WO1998036628A1

WO1998036628A1 - Apparatus and method of protecting electronics

Info

Publication number: WO1998036628A1
Application number: PCT/US1998/002385
Authority: WO
Inventors: Jeremy L. Walter; James T. Moore; Timothy A. Reese
Original assignee: The Penn State Research Foundation
Priority date: 1997-02-05
Filing date: 1998-02-05
Publication date: 1998-08-20
Also published as: AU6534298A

Abstract

Disclosed is an apparatus and method of protecting mass-produced electronics in harsh environments for which they were not originally designed to function. The invention provides for the electronics (16) to be encapsulated by a thermally conductive silicone compound (20), thereby forming an encapsulated electronics assembly. The encapsulated electronics assembly is placed into and retained by a protective receiver (18) that is also thermally conductive. The combination of the receiver (18) and the encapsulated electronics assembly forms a protective module (12). A number of protective modules (12) can be fastened together to form a rigid electronics package (10). The combination of the silicone compound (20) and rigid (10) package formed by the modules (12) provides protection from harsh mechanical and thermal environments.

Description

APPARATUS AND METHOD OF PROTECTING ELECTRONICS

Government Sponsorship This invention was made with governmental support under Grant No.

N00039-92-C-0100 awarded by the Department of the Navy. The Government has certain rights in the invention.

Background The electronics market has evolved to a state where innovation and mass production have made possible computational power in relatively small packages. To feed the growing expectations in this market, the commercial electronics industry has continually reduced costs, reduced lead time to market, and improved reliability. As a result, new generations of affordable electronics are being produced on a regular basis. Unfortunately, some sectors of the electronics market place environmental requirements on a set of electronics which only partially coincide with the demands of the general, commercial market. These sectors of the market, particularly the military, automobile and aerospace markets, have been forced to employ specialized electronics hardware to meet their environmental requirements. This leads to extra expense for the purchaser of the electronics units not only in initial cost, but also in cost of customized software oriented to the custom hardware. Upgrades of electronics packages are also more costly and cannot be doϊie as often since new generations of the specialized hardware cannot be produced nearly as quickly or efficiently as those produced for the general, commercial market. Custom-built electronics have been, and continue to be designed to withstand extreme environments. These custom electronics and methods for packaging them have most of the advantages and disadvantages of anything which is "custom-made." It is easier to tailor a custom design to an unusual set of requirements, but the cost of the product is higher. In some cases the design parameters are unusual enough to justify the additional cost. However, in many cases it would be desirable from a cost perspective to use mass-produced electronics.

Some efforts toward the use of mass-produced commercial electronics in harsh environments have been made. However, these efforts often fall short of the goal when trying to use mass-produced circuit boards, better known as "Commercial Off The Shelf (COTS) circuit boards. This is because the mechanical shock, vibration, and thermal requirements necessary for survival in harsh environments are often much higher than for the COTS circuit boards. Currently the use of commercial electronics in harsh environments is in the form of individual COTS components on military-type circuit boards, or the use of conductive foils and stiffening frames on circuit boards containing more conventional substrates. These existing approaches for the use of COTS electronics do not allow the full economic benefits of a mass-produced circuit board to be realized since major changes of the COTS circuit board must be made so that the board is able to withstand the harsh environments. The approach of taking commercial components and placing them on military-type circuit boards requires considerable redesign of the circuit board, as well as a separate production run. The approach of using stiffening frames on circuit boards requires extra design time and separate production runs of a given circuit board. This is because space must be preserved on the surface of the circuit board for the stiffeners and may require the components be placed differently than on the COTS boards. Also, conductive foil may be required in the circuit board substrate for the removal of heat which can cause the components to be rearranged.

It is known that silicone gels and resins have been effective as encapsulants for integrated circuits (ICs) which are individual semi-conductor chips. In these cases, the silicone gel or resin is used to protect the IC within a plastic or ceramic case. The case must then be attached in some way to an electronic circuit or circuit board. Also, silicone compounds have been used as a cover coat over an entire flex circuit board to act as a moisture seal for the circuit board. These normally do not provide enough protection against thermal and vibrational concerns as the silicone compound is disposed as a relatively thin layer over the circuit board.

The object of the present invention is to protect mass-produced, commercial electronics in order to allow their use in harsh mechanical and thermal environments without changing the design of the circuit board.

Summary of the Invention

The present invention is an apparatus and method of protecting mass- produced electronics in harsh environments for which they were not originally designed to function. The invention provides for the electronics to be encapsulated by a thermally conductive silicone compound, thereby forming an encapsulated electronics assembly. The encapsulated electronics assembly is placed into and retained by a protective receiver that is also thermally conductive. The combination of the receiver and the encapsulated electronics assembly forms a protective module for the electronics. A number of protective modules can be stacked together in good thermal contact with each other, thereby forming a thermal conductive path so that heat may be removed from the encapsulated electronics. The modules are fastened together to form a rigid electronics package and connected to a backplane electronic circuit so that the electronics of each module can communicate with each other. The combination of the silicone compound and rigid package formed by the modules provides protection from harsh mechanical environments. Therefore, the present invention provides for the removal of heat from the electronics and protection of the electronics from mechanical shock and vibration.

Brief Description of the Drawings

Fig. 1 is a perspective view of an electronics package according to the present invention.

- Fig. 2 is a perspective view of an encapsulated electronics assembly partially inserted into a protective receiver. Fig. 3 is a perspective view of the encapsulated electronics assembly separated from the protective receiver.

Fig. 4 is a perspective view of the encapsulated electronics assembly that includes alternative heat removal methods. Fig. 5 is a perspective view of an electronics package according to the present invention which includes elastomer mounts and a heat removal system.

Detailed Description

The present invention provides an apparatus and method for protecting circuit boards and circuit board assemblies. This invention allows electronics on a circuit board to survive in harsh mechanical and thermal environments for which they were not originally designed to function. The invention features a number of advantages over alternatives such as custom-built electronics which are built specifically to withstand the abuse of harsh environments. The invention allows the use of commercial off-the-shelf (COTS) circuit boards in their entirety without any structural modifications. This use of COTS circuit boards in environments more harsh than they were designed to function allows for the cost savings realized from using such mass-produced boards. In addition, the COTS circuit boards tend to have a higher reliability rate than small-production-run custom circuit boards, including those custom boards which employ COTS electronic components. Also, software packages can be used which are available for the COTS circuit boards in the commercial marketplace, thereby saving the expense of producing software for a custom circuit board. Each application of the invention will share the common features discussed below, even though each use will require optimized dimensions and parameters for a specific application.

Figure 1 illustrates an electronics package 10 using the principles of the present invention. The electronics package 10 is made up of circuit board protective modules 12 connected to a backplane 14. Each protective module 12 is made up of a circuit board 16 and a circuit board receiver 18 as shown in Figs. 1-3. The circuit board 16 is encapsulated in a silicone rubber potting compound 20 which contains particles of a thermally conductive filler material such as Aluminum Oxide, thereby forming an encapsulated electronics assembly. The filler material is mixed into the silicone compound 20 before encapsulating the circuit board 16. This produces a more thermally conductive silicone compound 20 than the original compound 20 while maintaining the properties of being electrically insulative. The thermally conductive silicone compound 20 functions to limit forces and moments on the components 15 of the board 16, to limit circuit board deflection, and to form a moisture barrier, while promoting cooling of the electronic components 15 on the board 16 by conducting away the heat produced by the components 15. The silicone compound 20 can be applied to afford maximum shock protection and heat transfer by allowing it to surround all components 15 on the circuit boards 16, whereby it flows under leads and into gaps between the components 15 and the board 16. If desired, the circuit board 16 can be covered with a removable membrane (not shown) prior to the application of the silicone compound 20 in order to prevent flow of the compound 20 under components and leads, thereby allowing the compound 20 and membrane to be removed for circuit board repair.

Once the board 16 is encapsulated, it is placed inside the circuit board receiver 18 which is designed to mate with the encapsulated circuit board 16. It is also possible to place the board 16 in the receiver 18 before it is encapsulated and then encapsulate the board 16 while it is in the receiver 18. The receiver 18 performs the dual purpose of rigidizing the electronics package 10 while providing a thermal conductive path to the surface 22 of the package 10, whereby part of the receiver 18 forms the surface 22 of the package 10. One example of the receiver 18 is illustrated in Figs. 2-3. The receiver 18 is made up of a plate 24 having two end flanges 25 and one top flange 26 extending from the plate 24 in the same direction. The plate 24 has an inside wall 28 which is on the side where the flanges extend outward and an outside wall 30 which faces opposite the inside wall 28. The end flanges 25, top flange 26 and outside wall 30 of the plate 24 all form the surface 22 of the package 10. The encapsulated board 16 is placed against the inside wall 28 and is contained by the top flange 26 and end flanges 25. The encapsulated board 16 is placed in the receiver 18 so that backplane electrical connectors 32 which extend from the board 16 are positioned where there are no flanges in order to allow the board 16 to be connected to the backplane 14. Both the receiver 18 and silicone compound 20 can be formed to have engaging members and grooves to hold the two in position relative to each other. In the example shown in Fig. 3, the compound 20 has small nipples 34 and ribs 36 extending outwardly which engage matching depressions 38 and 40 respectively in the plate 24 of the receiver 18 in order to keep the encapsulated board 16 in contact with the receiver 18. The receiver 18 can be made of aluminum or other thermally conductive material. One example of a thermal conductive material which may be substituted for aluminum is pyrolytic graphite. Use of pyrolytic graphite requires it to be diffusion-bonded inside a jacket of aluminum in order to maintain proper strength, flexibility, and moisture resistance, but allows for a lighter weight receiver 18.

To assemble the electronics package 10, the modules 12 are stacked in good thermal contact with each other and are fastened together so that the modules 12 become an enclosure or in effect a "chassis" for the electronics as shown in Fig. 1.

The modules 12 are fastened by bolting sheet metal plates 42 having through-holes 44 to the receivers 18 which have tapped holes 46 to receive bolts 48. The number of modules 12 which may be stacked are limited only by structural and weight considerations, and the size of the backplane 14. The electronics of each module 12 are protected from mechanical shock and vibration because each electronic component 15 on the board 16 is held firmly in place by the encapsulating silicone compound 20 and because each board 16 is held within the matrix of receivers 18 and the silicone compound 20. The stack of modules 12 is connected to the backplane 14 through the backplane electrical connectors 32. The backplane 14 is an electrical circuit board which connects to all of the modules 12 so their respective circuit boards 16 may communicate with each other. The backplane 14 is held to the stack of modules 12 with brackets 50 by any conventional type of fastener means (not shown) depending on the materials used for the backplane 14 and the module 12. The brackets 50 that hold the backplane 14 in place should be of a non- conductive material or padded with a non-conductive material to prevent electrical shorts on the backplane 14. If desired, front-plane electrical connectors 52 on the circuit boards 16 may be extended through slots 54 in the top flange of the receiver 18 for exterior electrical connections to each board 16 for such things as power, input signals and output signals. Once assembled with the backplane 14, the stack of modules 12 becomes a fully functional electronics package 10. This combination provides an electronics package 10 where all the electronic components 15 on the circuit boards 16 are well protected from mechanical shock, vibration, and moisture. It also provides for the conduction of heat generated at each electronic component 15 to the surface 22 of the electronics package 10.

Thermal management is possible via the thermally conductive filler material in the silicone compound 20 and the placement of the receivers 18 between each circuit board 16. The heat is conducted from an electronic component 15 across the silicone compound 20 to the receiver 18 and thereon to the surface 22 of the so called

"chassis". The heat flow path from the electronic components 15 on the circuit board

16 to the surface 22 of the electronics package 10 is explained by considering Figs. 2-3. Recall that the encapsulated board 16 is surrounded by a heat conductive receiver 18. In the receiver 18, a relatively small temperature difference occurs between the outside wall 30 and the material between it and the inside wall 28 of the receiver plate 24. The silicone compound 20 rests in good thermal contact with the inside wall 28 over its entire surface area so that the heat from any component 15 will conduct through the silicone compound 20 over the relatively short distance to the receiver plate 24. Once the heat reaches the inside wall 28 of the receiver plate

24, it is readily conducted to the outside wall 30 and flanges 25, 26 which form the surface 22 of the package 10. This heat can then be transferred from the surface 22 to an outer enclosure, if available. Additional heat flow paths to aid in the transfer of heat from a high powered component 56 may be added by one of several methods. Two of these methods are illustrated in Figure 4, which shows a "cut-away" view of a silicone-encapsulated circuit board 16. One method shown in Fig. 4 is to lower the heat flux from the component 56 by increasing its effective surface area for heat conduction. The increase in surface area is performed by direct attachment of a heat spreader 58 made from thin sheets of copper, tungsten-copper, or other heat conductive material. The heat conductivity of the heat spreader 56 is much greater than the heat conductivity of the surrounding silicone compound 20, so that the heat from the component 56 may be spread evenly over a larger area before being conducted to the receiver plate 24 via the silicone compound 20. This reduces the temperature drop across the silicone compound 20 and therefore reduces the temperature of the component 56. Another method of heat removal from the high powered component 56 is the use of a small heat pipe 60. The small heat pipe 60 may be attached to the surface of the component 56 in order to move the heat away from the component 56. The heat pipe 60 is known for the transfer of heat down a tube 62 length by evaporating a working fluid at an evaporator plate 64 and condensing the evaporated fluid at a condenser plate 66, where the condensed fluid is passively pumped back to the evaporator plate 64 via a porous wick (not shown) inside the tube 62. In this application, the evaporator plate 64 is placed into contact with the surface of the high powered component 56 and the condenser plate 66 is placed at the surface of the encapsulating silicone compound 20 for direct contact with the inside wall 28 of the receiver plate 24. The heat pipe 60 provides a low-thermal resistance path across the silicone compound 20 and thereby reduces the maximum temperature of the high powered component 56 to which it is attached.

Depending on the requirements of a particular application, the electronics package 10 may be further isolated from mechanical shock by suspending it from an outer. enclosure (not shown) on elastomer mounts 69 which may be engineered to achieve the optimum frequency response and attenuation of shock at the package 10 for a given set of shock inputs, as shown in Fig. 5. In this way, a set of standard, commercial electronics may be given even more protection, if necessary, than that given by the matrix of modules 12. If the electronics package 10 is suspended from an outer enclosure, the heat from the electronics package 10 needs to be transferred across the mount system from the package 10 to the outer enclosure without affecting the function of the elastomer mounts 69. One way to passively cool the electronics package 10 is to use a flexible heat pipe 70 to move the heat from the surface 22 of the electronics package 10 to the outer enclosure. The flexible heat pipe 70 is a larger version of the small heat pipe 60 described above and has a long adiabatic section made from a flexible hose 70 to connect the evaporator plate 64 and the condenser plate 66. The heat is moved by evaporating a working fluid at the evaporator plate 64 fastened to the surface 22 of the electronics package 10 and condensing it at the condenser plate 66 which is to be placed in contact with the inner surface of the outer enclosure. While an embodiment of the invention has been described in detail herein, it will be appreciated by those skilled in the art that various modifications and alternatives to the embodiment could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements are illustrative only and are not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

WE CLAIM:

1. A protection apparatus for protecting electronics comprising: said electronics; an encapsulate which surrounds said electronics, thereby forming an encapsulated electronics assembly; and a receiver which accepts and retains said encapsulated electronics assembly.

2. The protection apparatus of claim 1, wherein said encapsulate is a silicone rubber potting compound.

3. The protection apparatus of claim 1, wherein said encapsulate includes a thermally conductive filler material to aid in the conduction of heat from the electronics to said receiver.

4. The protection apparatus of claim 3, wherein said filler material is Aluminum Oxide.

5. The protection apparatus of claim 1, wherein said receiver is of a thermally conductive material.

6. The protection apparatus of claim 5, wherein said receiver and said encapsulated assembly are thermally interconnected so that heat from the electronics is conducted by said encapsulate and transferred to said receiver.

7. The protection apparatus of claim 1, wherein said encapsulated assembly includes at least one heat pipe assembly connecting at least one component of said electronics to said receiver.

8. The protection apparatus of claim 1, wherein said encapsulated assembly includes at least one heat spreader connected to at least one component of said electronics.

9. The protection apparatus of claim 1, wherein said electronics is a circuit board.

10. A protective electronics package comprising at least one electronics protective module, wherein said electronics protective module includes electronics; an encapsulate which surrounds said electronics, thereby forming an encapsulated electronics assembly; and a receiver which accepts and retains said encapsulated electronics assembly.

11. The protective electronics package of claim 10, wherein said electronics is a circuit board which include electrical connectors for interconnecting each of said ^Γûá protective modules to a backplane electronics circuit assembly.

12. The protective electronics package of claim 10, wherein an outer surface of said package is formed by each of said receivers of the protective modules.

13. The protective electronics package of claim 12, wherein each of said receivers and said encapsulated assembles are thermally interconnected so that heat from the electronics is conducted by said encapsulate to said receiver, thereby allowing the heat to be conducted by said receiver to said outer surface.

14. The protective electronics package of claim 13, wherein each of said protective modules are interconnected to form a rigid electronics package.

15. A method of protecting electronics comprising the steps of: a) encapsulating said electronics, thereby forming an encapsulated electronic assembly; and b) inserting said encapsulated electronic assembly into a protective receiver.

16. The method of claim 15, wherein said encapsulate is a silicone compound which includes a thermally conductive filler material to aid in the conduction of heat from the electronics to said receiver

17. A method of protecting electronics in a package comprising the steps of: a) encapsulating said electronics, thereby forming an encapsulated electronic assembly; b) inserting said encapsulated electronic assembly into at least one protective receiver, thereby forming at least one protective module; and c) connecting said electronics of said protective module to a backplane electronic circuit assembly.

18. The method of claim 17, further including the steps of stacking the protective modules together in thermal contact and fastening the protective modules together to form a rigid electronics package.