Abstract:
An electronic assembly having one or more heat producing semiconductor components in the form of a chip or die has improved heat dissipation properties. The semiconductor components are formed into subassemblies comprising one or more of the semiconductor components bonded on opposite planar surfaces to metal lead frames. Each lead frame is bonded to a substrate which is electrically insulating and thermally conductive. The resulting assembly is submerged into a central channel of a heat sink. The channel is adapted to receive the assembly such that the channel walls compressingly engage the assembly. Heat is thereby dissipated from the semiconductor components from both planar surfaces of the component.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to arrangements for enhancing heat transfer from semiconductor power switching devices. 
     In solid state relays and power bridge circuits relatively high current power is conducted through semiconductor devices. The transfer of heat from such semiconductor devices is essential to their operation. Heat conducting, electrically insulating substrates have been advantageously employed to retain heat generating electrical components along a first substrate side while the opposite side of the substrate is mounted on a heat sink to permit the heat generated by the device to be dissipated at the external heat sink. In these arrangements, approximately 90% of the heat generated by the device is transferred through the substrate to the heat sink. The ability of these one sided heat transfer dissipation arrangements to transfer heat limits the use of these arrangements to lower power devices. 
     For high current, high power devices such as devices handling currents of 200 amps or higher, various arrangements have been provided to dissipate heat from two sides of the semiconductor device. In these arrangements, solder is ineffective to bond the device electrically because of the temperatures involved and a mechanical connection is made to the device. The device is clamped between two plates which act as electrical contacts and also function as heat sinks such that the device dissipates heat from both sides. These very high current arrangements are available commercially and one type is known as a press pack arrangement. 
     In certain circuit modules the currents are high currents and the heat to be dissipated is too high to permit compact construction using one sided heat dissipation but the amount of heat to be generated is not at the levels for which press pack construction is desirable. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to an arrangement for the extraction of heat from semiconductor devices. In accordance with the invention, one or more devices are mounted in a sandwich like construction which is then mounted in a center channel of a heat sink. The sandwich like construction comprises a two lead frames, each bonded to the opposite surface of the semiconductor device. The lead frames are each bonded to a respective one of two electrically insulating but heat conducting insulators or substrates. The resulting subassembly is submerged in and captured in a central channel formed in a heat sink. The heat sink channel is adapted such that its side walls compressingly engage the insulators or substrates. With this construction, heat is extracted from both planar surfaces of the semiconductor device through the lead frames and through the substrates to the heat sink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawing in which like reference designators are used to identify like elements, and in which: 
     FIG. 1 illustrates a portion of a circuit module in accordance with the principles of the invention; 
     FIG. 2 illustrates a lead frame utilized in a module in accordance with the invention; 
     FIG. 3 illustrates in planar view various elements of a circuit module in accordance with the principles of the invention; 
     FIGS. 4, 5 and 6 illustrate a heat sink in accordance with the principles of the invention; 
     FIGS. 7 and 8 illustrate an assembled circuit module in accordance with the principles of the invention; and 
     FIG. 9 illustrates a portion of the assemblage of FIGS. 7 and 8 in enlarged cross section taken along lines 9--9 of FIG. 7. 
    
    
     DETAILED DESCRIPTION 
     A circuit module in accordance with the invention will be described in conjunction with and illustrative embodiment of the invention which is a high current solid state relay module which utilizes two identical circuit subassemblies. Each circuit subassembly includes two semiconductor devices in the form of a chip or die mounted between two lead frames. In the illustrative embodiment, the semiconductor devices are silicon controlled rectifiers (SCR&#39;s), but may, for example, be semiconductor diodes or other semiconductor devices in other circuits. A circuit module in accordance with the invention comprises a sandwich construction in which the outer layers each comprise an insulating polymid layer as shown in FIG. 1. The insulating layer has high thermal conductivity and may be of another material such as ceramic or any other electrically insulating, heat conducting material. The insulating layer 2 has a copper metallization pattern or layer 4 formed on one surface thereof. In the relay circuit of the illustrative embodiment, the insulating layer is a 0.001 inch thick layer of polymid to which a copper layer 0.0007 inch thick is bonded. The copper layer 4 includes a first portion 6 to which a lead frame is soldered and a second portion 8 to which the gate area of the semiconductor device is electrically connected to and to which a wire lead is solder bonded. The lead frame 20 is shown in FIG. 2 and includes a first portion 22 which is sized to substantially overlay the copper layer 6 on the insulating layer or substrate 2. A contact arm 24 extends from the portion 22. The arm 24 includes a narrow neck region 26 which facilitates bending of the arm 24 such that the upper portion 29 of arm 24 is perpendicular to the lower portion 28 of arm 24. The lead frame is of copper alloy and is 0.032 inches thick. A second lead frame 20A is shown in FIG. 3 and includes a first portion 22A which is comparable to portion 22 of lead frame 20. An arm 24A extends from the portion 22A and includes a narrow neck region 26A to facilitate bending of the arm 24A such that the upper portion 29A is perpendicular to the lower portion 24A in the assembled module. The arms 24 and 24A are made different in the illustrative embodiment as a matter of design choice determined by the spacing of the upper arm portions 29 and 29A in the final assembly. In other embodiments, the lead frames may be made identical in configuration. 
     Turning now to FIG. 3, the various components which are assembled together to form a circuit assembly in accordance with the principles of the invention are shown. In the illustrative solid state relay device, four identical solid state devices are utilized. Two of the devices 31, 33 are used in each of the two circuit assemblies 32, 34. 
     Each circuit module comprises a first lead frame and substrate subassembly 39 and a second lead frame and substrate assembly 37. The first subassembly 39 includes lead frames 20 solder bonded to the copper layer 4 of an insulating layer or substrate 2. Two semiconductor devices 31 and 33 are bonded to the copper layer. An insulating spacer 302 having apertures 331 and 333 adapted to surround the devices 31 and 33, respectively, is disposed on the surface of the lead frame 20 of each circuit assembly 32 and 34 although only one insulating spacer 302 is shown in FIG. 3. The insulating spacer 302 is formed from an insulating material available from Dupont Corporation and known as Kapton. The spacer 302 is 0.003 inches thick. As shown in FIG. 3, the semiconductor devices 31 are disposed on the substrate 2 of subassembly 39 with a control gate portion 311 of the device 31 facing downward to second copper portion 8. The semiconductor devices 33 are each disposed on lead frame 20 with their respective gate portions 311 facing up. Lead frames 20A are similarly bonded to substrates 2 to form subassemblies 37. The substrate and lead frame subassemblies 37 are disposed on the subassemblies 39 and the semiconductor devices 31 and 33 are bonded to the lead frames 20A. Each resulting circuit assembly 32, 34 is thus of a sandwich construction. 
     A circuit board 310 is also shown in FIG. 3 The circuit board includes appropriate circuitry to operate with the semiconductor devices 31, 33 of the circuit subassemblies 32 and 34. The specific circuitry which is included on the circuit board 310 does not form part of the present invention and therefore is not shown so that the present invention may be shown with greater clarity. The circuit board may be any circuit board type generally utilized and would typically include printed circuit wiring on at least one surface and may also include various discrete components. Although as will be appreciated by those skilled in the art, the advantages of the present invention are such that the inventive aspects are not intended to be limited to the use of any particular type of circuit board or circuit, nor is it intended to limit the invention to assemblies which utilize any circuit board. 
     Each of the circuit subassemblies 32 and 34 are assembled to the circuit board 310. To assemble the circuit subassemblies 32 and 34 to the circuit board 310, the contact arms 20 and 20A are respectively inserted through the circuit board through holes 320 and 320A, respectively. The contact arms may be solder bonded to circuit traces on the circuit board 310. After the arms 20 and 20A are inserted through the circuit board 310, the upper arm portions 29 and 29A may be bent perpendicular to the lower arm portions 28 and 28A, respectively, of the subassemblies 32 and 34. 
     The resulting assemblage of the circuit subassemblies 32 and 34 with the circuit board 310 is then assembled to the heat sink shown in FIGS. 4, 5 and 6. The heat sink 400 is an aluminum extrusion. The heat sink includes a central portion 402. Extending from the central portion are heat transfer fins 404. The heat fins 404 are of equal size and number on both sides of the heat sink 400. The top of the heat sink as shown in the drawing figures includes a channel 406 which is sized to receive the circuit board 310 shown in FIG. 3. The central portion 402 includes a channel 408 which extends along the entire length of the heat sink 400. The channel 408 has a depth into the central portion 402 which is at least equal to the distance that the subassemblies 32 and 34 extend below the circuit board 310. The width of the channel 408 when the subassemblies 32 and 34 are not inserted into channel 408 is less than the thickness of the subassemblies 32 and 34. The width of the channel 408 is such that the side walls 409 of the channel 408 will compressingly engage the insulating or substrate layers of the circuit subassemblies 32 and 34 such that heat will transfer through the substrates 2 on both sides of the subassemblies 32 and 34 to the side walls 409 such that heat will dissipate through the heat sink 400. The dimensions of the heat sink are that it has a length of 4.5 inches, a height of 1.638 inches, a central portion thickness of 0.27 inches, a channel 408 depth of 0.964 inches and a channel 408 width of 0.095 inches. The central portion 402 has an overall width of 0.338 inches. 
     A circuit module assembly in accordance with the invention is shown in FIGS. 8 and 9. To assemble the unit shown in FIGS. 8 and 9, a tool is used to engage the upper walls 411 of the channel 406. The walls 411 each include a notch 412 to facilitate engagement of the walls 411 by a spreading tool. By exerting outward directed forces against the walls 411, the channel 408 is spread to a width greater than the thickness of the circuit subassemblies 32 and 34. The printed circuit board 310 is disposed above the channel 408 and the circuit assembly is lowered into the channel 408. After the circuit board is moved downward so that the subassemblies 32 and 34 below the circuit board 310 are submerged into the channel 408 and the bottom of the circuit board 310 engages or is in close proximity to the upper surface of the heat sink channel 402, the spreading tool releases the heat sink 400 from the spread apart condition. The memory and inherent resiliency of the heat sink 400 is such that the side walls 409 of the channel 408 when released attempt to return to the original relaxed position and engage the exterior surfaces the substrates or insulating layers of the circuit assemblies 32 and 34. The side walls 409 firmly and compressingly engage and grip the circuit assemblies 32 and 34 such that heat transfer is achieved between the circuit assemblies 32 and 34 and the heat sink 400 from the two planar sides of the circuit assemblies 32 and 34 and thereby dissipating heat from both planar sides of the circuit assemblies 32 and 34. With this arrangement, a higher level of heat transfer may be obtained from semiconductor devices, thereby increasing the heat transfer efficiency from such devices in a manner that has not been previously obtained. 
     The sandwich like construction of the arrangement of the invention is more clearly shown in FIG. 9. As will be readily appreciated, the heat from semiconductor device 33 is extracted from both planar surfaces thereof in the direction of heat transfer shown by arrows 99. 
     As will be appreciated by those skilled in the art, the invention has been described in conjunction with a specific illustrative embodiment and various changes and modifications may be made without departing form the spirit or scope of the invention. It is intended that the invention be limited only by the claims appended hereto.