Interface Device for Thermally Coupling an Integrated Circuit to a Heat Sink

This invention relates to an interface device used to transfer heat away from a mounted electronic component. In particular, this invention relates to an interface device for transferring heat generated by a high power silicon integrated circuit chip to a heat sink characterized by a high coefficient of thermal conductivity.

The environmental conditions which integrated circuits must endure during operation are severe, particularly in an automotive environment. During the operation of an automobile, the integrated circuit chips are exposed to large variations in temperature arising from not only the changes in ambient atmospheric temperature, but also from the variations in the operating temperature generated by the automobile itself. Generally, an automobile manufacturer requires these integrated circuits to reliably perform while experiencing repeated cycling between ambient temperatures of -40.degree. C. and +125.degree. C. Therefore, it is necessary that the integrated circuit be capable of withstanding these temperature extremes without detriment to its structural or electrical integrity.

In addition, power generating integrated circuits are particularly problematic in that they accordingly also, generate substantial amounts of heat during generation of the power. This heat generation increases the temperature of the chip and may detrimentally affect circuit performance and/or cause chip failure. In addition, the power generating integrated circuit may need to be dielectrically isolated from the surrounding components. These detrimental effects are compounded when combined with the temperature extremes experienced by the integrated circuit during operation of an automobile. Therefore, it is desirable to provide means for maintaining a constant temperature of the integrated circuit chip despite any variations in temperature within and surrounding the chip. A common solution involves soldering a chip to a heat sink. Generally the heat sinks are formed from aluminum, which is relatively inexpensive, or alternatively copper, which is relatively expensive.

However, a shortcoming of this approach is that during the exposure to the variations in temperature, the solder used to couple the integrated circuit chip and heat sink will fracture due to stresses arising from the differences in thermal expansion rates of the various materials. The coefficient of thermal expansion for silicon is approximately equal to 3.5.times.10.sup.-6 /.degree. C., while the coefficients of thermal expansion for copper and aluminum are approximately equal to 17.7.times.10.sup.-6 /.degree. C. and 24.times.10.sup.-6 /.degree. C., respectively. Generally, the solder will fracture most often at the corners of the chip where the stresses are concentrated These fractures are particularly undesirable as they increase the thermal resistance between the chip and the heat sink, which in turn causes the temperature of the chip to rise because of retarded heat transfer to the heat sink. This phenomenon further adversely affects the performance of the integrated circuit thereby promoting failure of the integrated circuit chip

Alternative solutions to the temperature problem have also been disclosed by the prior art and generally involve the use of an interface device, or buffer, between the integrated circuit chip and heat sink which is characterized by a high thermal conductivity, or correspondingly, relatively low thermal resistance. In addition, it is desirable that the interface device have a coefficient of thermal expansion which is intermediate between that of the integrated circuit chip and that of the heat sink. In such a structure, the integrated circuit is soldered to one side of the interface device and the heat sink to the other side of the interface device Molybdenum and tungsten have been commonly used for interface devices because of their appropriate heat transfer and expansion characteristics. These interface devices have been a satisfactory solution to the problem, however it is desirable to provide a less expensive alternative In particular, for automotive applications, molybdenum and tungsten are considered too expensive for commercial use.

Another type of interface device has been proposed in United States patent application Ser. No. 191,441, entitled "Integrated Circuit Heat Sink Interface Apparatus" to Akin et al, wherein an inner core of low expansion material is surrounded everywhere by a copper outer layer characterized by high thermal conductivity. The integrated circuit contacts the copper layer at a region which is over the inner core layer of low expansion material. The heat sink contacts the copper layer on an opposite side of the integrated circuit. This arrangement is of relatively low cost and permits rapid conductivity of the generated heat from the integrated circuit through the copper outer layer around the inner core layer, while the low expansion material retards the overall thermal expansion of the interface device. Although this design is satisfactory, it would be desirable to provide an interface device which permits the most efficient heat transfer directly from the integrated circuit to the heat sink, rather than around the inner core layer as in this previous method.

Therefore, it is desirable to provide an interface device which provides direct thermal coupling between an integrated circuit and heat sink, and which is durable, reliable over varying temperature conditions, characterized by an intermediate coefficient of expansion, and readily amenable to automotive production techniques.

It is, therefore, an object of this invention to provide an efficient interface device suitable for thermally coupling an integrated circuit directly to a heat sink, which is durable such that it can reliably withstand the extreme operating temperatures present in an automotive environment.

It is a further object of this invention to provide an interface device which is characterized by a coefficient of thermal expansion intermediate between the coefficients for the integrated circuit and the heat sink.

In addition, it is an object of this invention that such an interface device be compatible with means for providing dielectric isolation between an appropriate integrated circuit chip and heat sink.

Lastly, it is still a further object of this invention to provide an interface device which is of relatively low cost and readily amenable to automotive production techniques.

In accordance with the preferred embodiment of this invention, these and other objects and advantages are accomplished as follows.

In a preferred embodiment, the interface device for thermally coupling an integrated circuit to a heat sink comprises a first material characterized by high thermal conductivity such as copper, wherein the copper completely surrounds a plurality of inner core regions. The plurality of inner core regions comprise a low coefficient of expansion material, preferably an iron-nickel alloy, such as Invar or Kovar. In addition, the plurality of inner core regions are primarily disposed in the copper material in the region directly under the integrated circuit, thereby retarding expansion of the interface device during exposure to variations in the temperature.

In use, a silicon integrated circuit chip is soldered to an upper surface of the interface device, and an aluminum or copper heat sink is bolted to a lower surface of the interface device Preferably, silicon grease is placed between the lower surface of the interface device and the heat sink so as to promote heat flow through the interface device to the heat sink. Therefore, the heat generated by the integrated circuit chip passes directly into the interface device, and primarily into the high thermally conductive copper layer wherein the heat transfers rapidly through the copper into the heat sink. The low expansion material retards the overall expansion rate of the interface device.

An inventive feature of this device is that the plurality of low expansion regions retard the expansion rate of the device where it is most needed under the silicon chip, but do not hamper the heat flow from the silicon chip to the heat sink. With previous designs, the low expansion material which is typically characterized by high thermal resistivity, acted as a barrier against effective heat transfer out of the chip. The heat was forced to flow around the low expansion material/high resistivity material, resulting in an inefficient method of heat transfer. With this design, the heat is not required to flow around the low expansion material, as there are continuous paths of high conductivity material directly from the integrated circuit chip to the heat sink. This interface device is characterized by enhanced thermal dissipation yet provides a compatible coefficient of thermal expansion with the silicon integrated circuit and underlying heat sink.

The coefficient of thermal expansion of the preferred interface device having a cross-sectional area which is approximately 80 percent copper and approximately 20 percent an iron-nickel alloy characterized by about 42 percent nickel, was found to be approximately 9.times.10.sup.-6 /.degree. C. This coefficient of thermal expansion is sufficiently compatible with that of silicon to allow the solder joint between the chip and the interface device to remain intact during operation without fracturing for a substantial number of temperature cycles between -40.degree. C. and +125.degree. C.

Other objects and advantages of this invention will be better appreciated from a detailed description thereof, which follows

FIG. 1 is cross-sectional view of an interface device which may be used for thermally coupling an integrated circuit chip and heat sink in accordance with a preferred embodiment of this invention.

FIG. 2 is a cross-sectional view of an interface device in accordance with an alternative and equally preferred embodiment of this invention.

FIG. 3 is a cross sectional view of the interface device shown in FIG. 2 wherein dielectric isolation means is also provided between the interface device and the heat sink.

FIG. 4 is a plan view of the perforate layer of material characterized by a low coefficient of thermal expansion overlaying a layer of material characterized by high thermal conductivity.

FIG. 5 is a cross sectional view of the interface device shown in FIG. 1.

This invention comprehends an interface device which is used to thermally couple an integrated circuit chip to a heat sink. The preferred interface device is thermally efficient in that heat generated from the integrated circuit passes directly through the interface device into the heat sink. In addition, the preferred device is characterized by a coefficient of thermal expansion intermediate between the coefficients for the integrated circuit and the heat sink. The preferred interface device is durable in that it can reliably withstand the extreme operating temperatures present in an automotive environment without detriment to its structural or electrical integrity. In addition, this preferred interface device is conducive in cost and manufacturability to automotive production techniques.

A preferred embodiment of the interface device 10 is shown in FIG. 1. The interface device 10 is coupled to an integrated circuit 12 and an underlying heat sink 14. The interface device 10 primarily comprises a first material 16 which is characterized by high thermal conductivity or low thermal resistance. A second material 18 which is characterized by a low coefficient of thermal expansion is dispersed throughout the first material 16. This first material 16 completely surrounds each of the plurality regions of second material 18.

The preferred first material 16 which is characterized by a high thermal conductivity is copper, however other materials characterized by high thermal conductivity, such as aluminum, may also be used. In addition, a consideration when choosing the first material 16 is that the first material 16 must be compatible with the second material 18 which will be dispersed throughout the first material 16. For the first material 16, copper is preferred because of its high thermal conductivity, cost and ease of manufacturability.

The plurality of second material 18 regions comprise a material characterized by a low coefficient of thermal expansion, such as an iron-nickel alloy. In particular, an iron-nickel alloy comprising approximately 42 percent nickel and and approximately 58 percent iron such as Invar is preferred. However, other materials which are characterized by a low coefficient of thermal expansion and which are compatible with the surrounding first material 16, may also be used.

As shown in FIG. 1, a silicon integrated circuit chip 12 is soldered, the solder being denoted as layer 20, to an upper surface 22 of the primarily copper interface device 10 and an aluminum or copper heat sink 14 is bolted to a lower surface 24 of the primarily copper interface device 10. The preferred solder is a 25/75 tin-lead type for manufacturability and availability purposes, however other suitable solders may also be used. Preferably, silicon grease (denoted as layer 26) is placed between the lower surface 24 of the interface device 10 and the heat sink 14. The silicon grease 26 is preferred since it permits heat transfer through to the heat sink 14 from the interface device 10, however, other suitable materials which will provide the appropriate heat transfer through to the heat sink 14 from the interface device 10 may also be used, such as suitable epoxies or solder.

Preferably, the plurality of low expansion second material regions 18 are primarily located under the silicon integrated circuit chip 12 in the surrounding copper or other appropriate first material 16. It is desirable to position the plurality of low expansion second material regions 18 primarily under the silicon chip 12 because the silicon expands at a much lower rate as temperature is increased, than the other materials. Therefore, the expansion of the interface device 10 is retarded most where it is required, i.e., under the silicon chip 12, with this design.

In addition, the number of low expansion second material regions 18 may vary within the surrounding copper material 16. As shown in FIG. 1, there are five low expansion regions 18 primarily situated under the integrated circuit chip 12 within the surrounding copper material 16. The arrows indicate the pattern of heat flow from the integrated circuit chip 12 to the heat sink 14 through the interface device 10. With this device 10, heat transfer occurs most efficiently. The heat generated by the integrated circuit chip 12 flows directly into the interface device 10, dissipating through the highly conductive surrounding copper layer 16 wherein the heat transfers rapidly through the copper layer 16 into the heat sink 14.

A truly inventive feature of this device 10 is that the plurality of low expansion regions 18 retard the expansion rate of the device 10 where it is most needed under the silicon chip 12, but do not hamper the heat flow from the silicon chip 12 to the heat sink 14 since the highly conductive surrounding copper layer 16 is provided generally everywhere throughout the device 10. With previous designs, the low expansion material which is typically characterized by high thermal resistivity, acted as a barrier against effective heat transfer out of the chip 12. The heat was forced to go around the low expansion material/high resistivity material, resulting in an inefficient method of heat transfer With this design, the heat does not need to go around the low expansion second material 18, as there are continuous paths of high conductivity copper straight from the integrated circuit chip 12 to the heat sink 14.

Therefore, this interface device 10 is characterized by enhanced and efficient thermal dissipation yet provides a compatible coefficient of thermal expansion with the silicon integrated circuit 12 and underlying heat sink 14.

As shown in FIG. 2, an alternative and equally preferred embodiment 110 of this invention is also illustrated. In this interface device 110, there are essentially two rows of the low expansion second material 118 enveloped by the surrounding copper or other high conductivity material 116. This device 110 permits enhanced heat transfer through the surrounding copper 116 and is characterized by an appropriate expansion rate. Theoretically, a device 110 in accordance with this invention, could have as few as two low expansion regions 116 separated by the copper 118 under the silicon chip 112 thereby providing a single direct path for heat transfer from the silicon chip 112 to the heat sink 114, or a multitude or regions wherein the only limitations would be manufacturing and feasibility considerations.

This interface device 210 shown in FIG. 3 may also be used when dielectric isolation is required, such as between a power producing integrated circuit 212 and a heat sink 214. In this preferred embodiment, the dielectric isolation is provided by a polyimide layer 228, which has a thermally conductive material dispersed throughout for enhanced thermal conductivity characteristics. The polyimide layer 228 is attached to the heat sink 214 by a very thin layer of thermally conductive adhesive 230.

These two relatively high thermal resistance layers, i.e., the polyimide layer 228 and the conductive adhesive layer 230, are manufactured to be very thin to minimize the detrimental effects associated with their inherently high thermal resistance. The dielectric means, or polyimide layer 228, is preferably formed from a plastic dielectric material, such as Kapton. It is even more preferred that this dielectric layer 228 have dispersed throughout a suitable dielectric material having relatively high thermal conductivity, such as alumina, aluminum nitride or beryllium oxide. The thermally conductive adhesive layer 230 used to attach the dielectric layer 228 to the heat sink 214, is preferably formed from an epoxy paste having silver or other appropriate thermally conductive medium dispersed throughout. This medium may also be electrically conductive since the dielectric isolation is provided by layer 228. The silver filled epoxy layer 230 is used for its convenience and availability, however other materials such as the commercially available B-staged epoxies which are partially cured pastes, may also be used.

As shown in FIG. 3, a layer 226 of solder or other attachment means is required between the interface device 210 and dielectric layer 228. It is preferred that this solder layer 226 be formed from a 60/40 tin-lead solder. This is preferred because the solder layer 220 used to attach the integrated circuit 212 to the interface device 210 is a 25/75 lead-tin solder and this combination of solders having differing reflow temperatures permits ease of manufacturability without detrimental effects to the complete structure. In addition, a flash layer (not shown) of metallization may be required on the dielectric layer 228 to promote adhesion between the dielectric layer 228 and the solder layer 226, as well as between the dielectric layer 228 and the thermally conductive adhesive layer 230. The structure shown in FIG. 3 provides dielectric isolation when necessary between an integrated circuit chip 212 and heat sink 214.

The coefficient of thermal expansion of the preferred interface device should be no more than about 10.times.10.sup.-6 /.degree. C. This is the maximum expansion rate that is desirable. The coefficient of thermal expansion for the silicon integrated circuit chip is approximately equal to about 3.5.times.10.sup.-6 /.degree. C., while the coefficient for an aluminum heat sink is approximately 24.times.10.sup.-6 /.degree. C. In addition, the coefficients for copper and an iron-nickel alloy characterized by approximately 42 percent nickel are 17.7.times.10.sup.-6 /.degree. C. and about 2.7.times.10.sup.-6 /.degree. C., respectively.

An empirically derived equation to determine the average coefficient of thermal expansion (CTE) of the interface device is: ##EQU1## where 0.62 is the Young's Modulus of elasticity of copper to that of an iron-nickel alloy having approximately 42 percent nickel.

An interface device having a cross-sectional area which is approximately 80 percent copper and approximately 20 percent an iron-nickel alloy characterized by about 42 percent nickel, such as either device 10, 110 or 210 shown in FIGS. 1, 2 and 3, was found to be approximately 9.times.10.sup.-6 /.degree. C. This coefficient of thermal expansion is sufficiently compatible with that of silicon to allow the solder joint 20, 120 or 220 between the chip 12, 112 or 212 and the interface device 10, 110 or 210 to remain intact during operation without fracturing for a substantial number of temperature cycles between -40.degree. C. and +125.degree. C.

There are several methods proposed for forming this interface device. A first method comprises the steps of sandwiching a perforate layer of low expansion material between two outer layers of copper and appropriately heating the materials so as to metallurgically bond the materials together The perforate layer of low expansion material may be formed by punching, piercing, dissolution or any other suitable means. In addition, the perforate layer 50, as shown in FIG. 4 overlaying a single layer 52 of copper or other appropriate high thermal conductivity material, is preferably provided only in the center region of the sandwiched layers (only the bottom layer 52 is shown for clarity), so as to correspond to the region where the integrated circuit chip will be disposed. During the heating step, the copper flows into the perforate areas 54 of the low expansion material 50, thereby completely enveloping the low expansion material 50. The resulting material is then sliced appropriately through all three sandwiched layers such that a resulting cross section through the device is illustrated in FIG. 5. FIG. 5 shows the interface device 10 illustrated in FIG. 1 which comprises a plurality of low expansion second material regions 18 which were formed from the perforate layer (50 in FIG. 4), surrounded by the high thermal conductivity material 16 which flowed into the perforations during heat treatment.

Alternatively, the device having a cross section as shown in FIGS. 2 or 3 may be formed by sandwiching a plurality of perforate layers between alternating layers of copper. Theoretically, the device could have a multitude of these layers, the only limit being manufacturability and feasibility. However, a device having two perforate layers sandwiched between three alternating layers of copper is preferred.

As another alternative, the device could be fabricated by sandwiching strips of low expansion wire between alternating layers of copper, and subsequently heat treating to metallurgically bond the materials. Again this would produce the desired result wherein the high conductivity copper completely surrounds a plurality of regions containing the low expansion material. Although this alternative embodiment performs satisfactorily, it is not as preferred as the perforated embodiment which provides better control of the coefficient of thermal expansion properties of the device in both lateral directions, i.e., the x and y-axis of the device.

This invention readily provides an interface device which promotes efficient heat transfer between a heat generating integrated circuit chip and a heat sink, while also providing an intermediate and compatible coefficient of thermal expansion between the two components. While our invention has been described in terms of preferred embodiments, it is apparent that other forms of the device could be adopted by one skilled in the art, such as by substituting materials, or components. Accordingly the scope of our invention is to be limited only by the following claims.