Abstract:
Disclosed is method and system for packaging electronics with an electronic card rack of radial panels, an outer casing, a stabilizing ring, and a flex-circuit interconnect, whereby electronic cards inserted into the sectors formed by the radial panels do not carry the structural load. The card-mounted devices are optimally oriented for set back forces, and minimal potting and/or structural foam is required, if any. The present invention provides a lightweight, highly serviceable assembly that is relatively less expensive to produce, test and rework compared with potted assemblies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from the following U.S. Provisional Patent Application, the disclosure of which, including all appendices and all attached documents, is hereby incorporated herein by reference in its entirety for all purposes: U.S. Provisional Patent Application Ser. No. 60/338,435 of Ernest Steven Blazic, Kent Carl Nelson, and Farhad James Nekoogar entitled, “GUIDED MUNITIONS ELECTRONICS PACKAGE AND METHOD,” filed Nov. 30, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to housing and mounting assemblies for electronic systems and devices and methods of packaging thereof and more particularly to the packaging and assemblies of electronics of systems operable in environments subject to accelerations several thousands times that of gravity.  
         BACKGROUND OF THE INVENTION  
         [0003]    Guidance electronics for gun-launched vehicles must endure some of the most demanding environments of all flight vehicle avionics. The set back forces, balloting and spin rates are typically far beyond those experienced by tactical and in many cases strategic missiles. The structural support for the guidance electronics is unsurprisingly challenging. The practice of the prior art in guided munitions typically employs a parasitically complicated structural support for the electronics packaging. In many instances, the structural supports are inadequate, leaving the circuits assemblies to rely upon potting material encapsulation and/or structural foams in order to protect the circuitry from the gun launched, high-g, environment.  
           [0004]    Unfortunately, the potting material is difficult to apply and control in production. The coefficient of thermal expansion (CTE) mismatch, contributes to failures related to temperature cycling. Potted electronics assemblies and structural foams are problematic where rework or repair is required. In addition, the circuit card form factor of many guided munitions is driven by the orientation and location of the electronics packaging within the aerodynamic shell, often resulting irregular, or at least in nonrectangular, planforms that do not readily lend themselves to high volume, low cost production.  
           [0005]    Electronic packaging considerations for gun-launched guided munitions must include the survivability of each circuit card assembly and its interconnection with the rest of the electronic assembly. For comparison, tactical missile circuit card assemblies and interconnects may be designed to withstand 30 g acceleration, whereas a gun-launched electronics assembly will typically be designed to withstand up to 20,000 g acceleration. A typical practice in the art of gun launch survivability has circuit card assemblies made substantially rigid with metal or, as discussed above, fully encapsulated in potting materials and/or structural foam.  
           [0006]    Adding to the complicating challenges, some components, such as a crystal oscillator, require a specific orientation with respect to the setback forces generated during a gun launch. Board level interconnects (e.g., wire bonds, gull wing leads, solder balls, and similar electrical connections), die attachment means (e.g., adhesive or solder) and component package styles (e.g., bare die, small outline surface mount packages and the like) must be tailored to accommodate the high setback forces.  
           [0007]    Circuit card assembly costs are another important aspect of this art because a gun hardened electronics design experience considerably higher production volumes as compared to relatively limited volumes of tactical missile electronics produced over the life cycle of the avionics in question.  
         OBJECT AND ADVANTAGES  
         [0008]    A principal object of this invention is to provide circuit card technology that is capable of surviving a gun-launch environment (including setback, spin rate and balloting) such as that from a 155 mm canon by substantially isolating the electronic subsystems from nose and canard loading. That is, an object is to environmentally protect the electronic assembly by completely supporting single-sided circuit cards where the circuit cards are not structural elements and concurrently provide a relatively large area for heat dissipation. An additional object is to provide guided munitions electronic packaging and packages that are easier and less expensive to produce compared to the prior art. An additional object is to minimize the number of electrical interconnections (e.g., solder joints and mechanical connectors) and thereby reduce the number of failure points and streamline the manufacturing process. An additional object of the present invention is to minimize the use of potting materials, if any, and thereby obviate the need for structural foam. An additional object is to exploit the use of commercial off-the-shelf devices wherever practicable. An additional object of the present invention is to maximize the testability to efficiently support a streamlined manufacturing process. An additional object of the present invention is to isolate the electronic subsystems from aerodynamic and thermal battery heat loads.  
         SUMMARY  
         [0009]    The card rack and stabilizer structure of the several embodiments of the present invention comprise high strength titanium beta alloy, hot isostatic processing (HIP) eliminating voids and assuring uniform properties, structural support elements integrated into a monocoque structure, whereby the radial rib design provides structural rigidity and maximizes the circuit card assembly thermal heat transfer area. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, and in which:  
         [0011]    [0011]FIG. 1 is a graph of a sample acceleration and velocity profile of the gun-launched environment;  
         [0012]    [0012]FIG. 2 is an example layout of a circuit card assembly of the present invention;  
         [0013]    [0013]FIG. 3A is a perspective view of a circuit card assembly of the present invention;  
         [0014]    [0014]FIG. 3B is a perspective view of a circuit card assembly of the present invention;  
         [0015]    [0015]FIG. 4 is a perspective view of a card rack embodiment of the present invention;  
         [0016]    [0016]FIG. 5A is an example panel radiating pattern for a card rack embodiment of the present invention;  
         [0017]    [0017]FIG. 5B is an example panel radiating pattern for a card rack embodiment of the present invention;  
         [0018]    [0018]FIG. 6 is a perspective view of a flex circuit embodiment of the present invention;  
         [0019]    [0019]FIG. 7 is an exploded view of a card rack assembly embodiment of the present invention;  
         [0020]    [0020]FIG. 8 is a perspective view of a card rack assembly embodiment of the present invention;  
         [0021]    [0021]FIG. 9 is an exploded view of a card rack assembly and outer casing embodiment of the present invention;  
         [0022]    [0022]FIG. 10 is a perspective view of an outer casing embodiment of the present invention; and  
         [0023]    [0023]FIG. 11 is a cross-sectional view of an outer casing embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The present invention addresses the electronic assembly interconnections as part of the overall gun-hardened electronic packaging. The FIG. 1 depicts a high-g event for a modern cannon shell where the resulting environmental effects provide a satisfactory example of a typical gun-launched environment for guidance electronics. In this example, the projectile is despun from the typical 300 revolutions per second of a 155 mm cannon to around 30 revolutions per second. Electronic assembly interconnections between the guidance, navigation and control (GNC) processor circuits and other circuit card assemblies cannot be made using the connectors that are typical of tactical missiles.  
         [0025]    [0025]FIG. 2 illustrates a circuit card assembly  200  of an embodiment of the present invention including three sections integrated via rigid flexible ribbon. The first section  240  and second section  242  are in electrical communication via a first flexible ribbon  246 . The second section  242  and third section  244  are in electrical communication via a second flexible ribbon  248 . The example GNC processor located on this example circuit card assembly is comprised of the following devices:(1) a microprocessor chip  202 ; (2) a plurality of static random access memory (SRAM) chips  204 ; (3) a plurality of flash programmable read only memory (FPROM) chips  206 ;(4) a programmable logic chip  208 ; (5) a plurality of universal asynchronous receiver/transmitter (UART) chips  210 ; (6) a crystal oscillator  212  with a clock buffer  214 ; and (7) an input/output (I/O) interfacing element  216 . A first plurality of connector pins  260  is provided along the upper edge of the first section  240  and a second plurality of connector pins  262  is provided along the upper edge of the second section  242 .  
         [0026]    The example g-hardened GNC processor circuit incorporates all components that are needed to support GNC processing calculations. The example electronic assembly has a power supply circuit card assembly to condition and otherwise regulate the voltage forms (e.g., 5 and 12 volts) generated by the flight thermal battery after gun launch. The power supply for this example provides the necessary power forms (e.g., 3 and 5 volts) to the GNC processor circuit card assembly. The GNC processor circuit for this example is specified to use a maximum of 8 Watts of power and fit within 5 cm×8 cm×1 cm envelope. The GNC processor in this example is not required to operate during the high-g event, but must remain functional.  
         [0027]    Other than requisite conformal coatings, electronics survival in the high-g (acceleration) environment is accomplished without potting and/or structural foam augmentations. In addition, by practicing the teachings of the several embodiments of the present invention, no secondary processes are required for the sealing of the undersides of parts generally subject to potting.  
         [0028]    Modularity of the present invention lends itself to efficient subassembly testing, test survivability, and accordingly provides for high manufacturing yields. Modularity without potting also provides for the ready connectivity to external sources via a plug or port. Part of this high yield is accomplished by the elimination of a potting process that is difficult to control. Elimination of the potting material also contributes to a comparative reduction of failures related to the coefficient of thermal expansion (CTE).  
         [0029]    Single-sided passive elements may be surface-mounted components on the multi-layer polyimide rigid-flex circuit  200 . Alternative embodiments have epoxy laminates for the circuit cards  200 . Other embodiments have ceramic circuit cards  200 . The several embodiments of the present invention have the circuit completely bonded to the card rack  400  and accordingly the circuit cards do not carry substantial g-loads. Moreover, the thermally conductive adhesive may be employed to enhance the thermal path. Finally, the several embodiments of the present invention do not require potting separately or in combination with structural foam to support cards assemblies under gun-launched setback forces and other forces experienced within a gun barrel.  
         [0030]    The circuit card  200  orientation of the several embodiments of the present invention have the setback force from a gun launch acting in parallel to the circuit card plane. This orientation lends itself to rectangular circuit card geometry that results in an optimal utilization of the circuit card materials and more efficient circuit assembly. An alternative circuit card orientation results in toroidal, donut-shaped, or disc-shaped geometry that is known to be relatively more expensive to produce as well as disc-shaped cards.  
         [0031]    [0031]FIG. 3A illustrates in a perspective view the orientation of the card assembly  200 . The first section  240  and the second section  242  are oriented in planes parallel with the gun setback force vector  310 . The third section  244 , when installed, is oriented in a plane perpendicular to the setback force vector  310  and is therefore ideally suited for supporting an crystal oscillator  212 . FIG. 3B illustrates in a perspective view the acute angle formed between the first section  240  and the second section  242 .  
         [0032]    [0032]FIG. 4 illustrates in perspective view of an example cast card assembly rack  400 . The card assembly rack  400  is preferably a cast member and functionally has a base  430  perpendicular to the setback force vector  310 , fins  410  parallel to the setback force vector  310  and a substantially cylindrical portion  440  projecting from the base  430  and from which the fins  410  radiate. FIG. 5A illustrates an example fin radiation pattern for the card assembly rack as viewed from the top with each fin  410  perpendicular to the local tangent of the cylindrical portion  440 . FIG. 5B illustrates an alternative fin radiation pattern for the card assembly rack as viewed from the top with the fins  410  radiating parallel to the local tangent of the cylindrical portion  440 . In alternative embodiments, the card rack  400 , as a cast piece, has one or more removable planar spokes with or without dovetailing features in order to support special circuit cards. The orientation of panel spokes or fins  410 , their number and the diameter of the hub aperture  440  are all adjustable for particular applications. While the card rack  400  is preferably cast, alternative embodiments have it fabricated by machining.  
         [0033]    [0033]FIG. 6 illustrates in perspective view a flex circuit interconnect  600  connected to the card rack via a connector strip  620 . The flex circuit  600  provides electrical connectivity across the several circuit cards  200  (FIG. 2) via receptacles  610 . In alternative embodiments, the electronic assembly interconnect  600  may use a rigid or flex printed circuit that is hard-wired or soldered to the GNC processor circuit card assembly. An alternative embodiment of the electronic assembly uses connectors. If electronic assembly interconnections are hard-wired or soldered to each circuit card assembly then rework is made more difficult, but not impossible.  
         [0034]    [0034]FIG. 7 illustrates in exploded view a portion of an example payload assembly  700  including the flex circuit  600 , a stabilizing member  710 , an inertial measurement unit (IMU)  720 , a plurality of card assemblies  200 , a card assembly rack  400 , and a safe-arming unit (SAU)  730 . The stabilizing member  710  is patterned substantially after the flex circuit  600  in planform and provides a stabilizing mechanical interface for receiving circuit card connector pins between the circuit cards  200  and the flex circuit  600 . FIG. 8 illustrates in a perspective view the example payload assembly  700  showing the flex circuit  600 , IMU  720 , and plurality of card assemblies  200  assembled onto the card assembly rack  400 . FIG. 8 illustrates the circuit card assemblies  200  bonded to the card rack  400 . The stabilizer  710  is bonded to the card rack  400  for added support. The preferred embodiments uses all solder joint terminations. That is, no connectors are used.  
         [0035]    An embodiment of the present invention illustrated in FIG. 9 has a two-piece metallic structure comprised of a cast outer shell  900  and the card rack  400  whereby the structure supports all the electronic subsystems as a portion of the payload  700 . The example cast outer shell  900  is shown with a plurality of antenna apertures  925 . An example of a cast outer shell  900  is shown in perspective view in FIG. 10. The cast outer shell  900  is reinforced with several cast ribs  935 . The cast outer shell  900  shown in cross-sectional view in FIG. 11 is also reinforced with bulkhead features  930 ,  931 . For embodiments using alignment bushings  960 , they are positioned in the outer casing  900  portion for receiving the fasteners  910 . The reinforcing features support the prepackaged electronic subsystems, e.g., battery  920  attached with fasteners  930  (FIG. 9), IMU  720 , SAU  730  (FIG. 7), and antennas (not shown). The card rack  400  is a cast structure that preferably holds all the circuit card assemblies, e.g., GNC, Global Positioning Satellite (GPS) navigational receiver (not shown), and a power supply (not shown) and in alternative embodiments contains a prepackaged electronic subsystem as well, e.g., battery, IMU  720 , and SAU  730 . The example illustrated in FIG. 9, the card rack  400  and outer shell are mechanically fastened together with four fasteners  910 , preferably steel bolts, in tension that hold the two structures tightly together. In alternative embodiments, the card rack  400  and outer shell  900  are mechanically fused together with alignment bushings  960  and fasteners  910 , preferably steel bolts, in tension that hold the two structures tightly together. The alignment bushing  960  engage the fasteners  910  and in doing so align the card rack  400  to the outer shell  900 . The bushings are preferably cast or machined steel. The resulting structure is sufficiently stiff and stable providing ample support for the circuit card assemblies  200  and the other prepackaged electronic subsystems. The circuit assemblies  200  are bonded or alternatively fastened, or bonded and fastened, to the card rack structure  400 . Embodiments of the present invention have the card rack-mounted circuitry  200  employing edge-mounted pins  262  for power/signal interconnect that protrude through copper plated vias in the flex or rigid-flex circuit interconnect  600 . This entire card rack assembly  700  may be passed through a wave solder operation to make all card-to-card interconnections without using mating connectors. Connectors may be used if so desired in alternative embodiments, but the use of connectors adds assembly costs and works to diminish system reliability. While soldering is the preferred means for electrically connecting electrical components, alternative embodiments have connectors built into the multiple-part or single-part flex circuit. While multi-layered flex  600  is the preferred medium of connectivity, alternative embodiments use multi-piece flex or rigid flex as the medium of connectivity. In typical implementations of the several embodiments of the present invention, the bulkhead is welded onto the aft bulkhead or otherwise sealed. A stabilizer  710  may also be employed to provide additional support for the card rack and rigid-flex or flex circuit interconnect. In alternative embodiments, the stabilizer  710  is increased in size and circumferential capacity over the preferred embodiment and is used to encase the entire unit, particularly for environments of less acceleration than those illustrated in FIG. 1.  
         [0036]    The first section  240  and second section  242  planform for several embodiments of the present invention is rectangular, and for the example embodiment is approximately 2.5 inches by 2 inches in planform. In alternative embodiments, individual circuit cards assemblies  200  for the invention may be interconnected using a rigid flex circuit to form a larger folded circuit. The several embodiments of present invention also lend themselves to in-plane or normal loading of the circuit card assembly depending on which face of the card rack that the circuit card is attached. The simplicity of the two piece structure that make up the structural design of the invention also lends itself to low cost high volume fabrication. The two parts, the card rack and are castable to the required shapes with minimal machining required. The way in which the card rack  400  and aft bulkhead  420  are combined into one part tends to reduce part count and weight. The outer shell  900  also incorporates inner bulkheads  935  to support the packaged electronic subassemblies and card rack, and this integrated structural design approach tends to reduce part count and weight. The four bolts  910  and features  930 ,  931  holding and aligning the card rack assembly  700  and, if present, the SAU  730 , to the outer shell  900  couples the two structures tightly together and also makes for minimal structural mass. The use of a wave solder or equivalent process to interconnect all the card rack mounted circuit card assemblies improves over typical processes in that interconnecting the circuit card assemblies with a wave solder process or equivalent process has significant cost, weight and reliability advantages over the use of connectors.  
         [0037]    Titanium is the preferred material from which the two-piece housing of the several embodiments of the present invention is made, with stainless steel being an alternative material. In addition, the present invention naturally lends itself to electromagnetic interference (EMI) shielding. The card rack  400  and stabilizer structure  710  of the several embodiments of the present invention are made of high strength titanium beta alloy with hot isostatic processing (HIP) eliminating voids and assuring uniform properties. Structural support elements  935  are integrated into the monocoque structure of the casing  900 , whereby the radial rib design of the card rack  400  provides structural rigidity and maximizes the circuit card assembly  200  thermal heat transfer area. The method of wave solder or equivalent provides a one-shot production step with a safe and arming unit  730 , if needed, assembled after curing. In addition, passive components may be bonded to the forward or outward face of the flexure  600  prior to wave solder.  
         [0038]    Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention and its several embodiments disclosed herein. Therefore, it must be understood that the illustrated embodiments have been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, the services disclosed may be performed by processing hosted on one or several network devices such as servers.  
         [0039]    The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0040]    The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result.  
         [0041]    In addition to the equivalents of the claimed elements, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0042]    The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.