a sheet of copper or a copper alloy. By providing a thin malleable metal film  320  on the order of, for instance, 0.001″ thickness, an attempt to penetrate through tamper-respondent sensor  300 ″ would necessarily pass through malleable metal film  320 , and in so doing generate debris which would be carried along by the intrusion tool or drill. This metal debris would facilitate detection of the intrusion attempt by potentially shorting or otherwise damaging one or more of the tamper-respondent layers  305  within tamper-respondent sensor  300 ″. As a variation, the malleable metal film  320  could be applied directly to one side of a flexible layer  302  with the opposite side having circuit lines forming the at least one resistive network. Note that a similar concept applies where one or more of the layers of circuit lines  301  are formed of metal circuit lines, such as copper or silver, and other layers of circuit lines  301  are formed of, for instance, conductive ink. In such embodiments, clipping of one or more metal lines would generate metal debris that could carried along by the intrusion tool and ultimately interact with one or more other circuit lines of the tamper-respondent electronic circuit structure to enhance the likelihood of damage and thus detection of the intrusion attempt. 
     Based on the description provided herein, those skilled in the art will understand that the tamper-respondent sensors described above in connection with  FIGS. 3A-3E  may be employed with any of a variety of different tamper-respondent assemblies, and if desired, may be pre-formed in any of the various configurations described herein below. For instance, one or more of the tamper-respondent sensors of  FIGS. 3A-3E  could be used in conjunction with an electronic enclosure to enclose, at least in part, one or more electronic components to be protected, with the tamper-respondent sensor overlying or being adhered to an outer surface of the electronic enclosure. Alternatively, in one or more implementations, the tamper-respondent sensor could be provided to cover or line an inner surface of the electronic enclosure, such as in one or more of the tamper-respondent assembles described below. 
     In contrast to a prior tamper-respondent sensor which may utilize a single substrate of flexible dielectric with circuit lines, either on the upper or lower surface, or both surfaces, provided herein are tamper-respondent sensors which comprise, in one or more embodiments, multiple layers of materials and circuits to provide an enhanced tamper-proof, tamper-evident packaging, to meet the demands of ever-improving anti-intrusion technology requirements to protect encryption/decryption functions. By way of example,  FIGS. 4A &amp; 4B  depict tamper-respondent sensors comprising stacks of layers, each of which include at least one formed flexible layer, which may be configured, by way of example, as a corrugated layer of flexible dielectric with circuit lines on one or both sides. Note that as used herein, a “formed layer” refers to a specially-shaped layer manufactured with, for instance, curvatures extending, at least in part, out-of-plane. For example, in the case of a corrugated layer as shown, the curvatures have a vertical component that results in an undulation of the formed layer. 
     As illustrated in  FIG. 4A , a tamper-respondent sensor  400  may include, by way of example, a first sensor layer  410 , a second sensor layer  420 , and a third sensor layer  430 , with the second sensor layer  420  being sandwiched between the first and third sensor layers  410 ,  430 . In this configuration, second sensor layer  420  comprises a formed flexible layer  401  having opposite first and second sides with circuit lines  402  comprising, for instance, conductive lines, such as metal lines (e.g., Cu or Au lines), wires, printed conductive ink (e.g., carbon ink), resistive materials, etc., which form at least one resistive network on at least one of the first side or the second side of the formed flexible layer. In one or more embodiments, the circuit lines may comprise fine-pitched line circuitry, for instance, circuit lines in the 20-50 micron width range, and 20-50 micron spaces between the circuit lines. In one or more implementations, the formed flexible layer comprises, at least in part, a dielectric material (such as polyimide, Mylar™, Teflon™, etc.), with the layer in such an example being referred to as a corrugated layer of flexible dielectric that has the circuit lines overlying, at least in part, the curvatures of the corrugated layer of flexible dielectric, as illustrated. Note in the example of  FIG. 4A , a cross-section through the tamper-respondent sensor  400  intersects multiple layers of circuit lines on the different sensor layers. The wiring patterns of the circuit lines may be in any desired configuration. For instance, circuit lines may be orthogonal or angled, or randomly arranged, with respect to adjacent or underlying or overlying circuit lines of the tamper-respondent sensor. This option applies to any of the tamper-respondent sensors disclosed herein, where circuit lines are provided on multiple different surfaces of a tamper-respondent sensor. As a further variation, each tamper-respondent electronic circuit structure may have a unique circuit line configuration or set of circuit line configurations associated with, for instance, a serial number of the tamper-respondent electronic circuit structure being provided. Also, any desired number of sensor layers may be associated with the at least one formed flexible layer of the tamper-respondent sensor. 
     Therefore, in one or more embodiments, first sensor layer  410  and third sensor layer  430  may also each comprise a flexible layer of material having circuit lines forming one or more resistive networks disposed on the first and/or second sides thereof. For instance, conductive circuit lines may be provided on both the first and second sides of the flexible layers of the first sensor layer  410 , the second sensor layer  420 , and the third sensor layer  430 , such that a vertical cross-section through the stack of layers intersects multiple layers of circuit lines. In this configuration, forming the second sensor layer  420  with curvatures, for instance, forming the second layer to be corrugated, advantageously enhances protection against physical intrusion, such as by a drill, without detection by the resistive networks by making the location of the circuit lines defining the resistive network(s) harder to identify. 
     By way of example, the second sensor layer  420  may initially comprise a thin, flexible layer of material, such as a thin, flexible layer with a thickness comparable to the desired minimum radius of the bending curvature for the desired corrugation of the second sensor layer. In one or more implementations, the second sensor layer may be corrugated by obtaining a flat, flexible sensor which is then fed through a set of heated top and bottom rollers, each with mating gear teeth to create the desired sinusoidal pattern in the sensor layer. One or more outer circuit layers or films comprising the circuit lines forming the one or more resistive networks may then be laminated, as desired, to one or both of the first and second sides of the formed layer to define the formed, flexible layer. In one or more implementations, an adhesive may be employed to affix the circuit layers or films comprising the one or more resistive networks to the formed layer. By way of example, the adhesive could include a PSA, epoxy, acrylic, thermoset, thermoplastic, electrically conductive epoxy, thermally conductive epoxy, etc., one or more of which could also be employed to affix the multiple sensor layers  410 ,  420 ,  430  together within the stack of layers. 
     As illustrated in  FIG. 4B , multiple second, corrugated layers  420  of flexible dielectric with circuit lines may be provided in the stack of layers of the tamper-respondent sensor  400  with, for instance, adjacent corrugated layers of flexible dielectric being separated by a substantially flat flexible layer, with or without additional circuit lines defining one or more additional resistive networks. In the embodiment depicted, adjacent corrugated layers of flexible dielectric with circuit lines are separated by a sensor layer  425 , which again may include circuit lines on one or both sides thereof. 
     Connections of the tamper-respondent sensors, and sensor layers, described herein to, for instance, monitor circuitry disposed within the associated secure volume defined by the tamper-respondent electronic circuit structure may comprise input/output contacts or connectors formed on one or more edges of the tamper-respondent sensor (or sensor layer) or, for instance, one or more ribbon cables extending from the tamper-respondent sensor into the secure volume, as will be understood by one skilled in the art. 
       FIGS. 5A &amp; 5B  depict another embodiment of a tamper-respondent sensor  500 , in accordance with one or more aspects of the present invention. As illustrated, tamper-respondent sensor  500  includes at least one formed flexible layer  510  having opposite first and second surfaces  511 ,  512 . Circuit lines  501  forming at least one resistive network are provided on at least one of the first or second sides  511 ,  512  of formed flexible layer  510 . As noted, the circuit lines may comprise any desired pattern of conductive circuit lines advantageous for a particular tamper-respondent sensor technology, and may include multiple sets of circuit lines in different regions or zones of the formed flexible layer. By way of example, the circuit lines may comprise conductive lines such as metal lines (e.g., copper lines), wires, printed conductive ink (e.g., carbon ink), etc. provided on one or both of the first and second sides of formed flexible layer  510 . As illustrated in  FIG. 5A , the formed flexible layer again includes curvatures  513 , with the formed flexible layer with curvatures being collapsed as a flattened, folded layer in this embodiment. Note that circuit lines  501  forming the at least one resistive network on the first side  511  or second side  512  of formed flexible layer  510  overlie, at least in part, at least some of curvatures  513 , such that the circuit lines wrap over or within the curvatures  513  and in transverse cross-section view, provide multiple layers of circuit lines formed on the same curving surface of the formed flexible layer. In one or more embodiments, formed flexible layer  510  may be a corrugated layer that has been flattened by applying a z-direction force with a metered x-y shear force, creating a controlled, flattening collapse of the multi-dimensional, formed flexible layer  510 . 
       FIGS. 5C-5H  depict various examples of a tamper-respondent sensor comprising a stack of layers, with one or more of the layers comprising a formed flexible layer  510 , such as described above in connection with  FIGS. 5A &amp; 5B . By way of example,  FIG. 5C  depicts a stack of layers comprising formed flexible layer  510  with at least one other layer  520  overlying one or both of first side  511  and second side  512  of formed flexible layer  510 . In one or more embodiments, the at least one other layer  502  overlying the formed flexible layer  510  may be, or include, an opaque layer of material to mask location of the circuit lines, or a breakable layer of material that will shatter with tampering and facilitate damaging the circuit lines forming the at least one resistive network to assist with detection of the attack on the tamper-respondent sensor. In one or more implementations, the opaque layer of material may be dark, non-transparent material obscuring what lies beneath, or, in one or more embodiments, the opaque material could be the same color material as the resistive circuit lines of the underlying layer, both obscuring and camouflaging the covered circuit lines. 
     In  FIG. 5D , multiple other layers  521 ,  522  overlie one or both sides  511 ,  512  of formed flexible layer  510 . In one example, the multiple other layers may include both a breakable layer  521  and an opaque layer  522  disposed on one side of formed flexible layer  510 , or both sides of formed flexible layer  510 . 
     In further embodiments, one or more of the other layers may themselves comprise a flexible dielectric material with circuit lines forming at least one other resistive network on one of the first side or second side thereof.  FIG. 5E  depicts one other layer  520  overlying second side  512  of formed flexible layer  510 , and  FIG. 5F  depicts one other layer  520  overlying first side  511 , and one other layer  520  overlying second side  512  of formed flexible layer  510 . 
       FIGS. 5G &amp; 5H  depict additional embodiments of a tamper-respondent sensor  500  comprising a stack of layers. In these embodiments, multiple formed flexible layers  510  are provided along with one or more other layers overlying one or more sides  511 ,  512  of the formed flexible layers  510 . As noted, the one or more other layers may comprise a variety of layers, such as an opaque flexible layer, a breakable layer, or additional flexible layers with circuit lines forming additional resistive networks of the tamper-respondent sensor, as desired to provide enhanced tamper-proof, tamper evident packaging for a particular application. 
       FIGS. 6A-6G  depict various embodiments of a tamper-respondent assembly, generally denoted  600 , within which a secure volume is defined for protecting one or more electronic components or an electronic assembly, such as discussed herein. Referring to  FIG. 6A , tamper-respondent assembly  600  may include an electronic enclosure  601 , such as a rigid, conductive enclosure, and a tamper-respondent electronic circuit structure  602  associated with the electronic enclosure  601 . As shown, tamper-respondent electronic circuit structure  602  comprises a tamper-respondent sensor  605 . In one or more implementations, tamper-respondent sensor  605  includes at least one formed flexible layer having opposite first and second sides, circuit lines forming at least one resistive network disposed on at least one of first or second sides, and formed curvatures provided within the formed flexible layer, with the circuit lines overlying, at least in part, the curvatures of the formed flexible layer, such as described above in connection with the exemplary embodiments of  FIGS. 4A-5H . 
     In the implementation of  FIG. 6A , a single, continuous tamper-respondent sensor  605  is provided, which is wrapped around to encircle electronic enclosure  601 , and which includes overlaps  606  where the ends join along the electronic enclosure  601 . For instance, the single, continuous tamper-respondent sensor  605  could be folded around electronic enclosure  601  in any manner analogous to wrapping a present. By way of example only, electronic enclosure  601  may be a 6-sided metal container, sized to accommodate the electronic to be protected. Further, in this configuration, tamper-respondent sensor  605 , configured as described herein, may be employed in an electronic assembly package such as described initially in connection with  FIGS. 1 &amp; 2 . 
       FIGS. 6B-6G  depict further embodiments of tamper-respondent assembly  600 . In one or more of these embodiments, multiple discrete tamper-respondent sensors  610 ,  611 ,  612 , are illustrated. By way of example, each tamper-respondent sensor may include one or more formed flexible layers with circuit lines extending, at least in part, over the curvatures of the formed flexible layers, such as described herein. 
     In the example of  FIG. 6B  an upper tamper-respondent sensor  610  and a lower tamper-respondent sensor  611  are provided overlying the upper and lower main surfaces, respectively, of electronic enclosure  601 . Additionally, a sidewall tamper-respondent sensor  612  wraps around the edge of electronic enclosure  601 , and in this example, is of sufficient width to fold over and thus overlap  606  upper tamper-respondent  610  and lower tamper-respondent  611 , as illustrated. The extent of overlap  606  may be customized as desired to inhibit a line of attack through the tamper-respondent electronic circuit structure at the seams where different tamper-respondent sensors  610 ,  611 ,  612  meet. 
       FIG. 6C  is an upper plan view of one embodiment of the assembly of  FIG. 6B , with tamper-respondent sensor  612  shown wrapping over upper tamper-respondent sensor  610  provided over the upper main surface of electronic enclosure  601 . An analogous wrapping of tamper-respondent sensor  612  over the lower tamper-respondent sensor may also be employed. Note in this configuration the provision of diagonal folds  615  at the corners, where tamper-respondent sensor  612  overlaps upper tamper-respondent sensor  610 . This overlap and fold example of  FIG. 6C  is provided by way of example only, and other overlap and fold configurations may be employed, without departing from the scope of the claims presented herewith. In this six-sided enclosure example, sidewall tamper-respondent sensor  612  is a separate sensor that wraps around the perimeter of electronic assembly enclosure  601 , and overlaps the upper and lower tamper-respondent sensors  610 ,  611 , respectively. 
       FIG. 6D  depicts a variation on the tamper-respondent assembly  600  of  FIG. 6B , wherein upper tamper-respondent sensor  610  and lower tamper-respondent sensor  611  are extended past the upper and lower surfaces, respectively, of electronic enclosure  601  and folded to overlap sidewall tamper-respondent sensor  612  provided along the edge or perimeter of electronic enclosure  601 . 
       FIGS. 6E &amp; 6F  depict alternate embodiments of upper tamper-respondent sensor  610 , for use in various embodiments of a tamper-respondent assembly, including, for instance, an assembly such as depicted in  FIG. 6D , where upper tamper-respondent sensor  610  wraps over an edge of the enclosure and sidewall tamper-respondent sensor  612  is sized to the width of the edge or sidewall of electronic enclosure  601 . In the upper tamper-respondent sensor  610  embodiment of  FIG. 6E , corner cutouts or indents  607  are established to facilitate folding of the illustrated edge flaps over the sidewall tamper-respondent sensor  612  as illustrated, for instance, in  FIG. 6D . If desired, where the edge flaps meet at the corners of the assembly package, additional corner tamper-respondent sensors (not shown) may be used as patches over the corners to provide still further tamper-proof, tamper-evident packaging along the respective seams of the edge flaps. Note that, in this configuration, the sidewall tamper-respondent sensor  612  ( FIG. 6D ) wraps fully around the electronic enclosure  601 , and thus, necessarily provides coverage at the seams where the edge flaps meet. 
     As a variation,  FIG. 6F  depicts upper tamper-respondent sensor  610  with corner cutouts  608  reduced to slots or channels to define tabs from the edge flaps at the corners of the edge flaps, so as to allow for a further folding along the edge or sidewall of the electronic enclosure  601 . That is, the further tabs may be transversely folded over the seam between adjoining edge flaps at the corners when the upper tamper-respondent sensor  610  is folded over sidewall tamper-respondent sensor  612 , such as illustrated in  FIG. 6D . 
     Note that in the embodiments of  FIGS. 6A-6D , the tamper-respondent sensors may have respective input/output (I/O) cabling extending off one or more ends thereof in a region under an overlap area of two of the tamper-respondent sensors, with the I/O cabling extending into the secure volume of the package to make electrical connection with the tamper-detect circuitry. For instance, the resistive networks within the tamper-respondent sensors may be electrically coupled to circuitry within the secure volume for monitoring the networks. The monitor circuitry may include various bridge and/or compare circuits, and utilize conventional electrical interconnect inside the secure volume of electronic enclosure  601 . Any of a number of interconnect configurations may be employed dependent, for instance, on the number and characteristics of the resistive networks provided within the tamper-respondent sensors. 
     In the embodiment of  FIG. 6G , tamper-respondent sensor  612  ( FIG. 6D ) is removed, and size of the upper and lower tamper-respondent sensors  610 ,  611  is extended to allow the upper and lower tamper-respondent sensors  610 ,  611  to overlap along the edge or sidewall of electronic enclosure  601  where folded, as shown. 
     Note that although depicted in  FIGS. 6A-6G  as flattened, folded layers, one or more of the respective tamper-respondent sensors illustrated could be otherwise implemented. For instance, one or more of the upper, lower, or sidewall tamper-respondent sensors in the depicted examples could comprise other formed, flexible layers, such as depicted in  FIGS. 4A-5H , or other non-formed, flexible layers, with circuit lines forming resistive networks on one or both opposing sides of the flexible layer(s). Further examples of tamper-respondent sensors are described below in connection with  FIGS. 7A-7C . 
       FIGS. 7A-7C  depict a further embodiment of a tamper-respondent circuit structure comprising one or more multi-sensor interweaved layers. In particular, a first tamper-respondent sensor  700  is depicted in  FIG. 7A  as comprising at least one first layer  701 , such as a flexible dielectric layer having opposite first and second sides. First circuit lines  702  are provided on at least one of the first side or the second side of the at least one first layer  701  to form at least one first resistive network. Multiple slits  703  are provided within the at least one first layer  701 , with the first circuit lines being located back from the multiple slits  703  a specified minimum distance, such as 2-10 mils. 
     As illustrated in  FIG. 7B , the tamper-respondent electronic circuit structure may comprise a multi-sensor interweaved layer  720  defined by interweaving first tamper-respondent sensor  700  with a second tamper-respondent sensor  710  constructed, in one or more embodiments, similarly to first tamper-respondent sensor  700 . In particular, second tamper-respondent sensor  710  may include at least one layer, such as at least one flexible layer having opposite first and second sides, and second circuit lines disposed on at least one of the first or second sides of the at least one second layer, which define at least one second resistive network. Note that the at least one first resistive network and at least one second resistive network may be the same or differently patterned resistive networks. For instance, in one or more implementations, the width of the circuit lines or traces in the different resistive networks, as well as the pitch between lines, may be varied. By providing slits within first tamper-respondent sensor  700  and second tamper-respondent sensor  710 , the resultant fingers of the first and second tamper-respondent sensors may be interweaved to define multi-sensor interweaved layer  720  to comprise a checkerboard pattern, as illustrated by way of example in  FIG. 7B . 
     As depicted in  FIG. 7C , an enhanced tamper-respondent electronic circuit structure may be obtained by stacking two multi-sensor interweaved layers  720 , with the slit lines, and in particular, the intersections of the slit lines, offset to provide a more tamper-proof, tamper-evident electronic structure. For instance, the multi-sensor interweaved layer  720  of  FIG. 7B  may be a first multi-sensor interweaved layer, and the circuit structure may include a second multi-sensor interweaved layer overlying the first sensor interweaved layer, with the second multi-sensor interweaved layer being formed from additional discrete tamper-respondent sensors, in a manner such as described above in connection with  FIGS. 7A &amp; 7B . In this configuration, the second multi-sensor interweaved layer  720 ′ may be constructed such that the slit intersections between the discrete tamper-respondent sensors interweaved into that layer do not align with those of the first multi-sensor interweaved layer  720  when stacked. For instance, the slits in the second multi-sensor interweaved layer  720 ′ could be differently spaced apart from those in the first multi-sensor interweaved layer  720  such that the intersections of the slits in the second multi-sensor interweaved layer are offset from those in the first multi-sensor interweaved layer, or second multi-sensor interweaved layer  720 ′ could be identically constructed as first multi-sensor interweaved layer  702 , but offset slightly from the first multi-sensor interweaved layer when stacked. Advantageously, in one or more implementations, one or more multi-sensor interweaved layers  720 ,  720 ′ may be employed as one of the upper, lower, or sidewall tamper-respondent sensors described above in connection with  FIGS. 6A-6G , if desired. 
     By way of further example,  FIGS. 8A &amp; 8B  depict one embodiment of another tamper-respondent assembly, or tamper-proof electronic package  800 , which comprises an electronic circuit  815 , in accordance with one or more further aspects of the present invention. 
     Referring collectively to  FIGS. 8A &amp; 8B , electronic circuit  815  includes a multilayer circuit board  810  which has an embedded tamper-respondent sensor  811  therein that facilitates defining, in part, a secure volume  801  associated with multilayer circuit board  810  that extends into multilayer circuit board  810 . In particular, in the embodiment of  FIGS. 8A &amp; 8B , secure volume  801  exists partially within multilayer circuit board  810 , and partially above multilayer circuit board  810 . One or more electronic components  802  are mounted to multilayer circuit board  810  within secure volume  801  and may comprise, for instance, one or more encryption modules and/or decryption modules, and associated components, with the tamper-proof electronic package comprising, in one or more embodiments, a communications card of a computer system. 
     Tamper-proof electronic package  800  further includes an enclosure  820 , such as a pedestal-type enclosure, mounted to multilayer circuit board  810  within, for instance, a continuous groove (or trench)  812  formed within an upper surface of multilayer circuit board  810 . In one or more embodiments, enclosure  820  may comprise a thermally conductive material and operate as a heat sink for facilitating cooling of the one or more electronic components  802  within the secure volume. A security mesh or tamper-respondent sensor  821 , such as the above-described tamper-respondent sensors of  FIGS. 4A-7C , may be associated with enclosure  820 , for example, wrapping around the inner surface of enclosure  820  to facilitate defining, in combination with tamper-respondent sensor  811  embedded within multilayer circuit board  810 , secure volume  801 . In one or more implementations, tamper-respondent sensor  821  extends down into continuous groove  812  in multilayer circuit board  810  and may, for instance, even wrap partially or fully around the lower edge of enclosure  820  within continuous groove  812  to provide enhanced tamper detection where enclosure  820  couples to multilayer circuit board  810 . In one or more implementations, enclosure  820  may be securely affixed to multilayer circuit board  810  using, for instance, a bonding material such as an epoxy or other adhesive. 
     As depicted in  FIG. 8B , one or more external circuit connection vias  813  may be provided within multilayer circuit board  810  for electrically connecting to the one or more electronic components  802  ( FIG. 8A ) within secure volume  801 . These one or more external circuit connection vias  813  may electrically connect to one or more external signal lines or planes (not shown) embedded within multilayer circuit board  810  and extending, for instance, into a secure base region of (or below) secure volume  801 , as explained further below. Electrical connections to and from secure volume  801  may be provided by coupling to such external signal lines or planes within the multilayer circuit board  810 . 
     As noted with reference to  FIGS. 8A &amp; 8B , secure volume  801  defined in association with multilayer circuit board  810  may be sized to house electronic components  802  to be protected, and be constructed to extend into multilayer circuit board  810 . In one or more implementations, multilayer circuit board  810  includes electrical interconnect within the secure volume  801  defined in the board, for instance, for electrically connecting the multiple tamper-respondent layers of the embedded tamper-respondent sensor  811  to associated monitor circuitry also disposed within secure volume  801 . 
     Note that the embodiment depicted in  FIGS. 8A &amp; 8B  is presented by way of example only. In one or more other implementations, the electronic circuit may comprise multiple multilayer circuit boards, each with a tamper-respondent sensor embedded within the multilayer circuit board with an appropriate connector, located within a secure volume defined between two adjacent multilayer circuit boards, interconnecting selected wiring of the multilayer circuit boards. In such an implementation, the overlying multilayer circuit board could be hollowed out to accommodate, for instance, the connector and/or one or more other electronic components between the multilayer circuit boards. In addition, other configurations of enclosure  820 , and/or other approaches to coupling enclosure  820  and multilayer circuit board  810  may be employed. 
     By way of further example,  FIG. 9  depicts a partial cross-sectional elevational view of one embodiment of multilayer circuit board  810  and enclosure  820 . In this configuration, the embedded tamper-respondent sensor includes multiple tamper-respondent layers including, by way of example, at least one tamper-respondent mat (or base) layer  900 , and at least one tamper-respondent frame  901 . In the example depicted, two tamper-respondent mat layers  900  and two tamper-respondent frame  901  are illustrated, by way of example only. The lower-most tamper-respondent mat layer  900  may be a continuous sense or detect layer extending completely below the secure volume being defined within multilayer circuit board  810 . One or both tamper-respondent mat layers  900  below secure volume  801  may be partitioned into multiple circuit zones, as discussed further below. Within each tamper-respondent mat layer, or more particularly, within each circuit zone of each tamper-respondent mat layer, multiple circuits or conductive traces are provided in any desired configuration, such as the configuration described above in connection with  FIG. 3 . Further, the conductive traces within the tamper-respondent layers may be implemented as, for instance, a resistive layer which is difficult to attach shunt circuits to, as explained further below. 
     As illustrated, one or more external signal lines or planes  905  enter secure volume  801  between, in this embodiment, two tamper-respondent mat layers  900 , and then electrically connect upwards into the secure volume  801  through one or more conductive vias, arranged in any desired location and pattern. In the configuration depicted, the one or more tamper-respondent frames  901  are disposed at least inside of the area defined by continuous groove  812  accommodating the base of enclosure  820 . Together with security sensor  821  associated with enclosure  820 , tamper-respondent frames  901  define secure volume  801  where extending, in part, into multilayer circuit board  810 . With secure volume  801  defined, at least in part, within multilayer circuit board  810 , the external signal line(s)  905  may be securely electrically connected to, for instance, the one or more electronic components  802  ( FIG. 8A ) mounted to multilayer circuit board  810  within secure volume  801 . In addition, the secure volume  801  may accommodate electrical interconnection of the conductive traces of the multiple tamper-respondent layers, for instance, via appropriate monitor circuitry. 
     Added security may be provided by extending tamper-respondent mat layers  900  (and if desired, tamper-respondent frames  901 ) outward past continuous groove  812  accommodating enclosure  820 . In this manner, a line of attack may be made more difficult at the interface between enclosure  820  and multilayer circuit board  810  since the attack would need to clear tamper-respondent mat layers  900 , the bottom edge of tamper-respondent sensor  821  associated with enclosure  820 , as well as the tamper-respondent frames  901  of the embedded tamper-respondent sensor. 
     Variations on the multilayer circuit board  810  of  FIG. 8A  are possible. For instance, in this embodiment, the embedded tamper-respondent sensor include multiple tamper-respondent mat layers  900  and multiple tamper-respondent frames  901 , such as described above, and a tri-plate structure comprising one or more external signal lines or layers sandwiched between an upper ground plane and a lower ground plane. In this configuration, high-speed transfer of signals to and from the secure volume, and in particular, to and from the one or more electronic components resident within the secure volume, would be facilitated. 
     Note also that, once within the secure volume is defined within multilayer circuit board  810 , conductive vias within the secure volume between layers of multilayer circuit board  810  may be either aligned, or offset, as desired, dependent upon the implementation. Alignment of conductive vias may facilitate, for instance, providing a shortest connection path, while offsetting conductive vias between layers may further enhance security of the tamper-proof electronic package by making an attack into the secure volume through or around one or more tamper-respondent layers of the multiple tamper-respondent layers more difficult. 
     The tamper-respondent layers of the embedded tamper-respondent sensor formed within the multilayer circuit board of the electronic circuit or electronic package may include multiple conductive traces or lines formed between, for instance, respective sets of input and output contacts or vias at the trace termination points. Any number of conductive traces or circuits may be employed in defining a tamper-respondent layer or a tamper-respondent circuit zone within a tamper-respondent layer. For instance, 4, 6, 8, etc., conductive traces may be formed in parallel (or otherwise) within a given tamper-respondent layer or circuit zone between the respective sets of input and output contacts to those conductive traces. 
     In one or more implementations, the multilayer circuit board may be a multilayer wiring board or printed circuit board formed, for instance, by building up the multiple layers of the board.  FIG. 10  illustrates one embodiment for forming and patterning a tamper-respondent layer within such a multilayer circuit board. 
     As illustrated in  FIG. 10 , in one or more implementations, a tamper-respondent layer, such as a tamper-respondent mat layer or a tamper-respondent frame disclosed herein, may be formed by providing a material stack comprising, at least in part, a structural layer  1001 , such as a pre-preg (or pre-impregnated) material layer, a trace material layer  1002  for use in defining the desired trace patterns, and an overlying conductive material layer  1003 , to be patterned to define conductive contacts or vias electrically connecting to the pattern of traces being formed within the trace material layer  1002 , for instance, at trace terminal points. In one or more implementations, the trace material layer  1002  may comprise nickel phosphorous (NiP), and the overlying conductive layer  1003  may comprise copper. Note that these materials are identified by way of example only, and that other trace and/or conductive materials may be used within the build-up  1000 . 
     A first photoresist  1004  is provided over build-up  1000 , and patterned with one or more openings  1005 , through which the overlying conductive layer  1003  may be etched. Depending on the materials employed, and the etch processes used, a second etch process may be desired to remove portions of trace material layer  1002  to define the conductive traces of the subject tamper-respondent layer. First photoresist  1004  may then be removed, and a second photoresist  1004 ′ is provided over the conductive layer  1003  features to remain, such as the input and output contacts. Exposed portions of conductive layer  1003  are then etched, and the second photoresist  1004 ′ may be removed, with any opening in the layer being filled, for instance, with an adhesive (or pre-preg) and a next build-up layer is provided, as shown. Note that in this implementation, most of overlying conductive layer  1003  is etched away, with only the conductive contacts or vias remaining where desired, for instance, at the terminal points of the traces formed within the layer by the patterning of the trace material layer  1002 . Note that any of a variety of materials may be employed to form the conductive lines or traces within a tamper-respondent layer. Nickel-phosphorous (NiP) is particularly advantageous as a material since it is resistant to contact by solder, or use of a conductive adhesive to bond to it, making it harder to bridge from one circuit or trace to the next during an attempt to penetrate into the protected secure volume of the electronic circuit. Other materials which could be employed include OhmegaPly®, offered by Ohmega Technologies, Inc., of Culver City, Calif. (USA), or Ticer™, offered by Ticer Technologies of Chandler, Ariz. (USA). 
     The trace lines or circuits within all of the tamper-respondent layers, and in particular, the tamper-respondent circuit zones, of the embedded tamper-respondent sensor, along with the tamper-respondent sensor  821 , may be electrically connected into monitor or compare circuitry provided, for instance, within secure volume  801  of multilayer circuit board  810 . The monitor circuitry may include various bridge or compare circuits, and conventional printed wiring board electrical interconnect inside the secure volume  801 , for instance, located within the secure volume defined by the tamper-respondent frames  901  ( FIG. 9 ), and the tamper-respondent mat layers. 
     Note that advantageously, different tamper-respondent circuit zones on different tamper-respondent layers may be electrically interconnected into, for instance, the same comparator circuit or Wheatstone bridge of the monitor circuitry. Thus, any of a large number of interconnect configurations may be possible. For instance, if each of two tamper-respondent mat layers contains  30  tamper-respondent circuit zones, and each of two tamper-respondent frames contains  4  tamper-respondent circuit zones, then, for instance, the resultant  68  tamper-respondent circuit zones may be connected in any configuration within the secure volume to create the desired arrangement of circuit networks within the secure volume being monitored for changes in resistance or tampering. Note in this regard, that the power supply or battery for the tamper-respondent sensor may be located external to the secure volume, with the sensor being configured to trip and destroy any protected or critical data if the power supply or battery is tampered with. 
       FIG. 11  is a partial enlarged view of the tamper-respondent assembly of  FIGS. 8A-9 , with a tamper-respondent electronic circuit structure  1100  comprising multiple tamper-respondent sensors  821 ,  822  disposed on the inner surface  825  of enclosure  820 . As illustrated, in one embodiment, enclosure  820  and the inner sidewall-disposed, tamper-respondent sensor  821  align at the bottom edge and extend into a continuous groove or trench  812  in multilayer circuit board  810  comprising, or associated with, the electronic circuit to be protected, as explained above. Tamper-respondent sensors  821 ,  822  may overlap, for instance, a few millimeters, or more, in order to provide added tamper-proof protection along the seam, where the tamper-respondent sensors  821 ,  822  meet within the electronic assembly enclosure  820 . 
       FIG. 12  depicts one example of a process for adhering a tamper-respondent sensor  1200  to the inner surface  825  of enclosure  820 . In this example, a single tamper-respondent sensor  1200  replaces the tamper-respondent sensors  821 ,  822  in the example of  FIG. 11 . However, the adhering apparatus and approach of  FIG. 12  could also be applied to securing multiple tamper-respondent sensors  821 ,  822  to, for instance, the inner surface of electronic assembly enclosure  820 . As illustrated, a bounding fixture  1210  may be used in combination with a first push and clamp mechanism  1211 , and a second push and clamp mechanism  1212 , to hold the tamper-respondent sensor  1200  in place while an adhesive  1205  cures between the tamper-respondent sensor and inner surface  825  of enclosure  820 . Note that, in one or more embodiments, the radius of the tamper-respondent sensor  1200  in the corners of the enclosure can vary to account for tolerances of the flexible tamper-respondent sensor comprising the one or more formed flexible layers. 
       FIGS. 13A-13C  depict one example of a process for adhering tamper-respondent electronic circuit structure  1100  of  FIG. 11  to inner surface  825  of electronic enclosure  820 . As illustrated in the underside, isometric view of  FIG. 13A , inner surface  825  of electronic enclosure  820  includes an inner sidewall surface  1300  and an inner main surface  1301 . Tamper-respondent sensor  821 , also referred to herein as an inner-sidewall tamper-respondent sensor, may be secured to the inner sidewall surface of enclosure  820  using an adhesive, such as a thermoset adhesive, in combination with an appropriate bonding fixture and push and clamp mechanisms (not shown) to, for instance, align tamper-respondent sensor  821  to the bottom edge of inner sidewall surface  1300 . 
     As depicted in  FIG. 13B , in one or more implementations, tamper-respondent sensor  822  may concurrently or subsequently be affixed to main inner surface  1301  ( FIG. 13A ) of electronic enclosure  820  using a bonding agent such as a thermoset adhesive. Note that in this embodiment, tamper-respondent sensor  822  is sized to overlap, at least in part, tamper-respondent sensor  821  affixed to inner sidewall surface  1300  ( FIG. 13A ). This overlap is depicted in greater detail in the partial enlargement of an inner corner  1302  of electronic enclosure  820  in  FIG. 13C . Advantageously, by providing two or more discrete tamper-respondent sensors, securing of the tamper-respondent electronic circuit structure to an electronic enclosure is facilitated by allowing more flexibility to align a particular sensor to a particular portion of the electronic enclosure, such as to the bottom edge of the inner sidewall surface of electronic enclosure  820 , as in the example of  FIGS. 13A-13C . 
     As illustrated in  FIG. 13C , inner corner  1302  of electronic enclosure  820  may be shaped with one or more curved portions and one or more flat, angled-sidewall portions to facilitate wrapping of tamper-respondent sensor  821  over inner sidewall surface  1300  (as explained further below). Note further that, within electronic enclosure  820 , various tamper-respondent sensor overlap and folding arrangements may be employed, as the case with the external wrapping of sensors about the electronic enclosure depicted in  FIGS. 6A-6G . In the embodiment of  FIGS. 13A-13C , the connections to the resistive networks of the tamper-respondent sensors  821 ,  822  may be provided in the region of the overlap, between the sensors, which in the embodiment of  FIGS. 8A-10 , facilitate defining the secure volume between the electronic enclosure and the multilayer circuit board. 
     One consideration with a tamper-respondent assembly, and more particularly, a tamper-respondent electronic circuit structure such as described herein, arises from the need to transition the inner-sidewall tamper-respondent sensor through one or more inner-sidewall corners of an electronic enclosure such as described. As noted above, in one or more embodiments, the tamper-respondent electronic circuit structure comprises one or more tamper-respondent sensors, which are adhesively mounted or affixed to the inner surfaces of the electronic enclosure. These inner surfaces include an inner main surface, and an inner sidewall surface having, for instance, at least one inner-sidewall corner. As noted, the tamper-respondent sensor(s) may each be formed of one or more flexible layers having circuit lines on one or more layers which define tamper-detect networks, such as resistive networks, that may be connected to monitor circuitry for detection of intrusion attempts into the secure space defined by the tamper-respondent assembly. During fabrication, the flexible layers of the tamper-respondent sensor(s) could stretch and potentially buckle within one or more inner-sidewall corners of the electronic enclosure as the sensor is mounted to the enclosure. This stretching or buckling within the corner(s) could result in breaking one or more circuit lines defining the tamper-detect networks to be monitored, which would destroy the tamper-respondent assembly for its intended use. Further, any buckling of the tamper-respondent sensor(s) over the inner surface, such as at the inner-sidewall corner(s) of an inner sidewall surface, could result in potential breach points, which would cause the tamper-respondent assembly to fail a NIST FIPS 140-2 Level 4 security test. Described hereinbelow with reference to  FIGS. 14A-18  therefore, are various enhancements to tamper-respondent assemblies such as disclosed herein which address this concern. 
       FIGS. 14A &amp; 14B  depict in greater detail one embodiment of an electronic enclosure  1400 , similar to enclosure  820  described above in connection with  FIGS. 8A-13C . Referring collectively to  FIGS. 14A &amp; 14B , in this example, electronic enclosure  1400  includes an inner main surface  1401 , which may be substantially flat, and an inner sidewall surface  1402 , which in this example joins to inner main surface  1401  via a curved (or radiused) transition region  1403  around the inner perimeter of electronic enclosure  1400 . Region  1403  provides a gradual transition between inner sidewall surface  1402  and inner main surface  1401 , which in one or more embodiments, may be oriented orthogonal to each other. In the configuration depicted, electronic enclosure  1400  also includes multiple inner-sidewall corners  1410 , which may be configured to facilitate transition between adjoining sides of electronic enclosure  1400 . By way of example, in the enlarged depiction of  FIG. 14B , one inner-sidewall corner  1410  is illustrated, which joins a first side  1404  and a second side  1405  of electronic enclosure  1400 . In one or more implementations, each inner-sidewall corner may be similarly configured. In the exemplary embodiment, which is presented by way of example only, inner-sidewall corner  1410  includes a flat, angled-sidewall portion  1411 , and first and second curved-sidewall portions  1412 ,  1413  located at opposite sides of flat, angled-sidewall portion  1411  as shown. 
     As one example, flat angled-sidewall portion  1411  may be oriented at a 45° angle to the adjoining first side  1404  and second side  1405  of electronic enclosure  1400 , which in one or more embodiments may be perpendicular to each other. In one or more implementations, first and second curved sidewall portions  1412 ,  1413  may have a similar bend radius, which may be, for instance, approximately five times or greater the thickness of the tamper-respondent sensor being mounted to the inner sidewall surface of the electronic enclosure  1400 . In the illustrated example, transition region  1403  between inner sidewall surface  1402  and inner main surface  1401  continues within the inner-sidewall corners  1410 , where a lower part  1411 ′ of flat, angled-sidewall portion  1411  curves outward in transition to inner main surface  1401 , and lower portions  1412 ′,  1413 ′ of first and second curved-sidewall portions  1412 ,  1413  also further curve outward in transition to inner main surface  1401 , as illustrated. Note that the corner configuration of  FIGS. 14A &amp; 14B  is presented by way of example only, and that the present invention may be used with other corner designs without departing from the concepts disclosed herein. Also, as noted, in one or more implementations, electronic enclosure  1400  may be employed in combination with a multilayer circuit board, such as multilayer circuit board  810  described above in connection with  FIGS. 8A-12 , to define a secure volume about one or more electronic components or an electronic assembly, for instance, comprising an encryption and/or decryption module and associated memory. 
     Electronic enclosure  1400  may be fabricated of a variety of materials and have a variety of different configurations. In one or more implementations, the enclosure may be a rigid, thermally conductive enclosure (fabricated, for instance, of a metal material) to facilitate conduction of heat from one or more electronic components within the secure volume defined (at least in part) by the tamper-respondent assembly. Note also that the rectangular configuration of electronic enclosure  1400  could be replaced with any of a variety of different enclosure configurations, any one of which may include one or more inner-sidewall corners configured, by way of example, such as illustrated in  FIGS. 14A &amp; 14B . 
       FIGS. 15A &amp; 15B  depict underside, isometric views of a further embodiment of a tamper-respondent assembly employing electronic enclosure  1400 . Referring collectively to  FIGS. 15A &amp; 15B , in one or more implementations, tamper-respondent assembly  1500  includes electronic enclosure  1400  which, as noted, is to enclose, at least in part, one or more electronic components or an electronic assembly to be protected. Electronic enclosure  1400  includes an inner main surface, and an inner sidewall surface including at least one inner-sidewall corner, such as described above in connection with  FIGS. 14A &amp; 14B . Further, tamper-respondent assembly  1500  includes a tamper-respondent electronic circuit structure which includes at least one tamper-respondent sensor mounted to and covering, at least in part, the inner surface(s) of electronic enclosure  1400 . As explained further below, the tamper-respondent sensor(s) is configured so as to facilitate good contact, and good adhesion, of the sensor to the inner surfaces of the enclosure, such as, for instance, the one or more inner-sidewall corners of the electronic enclosure  1400 , to provide secure coverage of the tamper-respondent sensor(s) over the inner surface(s) of the electronic enclosure. 
     As illustrated, in one or more implementations, the tamper-respondent electronic circuit structure associated with electronic enclosure  1400  may include an inner-sidewall tamper-respondent sensor  1510  and an inner main surface tamper-respondent sensor  1520 , along with a security band  1530 . In the illustrated example, inner-sidewall tamper-respondent sensor  1510  may be formed with an integrated flex ribbon cable or extension  1511  to facilitate electrical connection of the at least one resistive network within inner-sidewall tamper-respondent sensor  1510  to appropriate monitor circuitry (not shown) disposed within, for instance, the secure volume defined, at least in part, by the tamper-respondent assembly of  FIGS. 15A &amp; 15B . Similarly, inner main surface tamper-respondent sensor  1520  may be configured with an integrated flex ribbon cable or extension  1521  to facilitate electrical connection of inner main surface tamper-respondent sensor  1520  to the monitor circuitry, as well. A bonding agent (not shown), such as a thermoset adhesive, may be employed to adhere inner-sidewall tamper-respondent sensor  1520  to inner sidewall surface  1402  ( FIG. 14A ) and to inner-sidewall corners  1410  ( FIG. 14A ). A similar adhesive could be used to adhere inner main surface tamper-respondent sensor  1520  to inner main surface  1401  ( FIG. 14A ) and to inner-sidewall tamper-respondent sensor  1510  where the sensors overlap. Security band  1530  may further be adhesively secured over the overlap between inner main surface tamper-respondent sensor  1520  and inner-sidewall tamper-respondent sensor  1510  covering, in one or more implementations, transition region  1403  ( FIG. 14A ) between the inner sidewall surface and the inner main surface around the inner perimeter of electronics enclosure  1400 . 
     Note that, in the example provided in  FIGS. 15A &amp; 15B , inner-sidewall tamper-respondent sensor  1510  and inner main surface tamper-respondent sensor  1520  are discrete tamper-respondent sensors that overlap, at least in part, and facilitate defining a secure volume about the at least one electronic component to be protected. For instance, the secure volume may be defined by flipping over and securing the illustrated tamper-respondent assembly of  FIGS. 15A &amp; 15B  to a multilayer circuit board with an embedded tamper-respondent sensor, such as described above. 
       FIG. 16A  depicts one embodiment of inner-sidewall tamper-respondent sensor  1510  of  FIGS. 15A &amp; 15B . In this embodiment, inner-sidewall tamper-respondent sensor  1510  includes at least one first layer  1600  having opposite first and second sides  1601 ,  1602 , and circuit lines  1605  extending substantially over all of the flexible layer, and forming at least one tamper-detect network, such as described herein. For instance, circuit lines  1605  may be disposed on at least one of first side  1601  or second side  1602  of the at least one flexible layer  1600 . Note that the at least one flexible layer  1600  may be fabricated as a conventional security sensor layer, or be fabricated as one of the enhanced, tamper-respondent sensors described herein. In particular, although illustrated as a non-formed, flexible layer, the at least one flexible layer  1600  of inner-sidewall tamper-respondent sensor  1510  could comprise a formed flexible layer, such as one of the sensors depicted in  FIGS. 4A-5H , or multiple weaved tamper-respondent sensors, such as illustrated in  FIGS. 7A-7C . As noted, extension  1511  may extend from inner-sidewall tamper-respondent sensor  1510  to facilitate electrical connection of the at least one resistive network of the inner-sidewall tamper-respondent sensor  1510  to appropriate monitor circuitry (not shown) disposed within, for instance, the secure volume defined, at least in part, by the tamper-respondent assembly of  FIGS. 15A &amp; 15B . As illustrated, in one or more implementations, inner-sidewall tamper-respondent sensor  1510  is of sufficient length to encircle the inside of the electronic enclosure, covering the inner sidewall surface thereof, and overlap at its ends. Further, multiple slots  1610  are provided within inner-sidewall tamper-respondent sensor  1510 . These multiple slots  1610  are sized and positioned along the inner-sidewall tamper-respondent sensor so as to approximately align (in one or more embodiments) to respective inner-sidewall corners of the electronic enclosure to facilitate good contact, and good adhering, and bending the sensor within the inner-sidewall corners of the electronic enclosure, for instance, by allowing for regions of overlap of the inner-sidewall tamper-respondent sensor on itself. 
       FIGS. 16B &amp; 16C  depict one embodiment of inner-sidewall tamper-respondent sensor  1510  mounted within electronic enclosure  1400 . As illustrated, in the exemplary embodiment, the inner-sidewall tamper-respondent sensor includes first and second slots that respectively overlie, at least in part, the first and second curved-sidewall portions  1412 ,  1413  ( FIG. 14B ) of the associated inner-sidewall corner  1410  to be covered. These first and second slots are spaced apart to reside at opposite sides of the flat, angled-sidewall portion  1411  ( FIG. 14B ) of the inner-sidewall corner  1410 , and facilitate reducing the amount of material in the corner and thereby enhance good contact and adhesion of the inner-sidewall tamper-respondent sensor  1510  to the inner sidewall surface  1402  ( FIG. 14A ) of the electronic enclosure, including at the inner-sidewall corners  1410  thereof, while also reducing stress on the sensor within the corner(s). For instance, the multiple slots  1610  allow for overlapping of the inner-sidewall tamper-respondent sensor on itself at the inner-sidewall corners, as illustrated. Note that, in this configuration, the inner-sidewall tamper-respondent sensor  1510  has a width which allows the sensor to cover the transition region  1403  ( FIG. 14A ), as well as extend over, in part, the inner main surface  1401  of electronic enclosure  1400 . Note also that one or more uncovered regions  1615  may result from the presence of the slots when the inner-sidewall tamper-respondent sensor  1510  is wrapped around the inner sidewall surface as shown, exposing portions of the inner sidewall surface at the inner-sidewall corner(s), for instance, along the seams where the inner-sidewall tamper-respondent sensor overlaps at the corner. As explained below, these regions  1615  may be covered or protected by inner main surface tamper-respondent sensor  1520  corner tabs once that sensor and its corner tabs are adhered to the assembly. This is illustrated, by way of example, in  FIGS. 17A-17C . 
     Referring collectively to  FIGS. 17A-17C , inner main surface tamper-respondent sensor  1520  includes at least one flexible layer  1700  having opposite first and second sides  1701 ,  1702 , and circuit lines  1705  extending substantially over all of the flexible layer  1700  and forming at least one tamper-detect network, such as described herein. For instance, circuit lines  1705  are disposed on one or both of first side  1701  and second side  1702  of the at least one flexible layer  1700 , as described. As noted above, the at least one flexible layer  1700  may be fabricated as a conventional security sensor layer, or be fabricated as one of the enhanced, tamper-respondent sensors described herein. In particular, although illustrated as a non-formed, flexible layer, the at least one flexible layer  1700  of inner main surface tamper-respondent sensor  1520  could comprise a formed flexible layer, such as one or more of the sensors depicted in  FIGS. 4A-5H , or multiple weaved tamper-respondent sensors, such as illustrated in  FIGS. 7A-7C . As noted, extension  1521  may be formed integral with inner main surface tamper-respondent sensor  1520  to facilitate electrical connection of the at least one associated resistive network to monitor circuitry (not shown) within the secure volume being defined, at least in part, by the tamper-respondent assembly of  FIGS. 15A &amp; 15B ; for instance, in association with a multilayer circuit board having an embedded tamper-respondent sensor therein, as described above. 
     In the depicted configuration, multiple corner tabs  1710  are provided, with at least one corner tab  1710  being provided at the at least one inner-sidewall corner. In the exemplary embodiment illustrated, two corner tabs  1710  are provided at each corner of the inner main surface tamper-respondent sensor  1520 . These corner tabs  1710  include circuit lines  1705  ( FIG. 17A ) and are sized to cover a respective one of the uncovered regions  1615  in inner-sidewall tamper-respondent sensor and enclosure assembly which remain after securing inner-sidewall tamper-respondent sensor  1510  to electronic enclosure  1400 , as illustrated in  FIGS. 16B &amp; 16C . In particular, those skilled in the art should understand that corner tabs  1710  include respective portions of the at least one tamper-detect network provided by inner main surface tamper-respondent sensor  1520 , such that if an attempt were made to breach the tamper-respondent assembly  1500  through the underlying, uncovered regions  1615  of the inner sidewall surface, the respective corner tab would be contacted, thereby resulting in detection of the attempted breach. 
     As noted above in connection with  FIGS. 15A &amp; 15B , reinforcement of the overlap between inner-sidewall tamper-respondent sensor  1510  ( FIG. 15A ) and inner main surface tamper-respondent sensor  1520  ( FIG. 15 ) may be provided, in one or more implementations, by one or more physical security structures, such as security band  1530 . One potential point of exposure for a tamper-respondent assembly such as described herein would be at an overlap between two or more tamper-respondent sensors, such as at an overlap between an inner-sidewall tamper-respondent sensor and an inner main surface tamper-respondent sensor. For instance, an attack on a tamper-respondent assembly could entail drilling through the enclosure and chemically attaching an overlapped bond area between two tamper-respondent sensors of the tamper-respondent electronic circuit structure, such as the overlap area where inner main surface tamper-respondent sensor  1520  is adhesively secured over inner-sidewall tamper-respondent sensor  1510 . To address this concern, a physical security structure, such as security band  1530 , may be provided. Note that security band  1530  is one embodiment only of a physical security structure which could be employed to overlie and physically secure in place, at least in part, one or more tamper-respondent sensors covering one or more inner surfaces of an electronic enclosure, such as described herein. 
     Generally stated, in one or more implementations, disclosed herein is a tamper-respondent assembly which includes an electronic enclosure to enclose, at least in part, at least one electronic component to be protected, wherein the electronic enclosure includes an inner surface. The tamper-respondent assembly also includes a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor lining and covering, at least in part, the inner surface of the electronic enclosure. The tamper-respondent sensor may include a flexible layer having opposite first and second sides, and circuit lines substantially covering at least one of the first side or the second side of the flexible layer, forming at least one tamper-respondent network, such as described herein. The flexible layer of the tamper-respondent sensor could be a non-formed sensor layer or a formed sensor layer, in accordance with one or more of the sensor layer embodiments described herein. 
     The tamper-respondent assembly further includes a physical security structure, such as at least one security element, that overlies and physically secures in place, at least in part, the tamper-respondent sensor covering, at least in part, the inner surface of the electronic enclosure. In the embodiment of  FIGS. 15A &amp; 15B , security band  1530  is illustrated which includes multiple security elements  1531 , as shown in enlarged view in  FIG. 18 . Note that the security structure, such as security band  1530 , could comprise a single element or multiple elements, depending on the desired configuration. In the example of  FIGS. 15A, 15B &amp; 18 , two substantially identical, U-shaped security elements  1531  are illustrated, by way of example only. In the depicted embodiment, security elements  1531  are spaced apart, with gaps  1532  therebetween. By providing two or more security elements  1531 , to define a desired physical security structure (such as security band  1530 ) manufacturing tolerances may be better accommodated within the tamper-respondent assembly. By way of example, the gaps  1532  between adjacent security elements of the multiple, distinct security elements  1531 , may be on the order of several millimeters. Note that although illustrated as two security elements  1531 , any number of physical security elements could be provided within the tamper-respondent assembly, and any number of security structures, such as multiple security bands or plates, could be provided within the tamper-respondent assembly as desired to provide additional mechanical securing of the tamper-respondent sensor(s) in place over the inner surface of the electronic enclosure. 
     In the example of  FIGS. 15A, 15B &amp; 18 , the physical security structure is configured as a security band or collar comprising multiple distinct security elements  1531  which extends substantially fully around the inner perimeter of the electronic enclosure  1400 . The security band  1530  is sized and positioned to overlie and physically secure in place at least the overlap of the inner main surface tamper-respondent sensor and the inner-sidewall tamper-respondent sensor, as illustrated in  FIG. 15A . In one or more implementations, security band  1530  may be adhesively secured to the tamper-respondent sensors. Note that in this example, security band  1530 , comprising security elements  1531 , extends around the inner perimeter, including through the inner-sidewall corners, as illustrated in  FIG. 15A . In this manner, security elements  1531  advantageously overlie and secure in place the overlap(s) of the inner-sidewall tamper-respondent sensor and the inner main surface tamper-respondent sensor at the inner-sidewall corners of the electronic enclosure. By way of example, in the embodiment depicted, security band  1530 , or more particularly, security elements  1531 , overlie and physically secure in place the multiple corner tabs  1710  ( FIG. 17C ) projecting from the inner main surface tamper-respondent sensor  1520  at the inner-sidewall corners of the electronic enclosure. This advantageously prevents an attack against the tamper-respondent assembly through the areas lined by the multiple corner tabs projecting from the inner main surface of the tamper-respondent sensor. The security band  1530  creates a mechanical barrier that prevents the tamper-respondent sensors from being separated. 
     In one or more enhanced embodiments, the security element(s) defining the security band, or more generally, the physical security structure, are formed (for instance, by stamping) a metal material, or metal alloy, such as copper, soft stainless steel, etc. Further, the metal security element(s) may advantageously be electrically connected to ground to further enhance detection capabilities of the tamper-respondent assembly. By forming the security element(s) of a metal that is difficult to drill through, then, if an attempt were made to drill through the security element, metal fragments would be created, which potentially could be pulled into the sensor layer(s) lining the inner surface of the electronic enclosure, which would result in a greater chance of shorting or otherwise damaging the circuit lines forming the one or more tamper-respondent networks of the sensor during the attack, and thus enhance detection capability of the tamper-respondent sensor. Further, by electrically grounding the security element(s), then a drill contacting the grounded security element(s) after drilling through one or more tamper-respondent sensors would be more likely to short one or more of the circuit lines forming the at least one tamper-detect network in the associated tamper-respondent sensor(s). By grounding the security element(s), another path for current to flow is established, which advantageously increases the likelihood of detecting an attempt to tamper with the tamper-respondent assembly. Note that grounding of the security element(s) could be by any means, such as by electrically connecting the elements to one or more ground lines on the electronic assembly being protected by the tamper-respondent assembly, or (in certain of the embodiments disclosed herein) by electrically connecting the elements to one or more ground planes within the multilayer circuit board forming, in part, the secure volume about the electronic assembly being protected. In one or more implementations, the security element(s), or more generally, the security band or physical security structure, may be pre-formed (e.g., by stamping) into the desired shape, for example, to accommodate and overlie the overlap between the inner-sidewall tamper-respondent sensor and the inner main surface tamper-respondent sensor, such as depicted in  FIG. 15A . 
     As a further enhancement, increased sensor sensitivity to a tamper event may be provided by fabricating the tamper-respondent assembly, and in particular, the tamper-respondent sensor, to include one or more regions with increased fragility or susceptibility to damage from mechanical stress resulting from or associated with a tamper event. By including circuit lines or traces of the tamper-respondent sensor at least in part within the one or more regions of increased susceptibility to damage, then there is an increased likelihood that one or more of the circuit lines may be damaged during the tamper event due the associated mechanical stress, thus enhancing ability to detect the tamper event. The one or more regions of the tamper-respondent sensor having increased susceptibility may be formed using a variety of approaches, such as selective laser ablation or etching of the sensor, configuring the sensor with cutout areas to define stress-concentrating inner edges, and/or directly adhering exposed circuit lines to a rigid surface of the tamper-respondent assembly. By way of example,  FIGS. 19A-20B  depict examples of tamper-respondent assemblies with one or more tamper-respondent sensors having one or more regions of increased susceptibility to damage due to mechanical stress associated with a tamper event. 
     Referring to  FIG. 19A , a tamper-respondent assembly  1900  is depicted comprising a structure  1901  having a rigid surface  1902 . By way of example only, structure  1901  could comprise an electronic enclosure or housing surrounding, at least in part, one or more electronic components to be protected. By way of specific example, rigid surface  1902  could be an outer surface of an electronic enclosure, or an inner surface of an electronic enclosure.  FIGS. 6A-6G  depict exemplary embodiments of one or more tamper-respondent sensors of a tamper-respondent electronic circuit structure being adhered to an outer surface of an electronic enclosure, while  FIGS. 8A-17C  depict exemplary embodiments of one or more tamper-respondent sensors being adhered to an inner surface of an electronic enclosure. Still further, structure  1901  might be or comprise any rigid structure of an electronic assembly, or cooling apparatus associated with an electronic assembly to be protected. In one or more specific instances, structure  1901  could comprise a heat sink, such as a thermal spreader. 
     As illustrated, tamper-respondent assembly  1900  includes a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor  1910 . In the depicted embodiment, tamper-respondent sensor  1910  includes one or more flexible layers  1911  having opposite first and second sides  1912 ,  1913 , and circuit lines  1915 ,  1915 ′ forming, at least in part, at least one tamper-detect network. For instance, the circuit lines may form, at least in part, at least one resistive network electrically coupled to monitor circuitry such as described herein. By way of example, first circuit lines  1915  are disposed on first side  1912 , and second circuit lines  1915 ′ are disposed on second side  1913  of flexible layer  1911 . The circuit lines may be in the same or different patterns, as described above. Further, note that there may be any number of flexible layers  1911 , and any number of circuit line  1915 ,  1915 ′ layers provided within tamper-respondent sensor  1910  as discussed above, for instance, in connection with  FIGS. 3A-3E , with the illustrated embodiment being provided by way of example only. 
     In accordance with one or more aspects of the present invention, circuit lines  1915  at first side  1912  of flexible layer  1911  are exposed, that is, any overlying protective layer, such as a polyimide layer, is at least partially, or fully, removed in the region to be secured to rigid surface  1902  of structure  1901 . Note that circuit lines  1915 ,  1915 ′ may be formed of a variety of materials including, for instance, a metal or metal alloy. For instance, first circuit lines  1915  could be formed from copper, silver, silver-carbon, nickel-phosphorous. Alternatively, other materials, such as conductive ink or Omega-Ply®, or Ticer™, could be employed. 
     In the exemplary embodiment, an adhesive  1920 , such as a thermoset material, is used to adhere or laminate tamper-respondent sensor  1910  to rigid surface  1902 . In particular, in the embodiment depicted, the exposed first circuit lines  1915  on first side  1912  are directly, adhesively secured to rigid surface  1902  of structure  1901 . In one or more implementations, adhesive  1920  is selected with a bond strength to secure first circuit lines  1915  to rigid surface  1902  that is equal or greater to a bond strength of first circuit lines  1915  to first side  1912  of one or more flexible layers  1911 . In this manner, should a tamper event occur comprising an attempt to physically separate tamper-respondent sensor  1910  from structure  1901 , one or more of first circuit lines  1915  will likely separate from flexible layer  1911 , and thus be broken, with the resultant broken lines enhancing the ability of the tamper-respondent electronic circuit structure to detect the tamper event. 
     In one or more embodiments, setoff elements could be provided between rigid surface  1902  of structure  1901  and first side  1912  of tamper-respondent sensor  1910  to ensure the presence of a gap between first side  1912  of tamper-respondent sensor  1910  and the rigid surface  1920 , and thus ensure that adhesive  1920  remains between the two surfaces during a fabrication approach where pressure may be applied to force the surfaces together to, for instance, facilitate the laminating of tamper-respondent sensor  1910  to structure  1901 . In one or more implementations, the setoff elements could be dispersed throughout adhesive  1920  and may comprise, for instance, spherical elements  1922 , such as glass spheres, disposed within the adhesive. Alternatively, in one or more implementations, rigid surface  1902  could be formed with one or more standoffs extending from the surface a desired gap distance. 
       FIG. 19B  depicts an alternate embodiment of a tamper-respondent assembly  1900 ′, in accordance with one or more aspects of the present invention. In this embodiment, structure  1901  of  FIG. 19A  is replaced by an electronic enclosure  1400 , such as described above in connection with  FIGS. 14A &amp; 14B , as a further example. Electronic enclosure  1400  includes a rigid surface  1902 ′, which may be, for instance, the inner main surface  1401  ( FIG. 14A ) or inner-sidewall surface  1402  ( FIG. 14A ) in the exemplary embodiment of  FIGS. 14A &amp; 14B . 
     As illustrated in  FIG. 19B , in one or more implementations, tamper-respondent assembly  1900 ′ may also include a tamper-respondent sensor  1910 ′, and another structure  1930 , with another rigid surface  1932 . By way of example, in one or more implementations, another structure  1930  may comprise a cooling structure, such as a thermal spreader, provided to facilitate cooling of one or more components within the electronic enclosure. 
     In one or more embodiments, tamper-respondent sensor  1910 ′ may comprise a stack of tamper-respondent layers secured together via an adhesive layer  1925 , such as a double-sided adhesive film, similar to the structure described above in connection with  FIG. 3C . However, in this example, the circuit line widths and line-to-line spacings may be of any desired dimension, with the above-described line widths and spacings of ≤200 μm, respectively, being one example. Tamper-respondent sensor  1910 ′ includes multiple layers of circuit lines  1915 ,  1915 ′ which may be electrically connected to define one or more tamper-detect networks, such as one or more resistive networks. Any of the various materials described herein, or other known materials, may be employed to form the circuit lines. The circuit lines are provided on one or both sides of respective flexible layers  1911 , with first circuit lines  1915  being provided on a first side of each flexible layer  1911 , and second circuit lines  1915 ′ being provided on a second side of the layer, as one example only. The flexible layers  1911  may be formed of any flexible, dielectric material, such as a polyester material. Alternatively, the flexible layers  1911  could be formed of a crystalline polymer, such as the above-described PVDF, or Kapton, or other crystalline-polymer material. 
     In accordance with one or more aspects of the present invention, a portion of circuit lines  1915  is exposed on the upper side and a portion of circuit lines  1915 ′ is exposed on the lower side of tamper-respondent sensor  1910 ′. The exposed circuit lines  1915 ,  1915 ′ are respectively laminated using an adhesive layer  1920 ,  1920 ′, to rigid surface  1902 ′ of electronic enclosure  1400 , or to rigid surface  1932  of another structure  1930 , as illustrated. That portion of tamper-respondent sensor  1910 ′ adhered to electronic enclosure  1400  and/or to another structure  1930  defines the one or more regions of the tamper-respondent sensor having the increased susceptibility to damage from mechanical stress associated with a tamper event. That is, should a tamper event attempt to delaminate the tamper-respondent sensor from one or both of electronic enclosure  1400  and another structure  1930 , then adhesive layers  1920 ,  1920 ′ bonding the structures to the first side and the second side of tamper-respondent sensor  1910 ′, will likely cause one or more of the exposed circuit lines  1915 ,  1915 ′ to delaminate from the tamper-respondent sensor as the structures are separated, causing breaks in the circuit lines, and thereby facilitating detection of the tamper event by monitor circuitry of the tamper-respondent electronic circuit structure. As noted, this result can be facilitated by selecting the adhesive  1920 ,  1920 ′ to have, for instance, an equal or greater bond strength than the bond strength holding circuit lines  1915 ,  1915 ′ to their respective sides of the tamper-respondent sensor  1910 ′. 
     As noted, in the embodiments depicted in  FIGS. 19A &amp; 19B , the circuit lines may comprise a metal, such as copper, laminated directly to a rigid surface of the tamper-respondent assembly to define regions of the tamper-respondent sensor that have increased susceptibility to damage from mechanical stress associated with a tamper event, such as a tamper event which physically disturbs the lamination of the tamper-respondent sensor to the rigid surface. In one or more other implementations, the region(s) of the tamper-respondent sensor with increased susceptibility may be provided by increasing fragility of the tamper-respondent sensor. This may be accomplished, for example, through the provision of cutout areas in the sensor, such as channels, notches, openings, etc., which define one or more stress-concentrating inner edges (or stress risers) within the tamper-respondent sensor. Various examples of this approach are depicted in  FIGS. 20A &amp; 20B . 
     Referring to  FIG. 20A , one embodiment of a tamper-respondent assembly  2000  is illustrated. As shown, tamper-respondent assembly  200  includes a tamper-respondent sensor  2001 , with multiple peripheral edges  2002  of tamper-respondent sensor  2001  being shown. As described herein, such as with reference to  FIGS. 3A-3E , tamper-respondent sensor  2001  includes one or more flexible layers  2011 , and one or more layers of circuit lines  2015  on one or both opposite sides of each flexible layer  2011 . The circuit lines  2015  may be electrically connected to define one or more tamper-detect networks, such as one or more resistive networks. The tamper-respondent layers within the stack may be adhesively laminated together using, for instance, a double-sided adhesive film  2025 , as discussed above. 
     Multiple regions  2032 ,  2042  within tamper-respondent sensor  2001  of increased susceptibility to damage from mechanical stress are defined in this example by providing the tamper-respondent sensor with one or more cutout areas  2030 ,  2040 . In the exemplary embodiment, cutout areas  2030  are illustrated as rectangular-shaped cutouts, extending into the sensor from one or more peripheral edges  2002 , and cutout areas  2040  are triangular-shaped cutouts, extending into the sensor from one or more peripheral edge  2002  of the tamper-respondent sensor  2001 . In each instance, cutout areas  2030 ,  2040  define stress-concentrating inner edges  2031 ,  2041 , which (in this example) extend vertically through tamper-respondent sensor  2001 . These edges  2031 ,  2041  are stress risers which define the regions  2032 ,  2042 , respectively, of increased susceptibility to damage within tamper-respondent sensor  2001 . The regions  2032 ,  2042  are locations within the sensor  2001  where stress is concentrated due to the provision of the stress-concentrating inner edges  2031 ,  2041 . For instance, circuit lines  2015  may be provided which follow the contours of tamper-respondent sensor  2001  and extend, at least in part, through the respective regions  2032 ,  2042  of increased susceptibility to damage. In this manner, should a tamper event occur, mechanical stress within the tamper-respondent sensor will be concentrated by the stress-concentrating inner edges  2031 ,  2041 , in regions  2032 ,  2042  of the sensor, thereby increasing a likelihood of a break in one or more of the circuit lines  2015  within those regions. This increased likelihood of a break facilitates detection of the tamper event by the monitor circuitry of the tamper-respondent electronic circuit structure. 
     Note that the circuit lines  2015  may be provided in any desired configuration or pattern on the various flexible layers  2011 , as described herein. For instance, a rectangular grid of circuit lines could be provided in one or more layers which intersects, at least in part, the regions  2032 ,  2042  of increased susceptibility to damage from mechanical stress associated with a tamper event. Note also that regions  2032 ,  2042  of the tamper-respondent sensor are distinct regions of the sensor, separate from the balance of the tamper-respondent sensor, which may be characterized as being a standard robustness region outside of the regions  2032 ,  2042  created with the increased susceptibility to damage from mechanical stress. Thus, the tamper-respondent sensor may be thought of as being divided into different regions, a standard robustness region, and the regions  2032 ,  2042  of increased susceptibility to damage resulting from the provision of stress risers within the tamper-respondent sensor. 
     Note that any stress in one or more of regions  2032 ,  2042  of increased susceptibility to damage may extend vertically across multiple layers of the tamper-respondent sensor  2001 , or be associated with one or more particular tamper-respondent layers (or flexible layers) within the tamper-respondent sensor  2001 , dependent, for instance, on the tamper event. In one or more implementations, the cutouts, comprising for instance cutout areas  2030  and/or cutout areas  2040 , may extend around a portion or all of the periphery of tamper-respondent sensor  2001 , in any desired configuration and number. Further, any cutout configuration may be used that provides stress-concentration regions within the tamper-respondent sensor, for instance, due to the presence of one or more stress-concentrating inner edges (or stress risers). Additional embodiments of this concept are depicted in  FIG. 20B . 
     Referring to  FIG. 20B , a tamper-respondent assembly  2000 ′ is illustrated as comprising a tamper-respondent sensor  2001 ′ of a tamper-respondent electronic circuit structure. The tamper-respondent sensor  2001 ′ is partially depicted, with a corner thereof shown defined by peripheral edges  2002 . In this implementation, a cutout area  2030  is again depicted, defining (for instance) multiple stress-concentrating inner edges  2031  which define an adjoining region or regions  2032  of the tamper-respondent sensor having increased susceptibility to damage from mechanical stress within the sensor. By way of further example, in line with cutout area  2030 , one or more openings  2050  are formed extending through tamper-respondent sensor  2001 ′. The one or more openings  2050  include one or more stress-concentrating inner edges  2051  extending vertically through tamper-respondent sensor  2001 ′, which respectively define one or more regions  2052  within the tamper-respondent sensor having increased susceptibility to damage from mechanical stress within the sensor accompanying a tamper event. The stress-concentrating inner edges  2051  associated with opening  2050  again increase likelihood of damage to those portions of circuit lines  2015  of the tamper-respondent sensor  2001 ′ extending through the one or more of the regions of higher susceptibility to damage. For instance, tamper-respondent sensor  2001 ′ may be more likely to delaminate in regions  2052  of increased susceptibility to damage in response to a tamper event. Note that any mechanical stress within the tamper-respondent sensor may propagate through the sensor and be concentrated by the stress-concentrating inner edges  2051  within regions  2052 , resulting in increased susceptibility to damage for any circuit lines extending through those regions. 
     In the exemplary embodiment of  FIG. 20B , tamper-respondent sensor  2001 ′ is illustrated as comprising one or more flexible layers  2011  having opposing first and second sides, with circuit lines  2015  disposed on the opposing first and second sides. Protective layers  2026  are disposed over the first and second sides of the one or more flexible layers  2011  and circuit lines  2015 . By way of example, protective layers  2026  may comprise a polyimide material, as one example only. 
     As illustrated, cutout areas  2060  may also, or alternatively, by provided as one or more channels extending through protective layers  2026  above and/or below the one or more flexible layers  2011  and circuit lines  2015 . These cutout areas  2060  again comprise stress-concentrating inner edges  2061 , which define regions  2062  adjoining the edges within the tamper-respondent sensor  2001 ′ that have increased susceptibility to damage from mechanical stress accompanying a tamper event against tamper-respondent sensor  2001 ′. As in the embodiments described above, circuit lines  2015  may extend, at least in part, through regions  2062  of increased susceptibility to damage, and thus, should a tamper event occur which generates mechanical stress within tamper-respondent sensor  2001 ′, any circuit lines within the regions of increased susceptibility to damage, for instance, in regions adjoining the stress-concentrating inner edges  2061 , will more likely be damaged or break (that is, compared with the balance of the tamper-respondent sensor, characterized above as the standard robustness region of the sensor), thereby enhancing likelihood of detection of the tamper event by the monitor circuitry of the tamper-respondent electronic circuit structure comprising the tamper-respondent sensor  2001 ′. 
     Those skilled in the art will note that, disclosed herein are various enhancements to creating a secure volume for accommodating one or more electronic components, such as one or more encryption and/or decryption modules and associated components of a communications card or other electronic assembly. In certain embodiments, the tamper-respondent assembly, or tamper-proof electronic package, includes a tamper-respondent electronic circuit structure comprising one or more tamper-respondent sensors in various configurations disposed, for instance, external to or internal to an electronic enclosure to contain the electronic component(s) or electronic assembly to be protected. The tamper-respondent electronic circuit structure may also include tamper-detect monitor circuitry which monitors, for instance, for changes in resistance in one or more tamper-detect networks, such as one or more resistive networks, defined (at least in part) by circuit lines within the tamper-respondent sensor(s). 
     In one or more embodiments, the tamper-respondent sensor may be implemented as a fine-pitch flex circuit being formed, for instance, of multiple stacked layers of flexible film, such as the above-described crystalline-polymer material, with Kapton being one specific example. On one or more sides of each flexible layer, circuit lines are provided and electrically connected to facilitate defining one or more tamper-detect networks of the tamper-respondent sensor. In one or more implementations, line width and pitch may be significantly reduced in size from conventional approaches to, for instance, ≤200 μm. The circuit lines may be formed of any appropriate material, including a metal or metal alloy such as copper, silver, nickel-phosphorous (NiP), Omega-Ply®, or Ticer™. The flexible layers with the circuit lines are enclosed, for instance, by polyimide, such that the circuit lines are unexposed within the tamper-respondent sensor, and the tamper-respondent sensor provides a secure defense against a mechanical or physical intrusion through the sensor into a secure volume defined, at least in part, by the tamper-respondent sensor. 
     A variety of tamper-respondent assembly configurations are disclosed herein which may employ an adhesive in one or more external bond regions on a surface of the tamper-respondent sensor(s) to secure, for instance, a tamper-respondent sensor in an operative position within the tamper-respondent assembly. By way of example, the adhesive may be employed to maintain a particular configuration of the tamper-respondent sensor about an electronic enclosure, or to bond two or more tamper-respondent sensors together in a multi-sensor configuration, or to position a tamper-respondent sensor relative to an electronic enclosure of a tamper-respondent assembly, such as over an inner surface of an electronic enclosure. These external bond regions could be susceptible to chemical attack against the adhesive. 
     Therefore, by way of further enhancement, increased sensitivity to a tamper event may be provided by fabricating the tamper-respondent assembly to include conductive traces positioned and fabricated to have increased mechanical and/or chemical fragility or susceptibility to damage from a tamper event, and in particular to a tamper event at an external bond or overlap region of the sensor. For instance, one or more conductive traces may be exposed within a bond region(s) of one or more tamper-respondent sensors, and an adhesive provided contacting the conductive trace(s) within the bond region(s) of the tamper-respondent sensor(s). By directly contacting the adhesive to the conductive traces, any attempt to mechanically and/or chemically tamper with the adhesive, to facilitate gaining access to the secure volume within the tamper-respondent assembly, is more likely to damage one or more conductive traces, and thus be detected. Note in this regard, that by forming the conductive traces of a chemically compromisable or dissolvable conductive material during a chemical attack on the adhesive, then the conductive traces will likely be damaged during the attack on the adhesive. In this manner, the exposed conductive traces provide increased fragility or susceptibility to mechanical and/or chemical attack at the external bond region(s). 
     As noted, the at least one external bond region may be a region of the tamper-respondent sensor(s) where the sensor adheres to another surface, such as the surface of a rigid structure of the tamper-respondent assembly, or the surface of another tamper-respondent sensor, in a multi-tamper-respondent sensor embodiment, or even to another region of the same tamper-respondent assembly, such as depicted (for instance) in  FIG. 6A . In each of these examples, one or more conductive traces could be provided to form, for instance, an outer tamper-detect network that is exposed, at least in part, on a surface of the one or more tamper-respondent sensors within the bond region(s) of the sensor(s). 
     In one or more embodiments, these one or more conductive traces may be distinct conductors from the unexposed circuit lines on the flexible layers within the tamper-respondent sensor(s). For instance, and as noted, conductive traces may be formed of a chemically compromisable or dissolvable material susceptible to damage during a chemical attack of the adhesive within the bond region(s) of the tamper-respondent sensor(s) to facilitate detecting the chemical attack, whereas the unexposed circuit lines forming the tamper-respondent sensor may be of a different conductive material (and may even be of different line width, and/or line-to-line spacing) to facilitate, for instance, securing the one or more electronic components to be protected against a mechanical tamper event through the sensor. In one or more examples, the circuit lines within the tamper-respondent sensor(s) may be smaller and of closer pitch than the conductive traces exposed on the surface of the tamper-respondent sensor(s) within the bond region(s). The one or more conductive traces forming the outer tamper-detect network may be placed on the tamper-respondent sensor(s) in any location susceptible to a chemical attack, such as where adhesive is employed to bond the tamper-respondent sensor to another surface of the tamper-respondent assembly, such as to an electronic enclosure, or to another tamper-respondent sensor of the assembly, or even to itself, depending on the implementation. 
       FIGS. 21A-21C  depict one embodiment of a tamper-respondent assembly having one or more tamper-respondent sensors with conductive traces positioned and fabricated to facilitate detection of a mechanical and/or chemical attack against an adhesive of the tamper-respondent assembly. 
     Referring to  FIG. 21A , a tamper-respondent assembly  2100  is depicted comprising a structure  2101  having a rigid surface  2102 . By way of example, structure  2101  is illustrated as an electronic enclosure or housing surrounding, at least in part, one or more electronic components  802  to be protected. By way of specific example, rigid surface  2102  is shown (by example) as an inner surface of the electronic enclosure, such as an inner surface of electronic enclosure  1400  described above in connection with  FIGS. 14A-15B . The embodiment of  FIG. 21A  is presented, by way of example only, in connection with the implementation of  FIGS. 8A-17C , where one or more tamper-respondent sensors are adhered to an inner surface of an electronic enclosure. In this example, conductive traces  2120  may be provided between discrete first and second tamper-respondent sensors (such as inner-sidewall tamper-respondent sensor  1510 ′ and inner main surface tamper-respondent sensor  1520 ′), or alternatively, or additionally, conductive traces  2120 ′ may be provided between one or more of the tamper-respondent sensors and the inner surface of the enclosure. 
     Note also that, in one or more other implementations, the conductive traces described herein as susceptible to damage during chemical attack of the adhesive, could be disposed between one or more tamper-respondent sensors and an outer surface of a structure, such as an outer surface of an electronic enclosure. In this regard, reference the embodiments of  FIGS. 6A-6G , where one or more tamper-respondent sensors of a tamper-respondent electronic circuit structure may be adhered to an outer surface of an electronic enclosure. Still further, the conductive traces forming the outer tamper-detect network might be between the tamper-respondent sensor and any rigid structure of an electronic assembly, or cooling apparatus of an electronic assembly to be protected. For instance, the conductive traces disclosed herein could be disposed between one or more tamper-respondent sensors and a heat sink, such as a thermal spreader. 
     As illustrated, in one or more implementations, the tamper-respondent electronic circuit structure of the tamper-respondent assembly  2100  may include an inner-sidewall tamper-respondent sensor  1510 ′ and an inner main surface tamper-respondent sensor  1520 ′, each comprising one or more tamper-detect networks, such as one or more unexposed tamper-detect networks formed by circuit lines on one or more flexible layers, such as described above. The one or more tamper-detect networks are electrically connected to appropriate monitor circuitry (not shown) disposed within, for instance, the secure volume  801  defined by tamper-respondent assembly  2100 . Note that in this example, inner-sidewall tamper-respondent sensor  1510 ′ and inner main surface tamper-respondent sensor  1520 ′ are discrete, first and second tamper-respondent sensors that overlap, at least in part, and facilitate defining the secure volume about the at least one electronic component  802  to be protected. For instance, the secure volume may be defined by securing the electronic enclosure to a multilayer circuit board  810  with an embedded tamper-respondent sensor  811 , such as described above in connection with  FIGS. 8A-10 . Note further, in the depicted configuration, inner-sidewall tamper-respondent sensor  1510 ′ may be bonded via an adhesive  2110  to an inner-sidewall surface of the electronic enclosure, wrapping partially around and over, onto the inner main surface of the electronic enclosure, as shown. This allows the overlap region  2105  of the inner-sidewall tamper-respondent sensor  1510 ′ and inner main surface tamper-respondent sensor  1520 ′ to occur at the flat, inner main surface portion of the structure  2101 . An adhesive  2115  is provided to bond inner main surface tamper-respondent sensor  1520 ′ to the inner main surface of structure  2101 , as well as to the inner-sidewall tamper-respondent sensor  1510 ′ in overlap region(s)  1205 . Adhesives  2110 ,  2115  may be the same or different adhesives. In one or more implementations, adhesives  2110 ,  2115  may be a thermoset material, such as a thermally conductive epoxy. 
     As noted, to provide enhanced tamper-detect protection, one or more conductive traces  2120  may be provided exposed, at least in part, on one or more of the tamper-respondent sensors  1510 ′,  1520 ′ of tamper-respondent assembly  2100 . For instance, one or more conductive traces  2120  are illustrated in the overlap region  2105  between inner-sidewall tamper-respondent sensor  1510 ′ and inner main surface tamper-respondent sensor  1520 ′, by way of example. Additionally, or alternatively, one or more conductive traces  2120 ′ could be provided, as part of the same or a different tamper-detect network(s), on a surface of inner-sidewall tamper-respondent sensor  1510 ′ between inner-sidewall tamper-respondent sensor  1510 ′ and an inner-sidewall of structure  2101 , and/or on a surface of inner main surface tamper-respondent sensor  1520 ′, between inner main surface tamper-respondent sensor  1520 ′ and rigid surface  2102  of structure  2101 . In one or more implementations, the conductive traces  2120 ,  2120 ′ in the bond region(s) are formed of a chemically compromisable or dissolvable conductive material susceptible to wetting or other damage during a chemical attack of the adhesive  2110 ,  2115  in direct contact therewith. The damage may result in dissolving one or more portions of the conductive traces, and thus, one or more portions of the associated tamper-detect network(s) defined (at least in part) by the traces and being monitored by the tamper-respondent electronic circuit structure, thereby facilitating detecting the tamper event. Note that the chemically dissolvable conductor used to form the conductive traces may be the same or a different material than the material used to form the unexposed circuit lines defining the one or more tamper-detect networks of the respective tamper-respondent sensor. 
     Stated generally, the conductive traces may be formed of a chemically compromisable conductive material, and may be provided in any bond region external to one or more sensors where, for instance, an adhesive bonds the respective tamper-respondent sensor to another surface, such as another surface of the tamper-respondent assembly. By way of example, the chemically dissolvable material used to form the conductive traces may comprise, at least in part, at least one of carbon, silver, or carbon-silver. For instance, the one or more conductive traces of the respective tamper-detect network(s) may be formed of a carbon-loaded conductive material, silver-loaded conductive material, or carbon-silver-loaded conductive material. Note also that different conductive traces may be in the same or different tamper-detect networks, and that conductive traces may be in the same or a different tamper-detect network than the network(s) defined by the sensor&#39;s unexposed circuit lines. 
       FIG. 21B  depicts a modified version of inner main surface tamper-respondent sensor  1520 ′ described above in connection with  FIG. 17A . As described above, inner main surface tamper-respondent sensor  1520 ′ includes one or more flexible layers having opposite first and second sides, and circuit lines on the flexible layer(s) forming, at least in part, at least one tamper-detect network, such as at least one resistive network, where the circuit lines may be disposed on at least one of the first side or the second side the flexible layer(s). For instance, multiple flexible layers could be provided within a stack, with circuit lines being defined on each side of each flexible layer within the stack, in any desired pattern and in any desired network configuration, to facilitate detection of a mechanical attempt to gain access to the secure volume through the tamper-respondent sensor. 
     In the example of  FIG. 21B , multiple conductive traces  2120  are provided about the periphery of inner main surface tamper-respondent sensor  1520 ′ in an area which aligns with a bond region of the tamper-respondent sensor, and in particular, aligns to an overlap region of, for instance, inner-sidewall tamper-respondent sensor  1510 ′, such as depicted in  FIG. 21A . Note in this regard, that two conductive traces  2120  are shown by way of example only. One, or more than two conductive traces in any desired pattern could be provided within the bond region(s). Note also that, in the example of  FIG. 21A , the bond region comprises substantially the entire upper surface of the inner main surface tamper-respondent sensor  1520 ′, since the inner main surface tamper-respondent sensor  1520 ′ is either bonded to the inner main rigid surface  2102  ( FIG. 21A ) or to inner-sidewall tamper-respondent sensor  1510 ′ ( FIG. 21A ). The conductive traces  2120  are illustrated in  FIG. 21B  in the overlap region  2105  ( FIG. 21A ) between the two tamper-respondent sensors of the tamper-respondent assembly shown in  FIG. 21A . Note that in this implementation, a tamper-detect network is formed, at least in part, by conductive traces  2120 , and that this network may further include unexposed circuit lines  2121 , such as unexposed circuit lines within the tamper-respondent sensor  1520 ′ itself. That is, a tamper-detect network may include both exposed conductive traces  2120  and unexposed circuit lines  2121 , as desired for a particular application. The unexposed circuit lines may be provided within the tamper-respondent sensor to, for instance, complete the tamper-detect network where not needed to be external for tamper-detect purposes. For example, in the implementation of  FIG. 21B , the unexposed circuit lines  2121  are shown extending within integrated flex ribbon cable or extension  1521 ′ of inner main surface tamper-respondent sensor  1520 ′. The integrated flex ribbon cable or extension  1521 ′ would be within the secure volume  801  in the implementation of  FIG. 21A  to, for instance, facilitate electrical connection of the inner main surface tamper-respondent sensor  1520 ′ to monitor circuitry within the secure volume. Note further, in one or more other implementations, where the bond region of the tamper-respondent sensor comprises only a portion of the outer surface of the tamper-respondent sensor, that both unexposed circuit lines and exposed conductive traces may be employed in forming one or more tamper-detect networks, with the exposed conductive traces only residing in the bond region to provide the enhanced tamper-detection capability disclosed herein in areas susceptible to mechanical and/or chemical attack of the adhesive. 
       FIG. 21C  illustrates a potential line of attack  2130  addressed by the enhanced tamper-respondent assembly of  FIGS. 21A &amp; 21B . In particular, line of attack  2130  may be attempted through structure  2101 , into adhesive  2115 , with the attack involving a chemical attack against the adhesive in order to, for instance, separate inner main surface tamper-respondent sensor  1520 ′ from inner main surface tamper-respondent sensor  1510 ′, and/or from the inner main surface of structure  2101 . By providing one or more of conductive traces  2120 ,  2120 ′ as illustrated, any chemical attack against adhesive  2115  will also wet or otherwise damage conductive traces  2120 ,  2120 ′, with the damage to the conductive traces facilitating detection of the chemical attack by the monitor circuitry, and thus, initiation of an alarm and/or triggering of an erasure of, for instance, encryption/decryption keys stored within the secure volume. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of one or more aspects of the invention and the practical application, and to enable others of ordinary skill in the art to understand one or more aspects of the invention for various embodiments with various modifications as are suited to the particular use contemplated.