A printed wire board assembly

Abstract: A printed wiring board having electronic circuit devices (13, 44) such as semi-conductors including a heat sink forming a housing (32, 36) for the printed wiring board, one of the electronic devices (13) being mounted on one portion (12) of a heat spreader that is formed of a material having high thermal conductivity, another portion (10) of the heat spreader forming a thermal energy flow path to the heat sink, thereby lowering the junction temperature at the electronic device (13) and increasing the reliability of the wiring board.

Description: This invention relates to electronic assemblies utilising printed wiring boards. It is common practice in the manufacture of electronic assemblies to provide a printed circuit board as a base or carrier for circuit elements such as microprocessor chips, power dye transistors, semi-conductors and other circuit components. The printed circuit board may comprise a sheet of glass-filled epoxy, paper, phenolic resin, Kalvar etc. The sheet is stamped in a stamping dye or drilled and cut to form openings for mounting circuit components. A screen is then applied to the sheet. Openings are formed in the screen at strategic locations for forming pads that define electrical connections with components. The screen acts as a stencil as tin lead alloy in the form of a cold paste is applied to the screen. The alloy is deposited at the strategic locations on the circuit board. The printed circuit is comprised in part of copper lines etched on the circuit board by an etching process, thus providing electrical connections between the pads, terminal pins and circuit components. The circuit components can be mounted on either side of the board. It is usual practice to mount circuit components, such as electronic silicon chips, on the circuit board by first forming a plastic package for each circuit component and assembling the packaged component on the board with various fastening techniques. Examples of this assembly procedure may be found in prior art patents 5,095,404 and 4,978,638. In the device described in the '404 patent, a packaged chip is mounted on a base having a pedestal that extends through an opening formed in a printed circuit board. Mounting pads are formed on one side of the board, and these are connected to the chip by leads in the usual fashion. The mounting plate for the chip is secured to a mounting pad by a thermally conductive epoxy. In the design of printed circuit boards of this kind, it is necessary to accommodate dissipation of thermal energy from the components, particularly power dyes. For example, if the capacity of a power dye is about 10 watts, it is not unusual for the junction at the power dye to operate at a temperature of approximately 150 °C. If, however, the power dye operates at high temperatures continuously over a period of time, the reliability of the dye would be adversely affected. It has been found that for every 10°C incremental increase in junction temperature, the reliability of the electronic devices decreases by 50%. Attempts have been made in prior art constructions to control temperature increase by dissipating thermal energy. In the case of the design of the '404 patent, the mounting plate, which carries a high density integrated circuit chip, is formed with a heat pipe under the printed circuit board. It is attached to a heat spreader which allows the heat produced by the chip to be removed by the heat pipe and distributed to cooling fins. The heat spreader is mounted on the bottom side of the chip. Thus, only a single heat flow path from the chip is established. In the design shown in the '636 patent, a packaged circuit element is moulded into a plastic cover together with a heat sink that forms a part of the package. A thermally conductive interface material is situated between the heat sink portion of the package and one surface of the plastic package. Prior art patents 5,168,926 and 5,175,613 are further examples of prior art teachings for forming an assembly of a heat sink with a packaged chip carrier. In each case, a thermally conductive tape or bonding material is situated on one side of the packaged part to form a heat flow path from the part to a heat sink, the latter being secured to the part by bonding material. Heat transfer thus occurs from the part to the sink through a single heat flow path from only one side of the part. The thermal conductivity of the printed wiring board carrier material is very low. Thus, if a power semiconductor is mounted directly on the board, it is difficult to keep the junction temperature low. It is necessary, therefore, to operate the device at a low power level. If a power semi-conductor is mounted directly on the heat sink, as in the case of prior art designs such as those discussed above, only a part of the area of the dye is in contact with the heat sink. Furthermore, special assembly equipment and processes are required in the manufacture of power due transistors of this kind. The complexity of the assembly procedure adds substantially to the time and materials required during manufacture. The improved construction of the present invention makes it possible to use simplified assembly techniques and equipment in the manufacture of the printed wiring board assembly while achieving maximum heat transfer performance. According to the present invention, there is provided a printed wiring board assembly comprising an electrically non-conductive component carrier sheet, multiple electronic components mounted on said carrier sheet and electrical wiring printed on said carrier sheet and defining with said components an electronic circuit; at least one heat spreader formed of thermally conductive material connected to said carrier sheet on the margin thereof, at least one of said components being attached to a first part of said heat spreader; a housing enclosing said carrier sheet and said components, including a base housing portion and a cover housing portion, at least one of said housing portions being formed of thermally conductive material a second part of said heat spreader extending beyond the periphery of said carrier sheet and being disposed between said housing portions whereby said housing acts as a heat sink as a thermal energy flow path is established between said one component and said housing through said heat spreader. The sheet is clamped between the base and cover housing portions. The clamping of the housing portions minimises thermal resistance to the transfer of thermal energy from the heat spreader to the base heat sink and to the top housing portion. This results in a lower junction temperature of the circuit devices. By forming the printed wiring board assembly with power transistor devices as an integral part of the printed wiring board assembly, the equipment and assembly procedure is simplified. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1A is a plan view of one side of a heat spreader, which forms a part of the printed wiring board assembly of the present invention; Figure 1B is an end view of the heat spreader shown in Figure 1A; Figure 1C is a plan view of the opposite side of the heat spreader; Figure 1D is a top view of the heat spreader shown in Figure 1A; Figure 1E is a sectional view as seen from the plane of section line 1E-1E of Figure 1A; Figure 2 is a perspective view of a glass filled epoxy sheet that forms the carrier for the printed wiring board assembly; Figure 2A is an enlarged view of a cut out section of the board material shown in Figure 2 for accommodating a heat spreader; Figure 3 is a perspective view showing the board of Figure 2 with the heat spreader of Figures 1A through 1E assembled in place together with bond pads, although active and passive circuit components are not illustrated; Figure 4 is an enlarged cross-sectional view taken along the plane of section line 4-4 of Figure 3; Figure 5 is an exploded assembly view of the printed wiring board assembly of the invention including the cover and base heat sink portions and the printed wiring carrier board; Figure 6 is a bottom view of the heat sink assembly; and Figure 7 is an enlarged cross-sectional view as seen from the plane of section line 8-7 of Figure 6. In the manufacture of the improved printed wiring board assembly, a bare dye 13 is mounted on heat spreaders formed of a material with high thermal conductivity, such as copper or a copper alloy. A heat spreader is shown in Figures 1A through 1E. The dye 13 may be attached to the heat spreader by soldering or by a thermally conductive epoxy or other attachment technique. The dye can be placed precisely on the heat spreader using simple, automated, pick-and-place equipment. Each heat spreader comprises a portion 12 that is adapted to be secured by soldering to the printed wiring board carrier. In the embodiment shown in Figures 1A through 1E, a portion 10 is included to reduce resistance to transfer of thermal energy to the base heat sink and the top cover, as described subsequently. The portions 10 and 12 are in the form of a "T" when viewed from either the top or the bottom. A bare dye 13 in the preferred embodiment is attached, as described above, to each portion 12. The margin of the portion 12 on three sides is formed with a shoulder 14, as seen in Figure 1E. Figure 2 shows a printed circuit board carrier, which is identified by reference numeral 16. It is generally rectangular in shape. Cut out portions or notches are formed in any margin, such as the right margin 18 and the left margin 20. In the particular embodiment shown, each margin 18 and 20 has two cut out portions 22 and two cut out portions 24, respectively. As seen in Figure 3, portion 12 of each heat spreader is received in one of the notches or cut outs of the carrier 16. The shoulder 14 on the sides of the portion 12 overlie the upper surface of the carrier 16. Active and passive components 44 and bond pads 28 are placed on the printed wiring board carrier after -solder paste is applied. Wire bonds 30 connect the dyes13 to the bond pads 28 as shown in Figures 3 and 5. The printed circuit board carrier 16 is laminated to heat sink base 32 by applying a thermally conductive, electrically insulating, laminating material to the base or to the undersurface of the carrier 16, or both. The base is provided with a plurality of recesses 34 for accommodating electronic printed circuit components at the bottom of the printed circuit board carrier. The recesses are best seen in Figure 5. The wire bonds 30 can be applied to the assembly either before or after the bonding of the carrier 16 to the base 32. A cover assembly, shown in Figure 5 at 36, is provided with cooling fins 38 extending between opposite margins of the rectangular assembly. Although the cooling fins of the disclosed embodiment extend between the left and right margins, they could extend in any other direction, including a diagonal direction. Thermally conductive tape 40 is secured to the underside of the cover assembly at opposite margins. When the cover assembly is secured to the heat sink base, the tape 40 engages the portions 10 of the heat spreaders. The heat spreaders thus are electrically insulated from the cover, although heat may be transmitted from the dyes through the thermally conductive tape to the cover assembly. Thermally conductive material, which is used to attach the underside of the printed wiring board carrier to the base assembly 32, is used also to bond the underside of the heat spreader to the base, thereby establishing a heat flow path from the dyes through the heat spreader and through the thermally conductive material to the heat sink base 32. When the cover assembly is placed over the heat sink base, the tape 40 will be in contact with the heat spreader portion 10, thereby ensuring positive clamping of the heat spreader to the base heat sink portion. The top finned cover acts as a clamp when pressure-sensitive thermally conductive materials are used to laminate the printed wiring board carrier to the base 32. The top finned cover may be secured to the base with screws or any other fastening device. Thermal resistance at the interface of the heat spreaders and the cover and the interface of the heat spreader and the base is minimised. Also, manufacturing tolerances of the individual components can be accommodated because of the flexibility of the thermally conductive tape. The printed wiring board carrier, prior to the lamination of the carrier to the base 32, is covered with insulting material such as a lacquer, thus providing electrical insulation for the copper wire circuitry 42. The wiring shown at 42 in Figure 5 connects the active and passive circuit components 44 to terminal pins 46. As seen in Figure 6, the bottom of the heat sink base 32 is provided with fins 48 extending from one margin of the assembly to the other. The cross-sectional view of Figure 7 shows the top and the bottom assembled together with the printed wiring board carrier 16 sandwiched between them. As seen in Figure 7, the heat spreader portion 10 is in contact with the thermally conductive tape 40 and with the thermally conductive material 50 so that heat is transferred through both sides providing two parallel flow paths to the adjacent surfaces of the cover assembly 36 and the base 32. The thermally conductive tape may be an epoxy with a filling of alumina (aluminium oxide). It is manufactured commercially and is available from 3M Industrial Specialties. The Bergquist Company, A.I. Technology Inc., Dow Corning, and Chomerics. The thermally conductive adhesive material 50 also is available commercially from these suppliers. The top cover and base heat sink assembly may be formed of aluminium or other material. If aluminium is used, an anodised finish can be used to improve heat transfer. The anodising process creates a thin layer of aluminium oxide on the surfaces, which improves the heat radiation characteristics of the material. During assembly, the portions 12 of the heat spreaders can be fitted into the recesses or cut out portions of the circuit board carrier, and they are held in place with a sung fit as the soldering or brazing occurs. Any number of active or passive circuit components can be mounted on the printed wiring board carrier. They can be populated on either side of the board and, as mentioned previously, the recesses 34 will accommodate these devices. The ridges between the spaces or recesses 34 provides support for the printed wiring board carrier. Our improved design makes it possible to mount a bare dye on the circuit board without the necessity of using a packaged part, which inherently is more costly. Further, since it is feasible with our design to use bare dies, it is possible to have a higher power rating while reducing the space for the circuit elements. As mentioned, alumina may be used in the epoxy, but other materials may be used as well, such as boron nitride and aluminium nitride. Solder paste is applied to the board using a stencil which has openings at the predetermined strategic locations where it is desired to make the electrical connections with the components. The conductive pattern lines 42 are obtained by etching a copper-clad substrate, which is referred to here as the carrier 16. The anodising process for the cover assembly and the base provides a dark colour, as indicated above, which increases the radiation properties for heat dissipation purposes. The coating applied to the aluminium alloy during the anodising process creates a thin skin which may reduce somewhat thermal conductivity at the interface of the cover assembly and the thermally conductive tape 40 and at the interface of the base and the thermally conductive material 50. Although there may be a slight performance trade off in this respect, the overall result is a significant improvement in the heat dissipation properties of the completed assembly compared to prior art devices. The aluminium oxide, which forms a skin during the anodising step, is only a few microns thick.

Claims: A printed wiring board assembly comprising an electrically non-conductive component carrier sheet (16), multiple electronic components (13, 44) mounted on said carrier sheet (16) and electrical wiring (42) printed on said carrier sheet (16) and defining with said components (13, 44) an electronic circuit; at least one heat spreader (10, 12) formed of thermally conductive material connected to said carrier sheet (16) on the margin (18, 20) thereof, at least one of said components (13, 44) being attached to a first part (12) of said heat spreader; a housing enclosing said carrier sheet (16) and said components (13, 44), including a base housing portion (32) and a cover housing portion (36), at least one of said housing portions (32, 36) being formed of thermally conductive material; a second part (10) of said heat spreader extending beyond the periphery of said carrier sheet (16) and being disposed between said housing portions (32, 36) whereby said housing acts as a heat sink as a thermal energy flow path is established between said one component and said housing through said heat spreader. An assembly as claimed in claim 1, wherein said carrier sheet comprises a plurality of conductive junction pads and wiring conductor elements; said first heat spreader part being secured by solder metal to said conductor element at said carrier sheet margin. An assembly as claimed in claim 2, wherein said carrier sheet margin has at least one opening formed therein, said first carrier sheet portion being received and secured in said opening, said solder metal surrounding said opening. An assembly claimed in any one of the preceding claims, wherein said one component is a semi-conductor forming a part of said electronic circuit. An assembly as claimed in any one of the preceding claims, wherein thermally conductive material is disposed between said second heat spreader part and at least one of said housing portions whereby a clamping force is applied by said housing portions through said thermally conductive material as said housing portions are joined together to form a unitary assembly. An assembly as claimed in claim 5, wherein said thermally conductive material is a yieldable tape adapted to compensate for assembly tolerances as said carrier sheet with said circuit components is secured within said housing portions. An assembly as claimed in any one of the preceding claims, wherein said housing has a coating of dark colour thereby increasing thermal radiation properties of said assembly. An assembly as claimed in any one of the preceding claims, wherein at least one of said housing portions is formed with cooling fins on the exterior thereof. A process for manufacturing a printed wiring board assembly comprising the steps of forming on an electrically non-conductive carrier sheet metallic junction pads and wiring conductor elements; forming on said carrier sheet metal in a defined pattern thereby forming a portion of an electronic circuit; mounting on said carrier sheet a predetermined population of circuit components; coating said carrier sheet and said wiring pattern with an electrical insulating material; mounting at least one of said circuit components on a metallic heat spreader; mounting a first part of said heat spreader on said carrier sheet with a second part of said heat spreader extending outward from the margin of said carrier sheet; and assembling a metallic housing over said carrier sheet and heat spreader with said heat spreader secured between separate portions of said housing whereby heat generated by said one component is transferred through said heat spreader to said housing, the latter forming a heat sink. A process as claimed in claim 9, including the steps of bonding one side of said heat spreader to one of said housing portions with a thermally conductive material and placing a thermally conductive material between another portion of said housing and the opposite side of said heat spreader. A process as claimed in claim 9 or 10, wherein said step of forming wiring metal on said carrier comprises etching metal cladding on said carrier sheet, said carrier sheet being pre-clad. A process as claimed in claim 9 or 10, wherein said step of forming metal in a pattern on said carrier sheet comprises etching copper cladding on said carrier sheet, said carrier sheet being pre-clad with copper.

Inventors: BAKER JAY DEAVIS, REDDY PRATHAP AMERWAI, SALISBURY KENNETH ALAN, TRUBLOWSKI JOHN, BAKER, JAY DEAVIS, REDDY, PRATHAP AMERWAI, SALISBURY, KENNETH ALAN, TRUBLOWSKI, JOHN
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1995
Main classification: H05K7
IPC codes: H05K1, H05K7