BUTTON RECTIFIER PACKAGE

Abstract: An axial lead package (20) meeting existing dimensional standards is provided with internal pedestals (22, 24) each having a cross-sectional area more than one and one half times the area of the post (21, 32).

Description: BUTTON RECTIFIER PACKAGE Background This invention relates to a package for semiconductor diodes and, in particular, to what are known as button diodes. As higher power diodes are developed, the package for these diodes have increased in size accordingly. The result is a proliferation of package types and sizes with which the user must contend when designing a system using the diodes. If improved, higher power diodes are developed, the change in package often necessitates a re-design of a portion of the system. In assemblies using several high power diodes, such as bridge rectifiers, it is desired that the resulting assembly be as compact as possible, preferably retaining the smallest possible package. One package achieving the goal of a compact package is disclosed in U.S. Patent 4,093,958. The improved, low cost package described therein comprises a semiconductor diode chip or die soldered between two heatsinks having good thermal conductivity. The heatsinks have a cross-sectional area slightly larger than the semiconductor die and a predetermined length. The die and the inner ends of the heatsinks are surrounded with suitable encapsulants which protect the chip and insulate the heatsinks from each other. While achieving its intended purpose, the package therein described is limited as to the amount of power which can be dissipated by the diode, particularly when the diode is used in one particular application, for example as a transient suppressor. Not unexpectedly, there is a continuing demand for increased performance. While increasing the size of the device might be an answer, it is also desired to achieve the increased performance in a package meeting existing standards, e.g. JEDEC case outlines. Presently made button diodes of the maximum power rating use a round chip or die, obtained by etch cutting the silicon wafer. While closely fitting the geometry of the heatsinks, the etch cutting process adds considerably to the cost of the die. Substituting a square die of about the same area would require a larger, non-standard package to accommodate the larger diagonal dimension of the square die. In view of the foregoing it is therefore an object of the present invention to provide an improved package for high power diodes. Another object of the present invention is to provide an improved transient suppressor having higher transient energy capability. A further object of the present invention is to provide a lower cost power diode assembly enabling the use of square die while maintaining the standard package outline. Summary The foregoing objects are achieved in the present invention wherein it has been found that the internal pedestal, and not the external post, portion of the heatsink can be increased in size to improve the heat dissipating and transient energy absorption capabilities of the package without changing the outside dimensions of the package or incurring thermal failures due to the encapsulant cracking under stress caused by the expansion and contraction of the die and heatsink portions within. Brief Description of the Drawings A more complete understanding of the present invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: FIGURE 1 illustrates a package of the prior art. FIGURE 2 illustrates a package in accordance with the present invention. Detailed Description FIGURE 1 illustrates a semiconductor diode package of the prior art wherein a first heatsink, comprising post 11 and pedestal 12, and a second heatsink comprising post 13 and pedestal 14, are interconnected by semiconductor chip 15. Surrounding semiconductor chip 15 and the inner ends of the heatsinks is plastic- encapsulant 16. The heatsinks, which typically comprise copper, have a very large tempera- ture coefficient of expansion as compared to encapsulant 16. In addition, it is desired that the package be able to withstand torsional stress, i.e., a twisting of post 11 relative to post 13 and/or the encapsulant about the axis of the package. For these reasons encapsulant 16 was made quite thick.relative to the diameter of pedestal 12 because only silicone plastic would work on this device. For example, in devices of the prior art, pedestal 12 has a diameter of 6.6 mm while the overall package diameter is approximately 10.2 mm. Thus the wall thickness between pedestals 12 and 14 and the outside of the package is approximately 1.8 mm. Recent developments in the encapsulant materials (such as epoxy encapsulants) has allowed the wall thickness to be reduced to 1.0 mm. In accordance with the present invention, as illus- trated in FIGURE 2, diode 20 is provided with a package -4- having a first heatsink comprising post 21 and pedestal 22 and a second heatsink comprising post 23 and pedestal 24. Interconnecting pedestals 22 and 24 is die 25. Surrounding the pedestals and die is encapsulant 26 which may, for example, comprise epoxy. Unlike devices of the prior art, pedestals 22 and 24 are substantially increased in area. Specifically, pedestals 22 and 24 have an area more than one and one-half times the area of posts 21 and 23, as measured in a plane orthogonal to the axis of device 20. Die 25 may comprise any suitable shape such as square, round and hexagonal geometries. For example, where cost considerations are paramount, a square die.can be used. * Where performance is paramount, a circular die slightly smaller in area than the pedestals can be used. _ Hexagonal die allows for maximum area on a round pedestal while still allowing for cheaper wafer dicing techniques. Unlike prior art devices, the area of die 25 may exceed the cross- sectional area of post 21 or 22 and, in particular, a dimension of die 25 in the plane orthogonal to the axis of the device can be greater than a corresponding dimension of post 21; for example, the side or diagonal of a square die. As a specific example of a device built in accordance with the present invention, a 7.1 mm diameter round semiconductor diode chip was soldered between copper heatsinks having a post diameter of 5.6 mm, and a pedestal diameter of 8.1 mm. The exposed edges of the diode chip were coated with a silicone elastomer. Epoxy was transfer molded about the pedestals and die and about a portion of the posts. The outside of the package complied with current specifications for outside dimensions of the standard devices. A severe test has recently been defined for devices to be used as surge suppressors in automotive applications, i.e. avalanche diodes used as the rectifier bridge of an alternator or, as a separate element. The test requires -5- that ten pulses at intervals of thirty seconds be applied to the suppressor while it is reverse biased at approximately fourteen volts. The avalanche voltage of the surge suppressor is typically from 26 to 30 volts. The goal is a device that can withstand pulses of eighty-five volts, simulating a "load dump", i.e. the transient resulting from disconnecting a battery from an alternator generating at its maximum current. In tests of devices built as described above, suppressors survived ten pulses at 70 volts. Commercially available devices could only withstand 62 volts under these conditions. While neither achieved the goal, devices in accordance with the present invention performed much better than prior art devices, yet have the same package outline. More significant, at least for surge suppressors, is the energy absorbed by devices in accordance with the present invention. These devices absorbed 39.2 joules (7007 peak watts for 5.6 milliseconds) whereas standard devices absorbed 27.9 joules (4975 peak watts for 5.6 milliseconds) under similar test conditions. While the data from these tests showed a greater variation (standard deviation) for devices in accordance with the present invention, this is not unexpected with prototype, as opposed to mass produced, devices. Further, while some devices failed while subjected to repeated pulses in excess of seventy volts, they did not fail after only one pulse, and were subjected to over two hundred amperes reverse current as compared to one hundred sixty amperes or so for prior art devices. Having thus described the invention it will be to ' those of skill in the art that various modifications can be made within the spirit and scope of the present invention. For example, while described in a preferred embodiment in connection with avalanche diodes used as surge suppressors, conventional power diodes, _?chottky diodes or other power devices can be used in the package.

Claims: -6- CLAIMS 1. A semiconductor device comprising: first and second conductive heatsinks, each having a post portion and a pedestal portion; a semiconductor die interconnecting the pedestal portion of each heatsink; a plastic encapsulant surrounding said die and said pedestals; and wherein the area of said pedestal portion is more than one and one-half times the area of said post portion in a plane orthogonal to the axis of said device. 2. The device as set forth in claim 1 wherein a dimension of said die parallel to said plane is greater than a dimension of said post as measured in the same •direction. 3. The device as set forth in claim 1 wherein the area of said die is greater than said area of the post portion.

Inventors: ELLSWORTH KENNETH A
Applicants: MOTOROLA INC
Application year: 1981
Main classification: H01L23
IPC codes: H01L23