Patent Publication Number: US-10327343-B2

Title: Applying pressure to adhesive using CTE mismatch between components

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. 
     SUMMARY 
     Provided herein, in one or more aspects, is an assembly apparatus comprising a pressure cure fixture. The pressure cure fixture is sized to reside within a container, and facilitate applying pressure to an adhesive disposed over an inner surface of the container. The pressure cure fixture is formed of a material with a higher coefficient of thermal expansion (CTE) than the container, and is sized to correspond, at least in part, to an inner space of the container while allowing for the adhesive and a surface-mount element to be disposed between the pressure cure fixture and the inner surface of the container. When heated, the pressure cure fixture expands greater than the container and imparts the pressure to the surface-mount element and the adhesive to facilitate securing the surface-mount element to the inner surface of the container. Within the assembly apparatus, the container is an electronic enclosure, and the surface-mount element includes a tamper-respondent sensor. 
     In one or more other aspects, an assembly apparatus is provided which includes a pressure cure fixture sized to reside within an electronic enclosure, and facilitate applying pressure to an adhesive disposed over an inner surface of the electronic enclosure. The pressure cure fixture is formed of a material with a higher coefficient of thermal expansion (CTE) than the electronic enclosure, and is sized to correspond, at least in part, to an inner space of the electronic enclosure while allowing for the adhesive and a tamper-respondent sensor to be disposed between the pressure cure fixture and the inner surface of the electronic enclosure. When heated, the fixture expands greater than the electronic enclosure and imparts the pressure to the tamper-respondent sensor and the adhesive to facilitate securing the tamper-respondent sensor to the inner surface of the electronic enclosure. Further, the inner surface includes an inner-sidewall of the electronic enclosure, and the tamper-respondent sensor includes an inner-sidewall tamper-respondent sensor sized to cover the inner-sidewall of the electronic enclosure, and wherein the adhesive includes an adhesive sheet layer, at least one side of the adhesive sheet layer including a pressure-sensitive adhesive. 
     In one or more further aspects, a fabrication method is provided which includes providing an assembly apparatus. Providing the assembly apparatus includes: providing a pressure cure fixture sized to reside within a container and facilitate applying pressure to an adhesive disposed over an inner surface of the container during curing, the pressure cure fixture being formed of a material with a higher coefficient of thermal expansion (CTE) than the container, and being sized to correspond, at least in part, to an inner space of the container while allowing for the adhesive and a surface-mount element to be disposed between the pressure cure fixture and the inner surface of the container; and wherein heating the container with the pressure cure fixture, surface-mount element and adhesive disposed therein expands the pressure cure fixture greater than the container and imparts pressure to the surface-mount element and the adhesive to facilitate securing the surface-mount element to the inner surface of the container, wherein the adhesive is a first adhesive, the pressure cure fixture is a first pressure cure fixture, the material of the first pressure cure fixture is a first material, and the surface-mount element is a first surface-mount element, and wherein the assembly apparatus further comprises a second pressure cure fixture to facilitate adhering a second surface-mount element to the inner surface of the container after the first surface-mount element has been adhered to the inner surface of the container, the second pressure cure fixture being formed of a second material with a higher coefficient of thermal expansion (CTE) than the container, the second pressure cure fixture being sized to correspond, at least in part, to the inner space of the container while allowing for the first surface-mount element, the second surface-mount element and a second adhesive to be disposed between the second pressure cure fixture and the inner surface of the container, wherein when heated, the second pressure cure fixture expands greater than the container and imparts pressure to the second surface-mount element and the second adhesive to facilitate securing the second surface-mount element to the inner surface of the container. 
     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. 3  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. 4A  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. 4B  is a top plan view of the multilayer circuit board of  FIG. 4A , 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. 5  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. 6  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. 7  is isometric view of one embodiment of a tamper-respondent assembly, or tamper-proof electronic package, which includes (in part) a multilayer circuit board with embedded tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 8A  depicts an underside, perspective view of one embodiment of a tamper-respondent assembly comprising an electronic enclosure and multiple tamper-respondent sensors, in accordance with one or more aspects of the present invention; 
         FIG. 8B  depicts an exploded view of the tamper-respondent assembly of  FIG. 8A , in accordance with one or more aspects of the present invention; 
         FIG. 9A  is an isometric view of one embodiment of an inner-sidewall tamper-respondent sensor for covering an inner-sidewall surface of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 9B  depicts an underside, isometric view of the electronic enclosure and inner-sidewall tamper-respondent sensor of  FIGS. 8A &amp; 8B , 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. 9C  is an enlarged depiction of the tamper-respondent assembly of  FIG. 9B , illustrating region  9 C thereof, in accordance with one or more aspects of the present invention; 
         FIG. 10A  is an enlarged depiction of the inner main surface tamper-respondent sensor embodiment illustrated in  FIGS. 8A &amp; 8B , in accordance with one or more aspects of the present invention; 
         FIG. 10B  depicts the inner main surface tamper-respondent sensor of  FIG. 10A , with the corner tabs shown raised for positioning, as illustrated in  FIGS. 8A &amp; 8B , in accordance with one or more aspects of the present invention; 
         FIG. 10C  depicts the tamper-respondent assembly of  FIGS. 8A &amp; 8B , 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. 11  is a perspective view of the security element(s) illustrated in  FIGS. 8A &amp; 8B  for the tamper-respondent assembly, in accordance with one or more aspects of the present invention; 
         FIG. 12A  is an exploded view of an assembly apparatus for affixing, for instance, an inner-sidewall tamper-respondent sensor of a tamper-respondent assembly to the inner-sidewall of an electronic enclosure such as depicted in  FIGS. 7-8B , in accordance with one or more aspects of the present invention; 
         FIG. 12B  depicts the assembly apparatus of  FIG. 12A , with the electronic enclosure, adhesive, inner-sidewall tamper-respondent sensor, and pressure cure fixture shown positioned within the base fixture, in accordance with one or more aspects of the present invention; 
         FIG. 12C  depicts the assembly apparatus of  FIGS. 12A &amp; 12B  assembled to undergo heating to secure the inner-sidewall tamper-respondent sensor to the inner-sidewall of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 13A  depicts an exploded view of an assembly apparatus for affixing, for instance, an inner main surface tamper-respondent sensor to an inner main surface of the electronic enclosure of  FIGS. 12A-12C , in accordance with one or more aspects of the present invention; 
         FIG. 13B  depicts one embodiment of the pressure cure fixture and upper fixture of the assembly apparatus of  FIG. 13A , in accordance with one or more aspects of the present invention; 
         FIG. 13C  depicts the assembly apparatus of  FIGS. 13A &amp; 13B  assembled to undergo heating to secure the inner main surface tamper-respondent sensor to the inner main surface of the electronic enclosure, in accordance with one or more aspects of the present invention; 
         FIG. 14A  depicts an exploded view of an assembly apparatus for affixing, for instance, a security element(s) about the inner perimeter of the electronic enclosure over the inner-sidewall tamper-respondent sensor and an inner main surface tamper-respondent sensor, in accordance with one or more aspects of the present invention; 
         FIG. 14B  depicts a partial assembly of the assembly apparatus of  FIG. 14A , with the pressure cure fixture and tamper-respondent sensors shown within the inner space of the electronic enclosure, in accordance with one or more aspects of the present invention; and 
         FIG. 14C  depicts the assembly apparatus of  FIG. 14A  assembled to undergo heating to secure the security element(s) about the inner perimeter of the electronic enclosure over, for instance, the inner-sidewall tamper-respondent sensor and inner main surface tamper-respondent sensor, 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 instance, 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 multilayer circuit board, such as a printed circuit board or other multilayer 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. 3-14C  are various approaches and/or enhancements to creating, for instance, 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. 3  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. 3 , 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. 3  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 1  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. 3  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 1  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 1 , 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 1  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, it is more likely to reduce the line width sufficiently to detect the intrusion by a change in resistance. A change in a narrower line width 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, intrinsically conductive polymers, carbon ink, 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 1  and line length L 1 , 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 1  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 OmegaPly® 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, 5-6 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, 20 mils in the case of a typical polyester film, to 10 mils or less with the use of crystalline polymer films. 
     By way of further example,  FIGS. 4A &amp; 4B  depict one embodiment of tamper-respondent assembly  400 , or tamper-proof electronic package, which comprises an electronic circuit  415  to be protected, in accordance with one or more further aspects of the present invention. 
     Referring collectively to  FIGS. 4A &amp; 4B , electronic circuit  415  includes a multilayer circuit board  410  which has an embedded tamper-respondent sensor  411  therein that facilitates defining, in part, a secure volume  401  associated with multilayer circuit board  410  that extends into multilayer circuit board  410 . In particular, in the embodiment of  FIGS. 4A &amp; 4B , secure volume  401  exists partially within multilayer circuit board  410 , and partially above multilayer circuit board  410 . One or more electronic components  402  are mounted to multilayer circuit board  410  within secure volume  401  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  400  further includes an enclosure  420 , such as a pedestal-type enclosure, mounted to multilayer circuit board  410  within, for instance, a continuous groove (or trench)  412  formed within an upper surface of multilayer circuit board  410 . In one or more embodiments, enclosure  420  may comprise a thermally conductive material and operate as a heatsink for facilitating cooling of the one or more electronic components  402  within the secure volume. A security mesh or tamper-respondent sensor  421 , such as the above-described tamper-respondent sensors of  FIG. 3 , may be associated with enclosure  420 , for example, wrapping around the inner surface of enclosure  420  to facilitate defining, in combination with tamper-respondent sensor  411  embedded within multilayer circuit board  410 , secure volume  401 . In one or more implementations, tamper-respondent sensor  421  extends down into continuous groove  412  in multilayer circuit board  410  and may, for instance, even wrap partially or fully around the lower edge of enclosure  420  within continuous groove  412  to provide enhanced tamper-detection where enclosure  420  couples to multilayer circuit board  410 . In one or more implementations, enclosure  420  may be securely affixed to multilayer circuit board  410  using, for instance, a bonding material such as an epoxy or other adhesive. 
     As depicted in  FIG. 4B , one or more external circuit connection vias  413  may be provided within multilayer circuit board  410  for electrically connecting to the one or more electronic components  402  ( FIG. 4A ) within secure volume  401 . These one or more external circuit connection vias  413  may electrically connect to one or more external signal lines or planes (not shown) embedded within multilayer circuit board  410  and extending, for instance, into a secure base region of (or below) secure volume  401 , as explained further below. Electrical connections to and from secure volume  401  may be provided by coupling to such external signal lines or planes within the multilayer circuit board  410 . 
     As noted with reference to  FIGS. 4A &amp; 4B , secure volume  401  defined in association with multilayer circuit board  410  may be sized to house electronic components  402  to be protected, and be constructed to extend into multilayer circuit board  410 . In one or more implementations, multilayer circuit board  410  includes electrical interconnect within the secure volume  401  defined in the board, for instance, for electrically connecting the multiple tamper-respondent layers of the embedded tamper-respondent sensor  411  to associated monitor circuitry also disposed within secure volume  401 , along with, for instance, one or more daughter cards, such as memory DIMMs, PCIe cards, processor cards, etc. 
     Note that the embodiment depicted in  FIGS. 4A &amp; 4B  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  420 , and/or other approaches to coupling enclosure  420  and multilayer circuit board  410  may be employed. 
     By way of further example,  FIG. 5  depicts a partial cross-sectional elevational view of one embodiment of multilayer circuit board  410  and enclosure  420 . 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  500 , and at least one tamper-respondent frame  501 . In the example depicted, two tamper-respondent mat layers  500  and two tamper-respondent frame  501  are illustrated, by way of example only. The lower-most tamper-respondent mat layer  500  may be a continuous sense or detect layer extending completely below the secure volume being defined within multilayer circuit board  510 . One or both tamper-respondent mat layers  500  below secure volume  401  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  505  enter secure volume  401  between, in this embodiment, two tamper-respondent mat layers  500 , and then electrically connect upwards into the secure volume  401  through one or more conductive vias, arranged in any desired location and pattern. In the configuration depicted, the one or more tamper-respondent frames  501  are disposed at least inside of the area defined by continuous groove  512  accommodating the base of enclosure  520 . Together with security sensor  421  associated with enclosure  420 , tamper-respondent frames  501  define secure volume  401  where extending, in part, into multilayer circuit board  410 . With secure volume  401  defined, at least in part, within multilayer circuit board  410 , the external signal line(s)  505  may be securely electrically connected to, for instance, the one or more electronic components  402  ( FIG. 4A ) mounted to multilayer circuit board  410  within secure volume  401 . In addition, the secure volume  401  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  500  (and if desired, tamper-respondent frames  501 ) outward past continuous groove  412  accommodating enclosure  420 . In this manner, a line of attack may be made more difficult at the interface between enclosure  420  and multilayer circuit board  410  since the attack would need to clear tamper-respondent mat layers  500 , the bottom edge of tamper-respondent sensor  421  associated with enclosure  420 , as well as the tamper-respondent frames  501  of the embedded tamper-respondent sensor. 
     Variations on the multilayer circuit board  410  of  FIG. 4A  are possible. For instance, in one embodiment, the embedded tamper-respondent sensor may include multiple tamper-respondent mat layers  500  and multiple tamper-respondent frames  501 , 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  410 , conductive vias within the secure volume between layers of multilayer circuit board  410  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. 6  illustrates one embodiment for forming and patterning a tamper-respondent layer within such a multilayer circuit board. 
     As illustrated in  FIG. 6 , 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  601 , such as a pre-preg (or pre-impregnated) material layer, a trace material layer  602  for use in defining the desired trace patterns, and an overlying conductive material layer  603 , to be patterned to define conductive contacts or vias electrically connecting to the pattern of traces being formed within the trace material layer  602 , for instance, at trace terminal points. In one or more implementations, the trace material layer  602  may comprise nickel phosphorous (NiP), and the overlying conductive layer  603  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  600 . 
     A first photoresist  604  is provided over build-up  600 , and patterned with one or more openings  605 , through which the overlying conductive layer  603  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  602  to define the conductive traces of the subject tamper-respondent layer. First photoresist  604  may then be removed, and a second photoresist  604 ′ is provided over the conductive layer  603  features to remain, such as the input and output contacts. Exposed portions of conductive layer  603  are then etched, and the second photoresist  604 ′ 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  603  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  602 . 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  421  ( FIG. 4A ), may be electrically connected into monitor or compare circuitry provided, for instance, within secure volume  401  ( FIG. 4A ) of multilayer circuit board  410 . The monitor circuitry may include various bridge or compare circuits, and conventional printed wiring board electrical interconnect inside the secure volume  401 , for instance, located within the secure volume defined by the tamper-respondent frames  501  ( FIG. 5 ), 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, Wheatstone bridge, or similar 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. 
     By way of example, an isometric view of one embodiment of a tamper-respondent assembly, or tamper-proof electronic package, is depicted in  FIG. 7 , wherein an electronic enclosure  700  is shown sealed to multilayer circuit board  410  to define a secure volume about one or more electronic components, as described herein. In the embodiment depicted, electronic enclosure  700  includes a main surface  701  and sidewall(s)  702  which include sidewall corners  710 . An inner surface of electronic enclosure  700  would include an inner main surface, and an inner-sidewall surface corresponding to main surface  701  and sidewall(s)  702 , respectively, with the inner main surface and inner-sidewall surfaces being covered, at least in part, by a tamper-respondent sensor such as described herein. A power supply  705  or battery for the tamper-respondent sensor is located, in this embodiment, external to the secure volume, with the sensor again being configured to trip and destroy any protected or critical data if the power supply or battery is tampered with. Electronic enclosure  700  may be adhered or mechanically affixed to multilayer circuit board  410 , which as noted above, may include its own embedded tamper-respondent sensor layer(s). 
     In one or more implementations, the tamper-respondent assembly may incorporate or include an air path to adjust air pressure within the secure volume of the assembly to equalize with air pressure external to the assembly. By way of example, an air vent may be provided through electronic enclosure  700 , and this air vent may be in fluid communication with a vent structure disposed between the electronic enclosure and the multilayer circuit board. For instance, the vent structure may comprise a low profile vent structure sandwiched between overlapping regions of a tamper-respondent sensor covering an inner-sidewall surface of the electronic enclosure. The vent structure includes at least one air passage coupling in fluid communication the secure volume defined by the assembly and the air vent in electronic enclosure  700 . Note that in this context “in fluid communication” refers to air or gas communication being established between the secure volume provided by the tamper-respondent assembly, and the air vent through the electronic enclosure. 
       FIGS. 8A &amp; 8B  depict underside, isometric views of one embodiment of a tamper-respondent assembly employing electronic enclosure  700 . Referring collectively to  FIGS. 8A &amp; 8B , in one or more implementations, tamper-respondent assembly  800  includes electronic enclosure  700  which, as noted, is to enclose, at least in part, one or more electronic components or an electronic assembly to be protected, and associated with, for instance, a multilayer circuit board. Electronic enclosure  700  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. 7 . Further, tamper-respondent assembly  800  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  700 . 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  700 , 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  700  may include an inner-sidewall tamper-respondent sensor  810  and an inner main surface tamper-respondent sensor  820 , along with a security band or element  830 . In the illustrated example, inner-sidewall tamper-respondent sensor  810  may be formed with an integrated flex ribbon cable or extension  811  to facilitate electrical connection of the at least one resistive network within inner-sidewall tamper-respondent sensor  810  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. 8A &amp; 8B . Similarly, inner main surface tamper-respondent sensor  820  may be configured with an integrated flex ribbon cable or extension  821  to facilitate electrical connection of inner main surface tamper-respondent sensor  820  to the monitor circuitry, as well. A bonding agent (discussed below), such as a thermoset adhesive, may be employed to adhere inner-sidewall tamper-respondent sensor  820  to the inner-sidewall surface and to inner-sidewall corners. A similar adhesive could be used to adhere inner main surface tamper-respondent sensor  820  to inner main surface and to inner-sidewall tamper-respondent sensor  810  where the sensors overlap. Security band  830  may further be adhesively secured over the overlap between inner main surface tamper-respondent sensor  820  and inner-sidewall tamper-respondent sensor  810  covering, in one or more implementations, transition regions between the inner-sidewall surface and the inner main surface around the inner perimeter of electronics enclosure  700 . 
     Note that, in the example provided in  FIGS. 8A &amp; 8B , inner-sidewall tamper-respondent sensor  810  and inner main surface tamper-respondent sensor  820  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. 8A &amp; 8B  to a multilayer circuit board with an embedded tamper-respondent sensor, such as described above. 
       FIG. 9A  depicts one embodiment of inner-sidewall tamper-respondent sensor  810  of  FIGS. 8A &amp; 8B . In this embodiment, inner-sidewall tamper-respondent sensor  810  includes at least one first layer  900  having opposite first and second sides  901 ,  902 , and circuit lines  905  extending substantially over all of the flexible layer, and forming at least one tamper-detect network, such as described herein. For instance, circuit lines  905  may be disposed on at least one of first side  901  or second side  902  of the at least one flexible layer  900 . Note that the at least one flexible layer  900  may be fabricated as a conventional security sensor layer, or be fabricated as an enhanced, tamper-respondent sensor such as described herein. As noted, extension  811  may extend from inner-sidewall tamper-respondent sensor  810  to facilitate electrical connection of the at least one resistive network of the inner-sidewall tamper-respondent sensor  810  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. 8A &amp; 8B . As illustrated, in one or more implementations, inner-sidewall tamper-respondent sensor  810  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  910  are provided within inner-sidewall tamper-respondent sensor  810 . These multiple slots  910  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. 9B &amp; 9C  depict one embodiment of inner-sidewall tamper-respondent sensor  810  mounted within electronic enclosure  700 . 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 of the associated inner-sidewall corner  710  to be covered. These first and second slots are spaced apart to reside at opposite sides of the flat, angled-sidewall portion of the inner-sidewall corner  710 , and facilitate reducing the amount of material in the corner and thereby enhance good contact and adhesion of the inner-sidewall tamper-respondent sensor  810  to the inner-sidewall surface of the electronic enclosure, including at the inner-sidewall corners thereof, while also reducing stress on the sensor within the corner(s). For instance, the multiple slots  910  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  810  has a width which allows the sensor to cover the transition regions, as well as extend over, in part, the inner main surface of electronic enclosure  700 . Note also that one or more uncovered regions  915  may result from the presence of the slots when the inner-sidewall tamper-respondent sensor  810  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  915  may be covered or protected by inner main surface tamper-respondent sensor  820  corner tabs once that sensor and its corner tabs are adhered to the assembly. This is illustrated, by way of example, in  FIGS. 10A-10C . 
     Referring collectively to  FIGS. 10A-10C , inner main surface tamper-respondent sensor  820  includes at least one flexible layer  1000  having opposite first and second sides  1001 ,  1002 , and circuit lines  1005  extending substantially over all of the flexible layer  1000  and forming at least one tamper-detect network, such as described herein. For instance, circuit lines  1005  are disposed on one or both of first side  1001  and second side  1002  of the at least one flexible layer  1000 , as described. As noted above, the at least one flexible layer  1000  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  1000  of inner main surface tamper-respondent sensor  820  could comprise a flexible layer, such as described above. As noted, extension  821  may be formed integral with inner main surface tamper-respondent sensor  820  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. 8A &amp; 8B ; 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  1010  are provided, with at least one corner tab  1010  being provided at the at least one inner-sidewall corner. In the exemplary embodiment illustrated, two corner tabs  1010  are provided at each corner of the inner main surface tamper-respondent sensor  820 . These corner tabs  1010  include circuit lines  1005  ( FIG. 10A ) and are sized to cover a respective one of the uncovered regions  915  in inner-sidewall tamper-respondent sensor and enclosure assembly which remain after securing inner-sidewall tamper-respondent sensor  810  to electronic enclosure  700 , as illustrated in  FIGS. 10B &amp; 10C . In particular, those skilled in the art should understand that corner tabs  1010  include respective portions of the at least one tamper-detect network provided by inner main surface tamper-respondent sensor  820 , such that if an attempt were made to breach the tamper-respondent assembly  800  through the underlying, uncovered regions  915  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. 8A &amp; 8B , reinforcement of the overlap between inner-sidewall tamper-respondent sensor  810  ( FIG. 8A ) and inner main surface tamper-respondent sensor  820  ( FIG. 8A ) may be provided, in one or more implementations, by one or more physical security structures, such as security band or element  830 . One potential point of attack 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  820  is adhesively secured over inner-sidewall tamper-respondent sensor  810 . To address this concern, a physical security structure, such as one or more security bands or elements  830 , may be provided. Note that security band  830  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. 
     In general, 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 or 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 tamper-respondent assembly may further include 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. 8A &amp; 8B , security band  830  is illustrated which includes multiple security elements  831 , as shown in enlarged view in  FIG. 11 . Note that the security structure, such as security band  830 , could comprise a single element or multiple elements, depending on the desired configuration. In the example of  FIGS. 8A, 8B &amp; 11 , two substantially identical, U-shaped security elements  831  are illustrated, by way of example only. In the depicted embodiment, security elements  831  are spaced apart, with gaps  832  therebetween. By providing two or more security elements  831 , to define a desired physical security structure (such as security band  830 ) manufacturing tolerances may be better accommodated within the tamper-respondent assembly. By way of example, the gaps  832  between adjacent security elements of the multiple, distinct security elements  831 , may be on the order of several millimeters. Note that although illustrated as two security elements  831 , 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. 8A, 8B &amp; 11 , the physical security structure is configured as a security band or collar comprising multiple distinct security elements  831  which extends substantially fully around the inner perimeter of the electronic enclosure  700 . The security band  830  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. 8A . In one or more implementations, security band  830  may be adhesively secured to the tamper-respondent sensors. Note that in this example, security band  830 , comprising security elements  831 , extends around the inner perimeter, including through the inner-sidewall corners, as illustrated in  FIG. 8A . In this manner, security elements  831  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  830 , or more particularly, security elements  831 , overlie and physically secure in place the multiple corner tabs  1010  ( FIG. 10C ) projecting from the inner main surface tamper-respondent sensor  820  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  830  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. 8A . 
     One fabrication issue with providing a tamper-respondent assembly such as described above in connection with  FIGS. 7-11  is that the adhesive used within the electronic enclosure may require a pressure cure at an elevated temperature to ensure intimate contact between, for instance, one or more of the sensors and the inner surface of the electronic enclosure; that is, to ensure that there are no voids between the surfaces. Existing techniques for achieving pressure cure adhesion may include vacuum bagging or use of a pressure cure oven, both of which are expensive. Further, the existing pressure cure approaches can be tedious; for instance, vacuum bagging a large volume of parts may be difficult, or even logistically prohibitive. 
     Described herein below with reference with  FIGS. 12A-14C  are enhanced techniques and assembly apparatuses for, for instance, aiding in good adhesion of tamper-respondent sensors to an electronic enclosure, such as a metal electronic enclosure, to provide a secure volume about one or more electronic components to be protected. The assembly apparatuses and processes disclosed may also be generalized to facilitating securing any surface-mount element to an inner surface of a container using a pressure-sensitive or pressure cure adhesive. 
     Generally stated, therefore, in one or more aspects, an assembly apparatus is disclosed herein which includes a pressure cure fixture or element that is sized to reside within a container, such as an electronic enclosure, and facilitate applying pressure during heat curing to an adhesive disposed over an inner surface of the container. The fixture is formed of a material with a higher coefficient of thermal expansion (CTE) than the container, and is sized to correspond, at least in part, to an inner space of the container, while allowing for the adhesive and a surface-mount element, such as a tamper-respondent sensor, to be disposed between the pressure cure fixture and the inner surface of the container. By way of example, the adhesive may comprise a pressure-sensitive adhesive, such as an adhesive sheet layer with a pressure-sensitive adhesive on at least one side. Such an adhesive may be particularly advantageous when applying a surface-mount element to an inner-sidewall of the container. The pressure cure fixture, which may be sized to substantially fill the inner space of the container, is configured and the material thereof is chosen so that when heated, the fixture expands greater than the container and imparts a desired pressure to the surface-mount element and adhesive to facilitate securing the surface-mount element to the inner surface of the container. Achieving a desired pressure on the surface-mount element and adhesive may be accomplished by selecting the material of the pressure cure fixture dependent, in part, on the material of the container, as well as forming or machining the fixture to have a desired minimal spacing or gap between the fixture and the surface-mount element at room temperature. For instance, when assembled at room temperature, the gap between the pressure cure fixture and surface-mount element or inner surface of the container may be only on the order of a few millimeters or less. 
     In one or more implementations, the adhesive may comprise an adhesive sheet layer, with at least one side of the adhesive sheet layer comprising a pressure-sensitive adhesive. Further, the material of the pressure cure fixture may be, for instance, a rubber or silicone material, such as silicone sponge rubber, or may be teflon, and the inner surface may be an inner-sidewall surface or inner main surface of the container, where the surface-mount element mounts to the inner surface of the container via the adhesive. Alternatively, in one or more implementation, the adhesive may comprise a liquid adhesive to be pressure cured. By way of example, the adhesive may also include one or more compounds, materials or elements to enhance thermal conductivity of the adhesive once cured in place. 
     In one or more embodiments, the assembly apparatus further includes a base fixture, upper fixture, and clamp. The base fixture includes an opening sized to receive the container with a pressure cure fixture, surface-mount element and adhesive disposed therein, and the clamp clamps or affixes the upper fixture to the base fixture. In one or more embodiments, the upper fixture may include or be subject to spring biasing (for instance, via a spring biasing plate disposed over the upper fixture, and affixed to the base fixture) to, for instance, apply uniform pressure to the pressure cure fixture to force the fixture towards, for instance, the inner main surface of the container. The base fixture, upper fixture and clamp facilitate, in part, alignment of the surface-mount element to an edge of the container during the heating or curing of the adhesive and imparting of the pressure to the surface-mount element and adhesive. 
     In one or more implementations, the adhesive is a first adhesive, the pressure cure fixture is a first pressure cure fixture, the material of the first pressure cure fixture is a first material, and the surface-mount element is a first surface-mount element. Further, the assembly may include a second pressure cure fixture to facilitate adhering a second surface-mount element to the inner surface of the container after the first surface-mount element has been adhered to the inner surface of the container. The second pressure cure fixture may be formed of a second material with a higher coefficient of thermal expansion (CTE) than the container. The second pressure cure fixture may be sized to correspond, at least in part, to the inner space of the container while allowing for the first surface-mount element, the first adhesive, the second surface-mount element, and the second adhesive to be disposed between the second pressure cure fixture and the inner surface of the container. When heated, the second pressure cure fixture expands greater than the container and imparts pressure to the second surface-mount element and the second adhesive to facilitate securing the second surface-mount element to the inner surface of the container. As one example, the first surface-mount element may be an inner-sidewall tamper-respondent sensor, and the second surface-mount element may be an inner main surface tamper-respondent sensor. Further, by way of example, the first material and the second material of the first and second pressure cure fixtures may be the same or different materials, and the first and second adhesives may be different types of adhesives. For instance, at least one of the different types of adhesives may include a pressure-sensitive adhesive. Further, the materials of the first and second pressure cure fixtures may include at least one of silicone or teflon. Still further, in one or more implementations, the container may be an electronic enclosure formed of a metal, such as aluminum or copper, or a metal alloy. 
     Note generally, the assembly apparatuses and processes disclosed herein utilize a specially configured high CTE material placed within a container or enclosure along with a surface-mount element, such as a tamper-respondent sensor, at room temperature prior to curing. The adhesive employed may include, for instance, a pressure-sensitive adhesive. Once assembled, the assembly is placed within an oven for curing of the adhesive, and as temperature rises, the high CTE material expands at a rate much higher than the container, and thereby exerts the desired pressure outwards towards the inner wall of the container, and thereby facilitates void-free bonding of the surface-mount element to the container. As the temperature cools after curing, the high CTE material shrinks back to its original size, and is readily removed from the container. The high CTE material(s) of the pressure cure fixture may be chosen, in part, so that the material does not adhere itself to the adhesive in the event that any adhesive contacts the fixture. Advantageously, the assembly apparatus disclosed herein is inexpensive, with the high CTE pressure cure fixture being reusable, and with the process employing, for instance, a standard batch oven. 
       FIGS. 12A-12C  depict one embodiment of an assembly apparatus  1200 , in accordance with one or more aspects of the present invention. By way of example, the assembly apparatus of  FIG. 12A  is configured to facilitate forming a tamper-respondent assembly, such as described above in connection with  FIGS. 7-11 . As noted, however, the assembly apparatus and process disclosed herein can be readily adapted to facilitate securing any surface-mount element to an inner surface of a recess or void in a structure, generally referred to herein as a container. 
     The electronic enclosure  700  depicted is presented as one example of a container having an inner surface to which one or more surface-mount elements, such as inner-sidewall tamper-respondent sensor  810  are to be affixed. As noted above, the tamper-respondent assembly includes a tamper-respondent electronic circuit structure which includes, in one or more embodiments, at least one tamper-respondent sensor  810  mounted to and covering, at least in part, the inner surface(s) of electronic enclosure  700 . The tamper-respondent sensor(s) is configured (for example, is flexible) so as to facilitate good contact, and good adhesion of the sensor to the inner surface of the enclosure. 
     By way of example,  FIGS. 12A-12C  depict an assembly apparatus and process for adhering inner-sidewall tamper-respondent sensor  810  to the inner-sidewall of electronic enclosure  700 , while  FIGS. 13A-13C  illustrate an assembly apparatus and process for adhering an inner main surface tamper-respondent sensor to the inner main surface of the electronic enclosure, and  FIGS. 14A-14C  depict one embodiment of an assembly apparatus and process for securing a security element(s) around the inner perimeter of the electronic enclosure over, for instance, the inner main surface tamper-respondent sensor and/or inner-sidewall tamper-respondent sensor. 
     Referring collectively to  FIGS. 12A-12C , assembly apparatus  1200  includes a pressure cure fixture or element  1210  designed and sized to reside within electronic enclosure  700  and facilitate applying pressure during curing to an adhesive  1201  overlying the inner-sidewall surface of electronic enclosure  700 . By way of example, adhesive  1201  may be an adhesive sheet layer that is similarly sized to inner-sidewall tamper-respondent sensor  810  to be secured to the inner-sidewall of electronic enclosure  700 . The adhesive sheet layer may be, for instance, a few millimeters smaller in width or height than the inner-sidewall tamper-respondent sensor  810 , with constant pressure desired during the curing process to ensure good adhesion of the tamper-respondent sensor to the inner-sidewall of the electronic enclosure. Commercially available adhesive sheet layers may be employed. For instance, a 3M Scotch-Weld™ Structural Adhesive Film AF163-2 could be employed for adhesive  1201 . This material, which is available from 3M Corporation of Saint Paul, Minn., USA, has good bond strength, and high fracture toughness and peel strength. Further, the material has good resistance to moisture, has a short cure time, for instance, 125° C. for 90 minutes, and is capable of low pressure bonding. The material is also X-ray opaque. Note, however, that this is only one example of adhesive  1201  which could be employed in the assembly apparatuses and processes disclosed herein. In one or more implementations, adhesive  1201 , such as the above-noted adhesive sheet layer, may be placed into the electronic enclosure over the inner-sidewall of the enclosure and the inner-sidewall tamper-respondent sensor  810  may then be placed over the adhesive  1201  within electronic enclosure  700 . 
     As depicted, pressure cure fixture  1210  is sized to correspond, at least in part, to the inner space of electronic enclosure  700 , while still allowing for adhesive  1201  and inner-sidewall tamper-respondent sensor  810  to be disposed between pressure cure fixture  1210  and electronic enclosure  700 . By way of example, pressure cure fixture  1210  could be configured to substantially fill the inner space of electronic enclosure  700  with adhesive  1201  and inner-sidewall tamper-respondent sensor  810  disposed therein. For instance, pressure cure fixture  1210  may be configured, designed and sized so that at room temperature, the fixture readily slides into the inner space of electronic enclosure  700 . Any spacing or gap between pressure cure fixture  1210  and, for instance, inner-sidewall tamper-respondent sensor  810  may be dependent, for instance, on the particular material of the pressure cure fixture selected as the high coefficient of thermal expansion (CTE) material, relative to the coefficient of thermal expansion of the electronic enclosure  700 . As one example only, the material of pressure cure fixture  1210  could be a silicone sponge rubber material, such as the sponge rubber materials offered by Diversified Silicone Products, Inc. of Santa Fe Springs, Calif., USA, and electronic enclosure  700  may be formed of a metal, such as aluminum. The density of the silicone sponge rubber may be selected to achieve a desired force exerted on the adhesive for a given set of cure conditions. For instance, an applied force in the range of 5-20 pounds may be achieved. 
     In the embodiment depicted, assembly apparatus  1200  may further include a base fixture  1220 , upper fixture  1230  and clamp  1240  with bolts  1241  to secure the assembly together in fixed relation, for instance, for placement within a curing oven. In one or more embodiments, upper fixture  1230  may be configured to accommodate the integrated flex ribbon cable or extension  811  of inner-sidewall tamper-respondent sensor  810 . For instance, upper fixture  1230  and/or pressure cure fixture  1210  could be provided with one or more recesses or cutouts to accommodate the extension when assembled as depicted in  FIG. 12C . Alternatively, upper fixture  1230  and/or pressure cure fixture  1210  could include an appropriately located recess to allow for integrated flex extension  811  to bend over within the assembly during the curing process. 
     As noted,  FIGS. 13A-13C  illustrate one embodiment of an assembly apparatus and process for adhering an inner main surface tamper-respondent sensor  820  to an inner main surface of electronic enclosure  700 . In the depicted implementation of  FIGS. 13A-13C , a different assembly apparatus  1300  is presented which includes a pressure cure fixture  1310 , base fixture  1320 , and upper fixture  1330 , which may be secured, such as in the depicted embodiment, to pressure cure fixture  1310 . A clamp or attachment mechanism, such as one or more bolts  1332 , may be employed to secure the assembly apparatus together, as explained further below. Note that the assembly apparatus  1300  and process of  FIGS. 13A-13C  are presented by way of example only as part of forming a tamper-respondent assembly, such as described above in connection with  FIGS. 7-11 . As noted above, the assembly apparatuses and processes disclosed herein could be readily adapted to facilitate securing any surface-mount element to an inner surface of many different types of containers, one embodiment of which would be electronic enclosure  700 . In  FIGS. 13A-13C , inner main surface tamper-respondent sensor  820  is to be adhered to an inner main surface of electronic enclosure  700  using an adhesive  1301 , such as a liquid adhesive. 
     Pressure cure fixture or element  1310  is configured, designed and sized to reside within electronic enclosure  700 , with inner-sidewall tamper-respondent sensor  810  positioned as depicted in  FIG. 13A , and with inner main surface tamper-respondent sensor  820  disposed between pressure cure fixture  1310  and electronic enclosure  700 . Pressure cure fixture  1310  may be formed of a material selected to have a high coefficient of thermal expansion (CTE) compared with the material(s) of electronic enclosure  700 . As noted, once assembled, the assembly may be placed within an oven for curing of the adhesive  1301 , and as temperature rises, the high CTE material of pressure cure fixture  1310  expands at a rate much higher than that of electronic enclosure  700 , and thereby exerts a desired force towards the inner surface of the electronic enclosure facilitating pressure curing of, for instance, the inner main surface tamper-respondent sensor  820  to the inner main surface of electronic enclosure  700 . 
     By way of example, adhesive  1301  may be a liquid adhesive with enhanced thermal conduction properties compared with, for example, adhesive  1201  ( FIG. 12A ) securing the inner-sidewall tamper-respondent sensor to the inner-sidewall of electronic enclosure  700 . In one or more implementations, the inner main surface tamper-respondent sensor  820  may be placed in direct contact with one or more heat generating electronic components within the secure volume being defined, in part, by the tamper-respondent assembly. Alternatively, one or more heat sinks may be in contact with the inner main surface tamper-respondent sensor  820  when the tamper-respondent assembly or tamper-proof electronic package is assembled. 
     As one example, adhesive  1301  may comprise a Master Bond Supreme 10AOHTXT black adhesive with, for instance, aluminum oxide as a filler to provide the adhesive with enhanced thermal conductivity properties. This adhesive is available from Master Bond, Inc. of Hackensack, N.J., USA. The material has good bond strength and a relatively short cure time, for instance, 125° C. for 90 minutes. As with the above-described material, adhesive  1301  is capable of relatively low pressure bonding (for instance, in the range of 5-20 pounds). Note, however, that this adhesive  1301  example is one example only of an adhesive which could be employed in the assembly apparatuses and processes disclosed herein. Further, note that in one or more implementations, the pattern of adhesive  1301  applied to the inner surface of electronic enclosure  700  may be configured or designed based upon placement of heat generating components within the secure volume. For instance, in the design depicted in  FIG. 13A , a higher heat generating electronic component in the middle of the container  700  is assumed such that the adhesive pattern facilitates spreading of any heat conducted across the inner main surface tamper-respondent sensor outward across the inner surface of the electronic enclosure  700 . 
     In operation, electronic enclosure  700  with inner-sidewall tamper-respondent sensor  810  affixed thereto may be prepared with adhesive  1301  and placed into base fixture  1320 , that is, into an opening configured and sized to receive electronic enclosure  700 . Inner main surface tamper-respondent sensor  820  may then be placed into electronic enclosure  700  over adhesive  1301 . Upper fixture  1330  with pressure cure fixture  1310  mounted thereto may then be placed in position over the subassembly using alignment pins  1321  projecting from base fixture  1320  and alignment openings  1331  in upper fixture  1330 . In this configuration, pressure cure fixture  1310  projects downward and substantially fills the inner space of electronic enclosure  700 , with inner main surface tamper-respondent sensor  1820  and inner-sidewall tamper-respondent sensor  1810  disposed between the fixture and the inner surface of electronic enclosure  700 . One or more bolts  1332  may be employed to clamp or affix the upper and lower fixtures together for placement of the assembly into a curing oven. 
     In one or more implementations, pressure cure fixture  1310  may be formed of teflon so that should any liquid adhesive  1301  contact the pressure cure fixture, it would not adhere to the fixture and prevent or inhibit disassembly of the assembly apparatus after curing of the adhesive. Note also, in one or more implementations, the upper fixture may include or be subject to spring biasing. For instance, a spring biasing plate (not shown) could be disposed over the upper fixture and be affixed to the base fixture, to facilitate spring biasing of the upper fixture towards the base fixture to, for instance, apply uniform pressure to the pressure cure fixture to force the fixture towards, for instance, the inner main surface of the container. 
     By way of further example,  FIGS. 14A-14C  depict one embodiment of an assembly apparatus  1400  and process for securing, for instance, security element(s)  831  around the inner perimeter of the electronic enclosure  700  over, for instance, the inner main surface tamper-respondent sensor  820  and/or inner-sidewall tamper-respondent sensor  810 . By way of example, in this configuration, the assembly apparatus of  FIGS. 12A-12C  may be reused with, for instance, a slightly reduced size pressure cure fixture  1410  compared with pressure cure fixture  1210  of  FIGS. 12A-12C . In particular, the assembly apparatus  1400  may include base fixture  1220 , upper fixture  1230  and clamp  1240  with bolts  1241  to secure the assembly together before placement within a cure oven. 
     Referring collectively to  FIGS. 14A-14C , assembly apparatus  1400  includes a pressure cure fixture  1410  designed and sized to reside within electronic enclosure  700  and facilitate applying pressure during curing to, for instance, security element  831  and adhesive  1401  overlying, for instance, one or more of the inner main surface tamper-respondent sensor and inner-sidewall tamper-respondent sensor. By way of example, adhesive  1401  may be an adhesive sheet layer that is similarly sized to security element(s)  831  to be secured about the inner perimeter of electronic enclosure  700 . The adhesive sheet layer may be, for instance, a few millimeters smaller in width or height than the security element(s)  831 , with constant pressure desired during the curing process to ensure good adhesion of the security element(s)  831  to, for instance, one or more of the inner-sidewall tamper-respondent sensor  810  and inner main surface tamper-respondent sensor  820 . Commercially available sheet layers may be employed, such as the above-referenced 3M Scotch-Weld™ Structural Adhesive Films. Note, however, that this is only one example of an adhesive  1401  which may be used to secure the security element(s)  831  within the electronic enclosure about the inner perimeter. 
     As with the examples above, pressure cure fixture  1410  is sized to correspond, at least in part, to the inner space of electronic enclosure  700 , while still allowing for the inner-sidewall tamper-respondent sensor  810 , inner main surface tamper-respondent sensor  820 , adhesive  1401  and security element(s)  831  to be disposed between the pressure cure fixture and the inner surface of the electronic enclosure, as illustrated. By way of example only, the material of pressure cure fixture  1410  could again be a silicone sponge rubber material, such as the above-discussed sponge rubber materials offered by Diversified Silicone Products, Inc. Alternatively, the material of pressure cure fixture  1410  could be formed of teflon, or any other available material with a higher CTE than that of the electronic enclosure  700  so that when placed within the cure oven, a desired pressure is generated from within the electronic enclosure outwards towards the surface-mount element to be adhered within the electronic enclosure  700 . As with the assembly apparatuses described above, once assembled together as depicted in  FIG. 14C , the assembly may be placed within a curing oven. In one or more embodiments, the pressure cure fixture  1410  and upper fixture  1230  are configured to accommodate the integrated flex ribbon extensions  811 ,  821 . For instance, one or more recesses or cutouts may be provided within the respective fixtures to allow the integrated flex extensions  811 ,  821  to project outward from the assembly. Alternatively, the upper fixture  1230  and/or pressure cure fixture  1410  could be configured with appropriately located and sized notches to allow for the integrated flex ribbon extensions to be bent over within the assembly during the curing process. 
     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.