LOW-STRESS-INDUCING OMNIDIRECTIONAL HEAT SINK WITH DOMED ATTACHMENT PORTION

Abstract: Disclosed is a heat sink for cooling an integrated circuit package. The heat sink (80) comprises a single thin sheet of material having two oppositely facing major surfaces. These two major surfaces have a common perimeter defining a plurality of spaced-apart finger-shaped portions of the thin sheet of material that extend radially from a central portion of the sheet. The central portion (81) is convex for attachment to the integrated circuit package (11); and the finger-shaped portions are formed to extend out of the plane of the central portion to become cooling fins for the central portion.

Description: BACKGROUND OF THE INVENTION This invention relates to the art of packaging and cooling integrated circuits. The Application contains material common with pre-published European Patent Application No. EPA.94,200 which shows and claims an Omni-Directional Heatsink. One commonly used integrated circuit package of the prior art is illustrated in cross section in Figure 1 where it is identified by reference numeral 10. Some of the major components of this package 10 are a substrate 11, a lid 12, a plurality of conductive leads 13, and an integrated circuit chip 14. As Figure 1 shows, substrate 11 is rectangular in cross section. Also it has a top major surface 11a and a bottom major surface 11b. Surface 11a has a cavity near its centre which is shaped to receive chip 14. Surface 11b, by comparison, is merely flat. A plurality of electrical conductors 11d lie within substrate 11 between the periphery of cavity 11c and the periphery of surface 11a. At the periphery of cavity 11c, electrical connections are made between the conductors 11d and chip 14 by a plurality of bonding wires 15. At the periphery of surface 11a, the conductors lid contact the leads 13 directly which in turn extend from surface 11a to make electrical connections to an external system (not shown). Lid 12 overlies cavity 11c and is rigidly attached to surface 11a at the periphery of the cavity by a lid attach material 16. This lid attach material 16 together with lid 12 and substrate 11 thus provide a hermetic enclosure for chip 14. One specific example of the materials and their dimensions in the above-described circuit package is as follows: Substrate 11 is made of ceramic; and it has a length of 2.413 cm (0.950 inches), a width of 2.413 cm. (0.950 inches) and a height of 0.152 cm. (0.060 inches). Chip 14 is made primarily of silicon; and it has a length of 0.762 cm. (0.300 inches), a width of 0.762 cm. (0.300 inches), and a thickness of 0.051 cm. (0.020 inches). Lid 12 is made of ceramic; and it has a length of 1.473 cm. (0.580 inches), a width of 1.473 cm. (0.580 inches), and a thickness of 0.076 cm. (0.030 inches). And the lid attach material 16 is made of a layer of glass having a thickness of 0.005 cm. (0.002 inches). When chip 14 in package 10 is of the type that uses a relatively small amount of power (e.g., less than one watt), then no heat sink needs to be attached to the package. However, as the amount of power which chip 14 uses increases, a point is eventually reached at which a heat sink must be attached to the package in order to insure that chip 14 does not overheat. Conventionally, the heat sink is made of metal, such as copper or aluminum; and it is rigidly attached by an epoxy or a solder to surface 11b directly below chip 14. During the attachment process, the epoxy or solder is heated to a fluid state whereupon it is dispersed in a thin smooth layer between surface 11b and the heat sink. Thereafter, the epoxy or solder is allowed to cool and harden. However, this cooling and hardening step also induces stresses in the package, and particularly in the lid attach material 16. These stresses vary in magnitude with the overall shape of the particular heat sink that is being attached. And, depending on the shape of the heat sink, the stresses can become so large as to cause cracks in the lid attach material 16. When this occurs, the hermetic seal for chip 14 is broken which makes the package inoperable. To further understand how the heat sink induces stresses in the lid attach material 16, reference should now be made to Figure 2. That Figure contains a graph wherein the temperature of the integrated circuit package is plotted on a horizontal axis, and stress in the lid attach material 16 is plotted on a vertical axis. In this graph, a curve 21 illustrates how stress in the lid attach material 16 varies as a function of temperature under the condition where no heat sink is attached to package 10; while another curve 22 shows how stress in the lid attach material 16 varies under the conditions where a heat sink is attached to package 10. Curves 21 and 22 begin at a temperature T LID , which is the temperature at which the lid attach material solidifies. For example, temperature T LID is approximately 320°C when the lid attach material is glass. In order to attach lid 12 to surface 11a, the lid attach material must be heated above temperature T LID , and typically it is heated to 420°C. Thereafter, the package is cooled to room temperature T RT . During this cooling, both substrate 11 and lid 12 contract. But this contraction induces only relatively small stresses in the attach material 16, because both lid 12 and substrate 11 are made of essentially the same material and thus they contract at nearly equal rates. Thereafter, package 10 is reheated to attach the heat sink to surface 11b. In this step, the temperature to which the package is heated must exceed the solidification temperature T HS of the heat sink attach material. For example, the temperature for solder is about 183°C, and for an epoxy is about 150°C. So long as the heat sink attach material remains liquid, the stresses induced in the lid attach material 16 remain relatively small. However, once the heat sink attach material solidifies at temperature T HS , the stresses in the lid attach material 16 rapidly increase. This rapid increase in stress is due to the fact that the heat sink contracts much more rapidly than the ceramic substrate. For example, the coefficients of thermal expansion for copper and aluminum respectively are about 2.6 and 3.6 times the expansion coefficient of ceramic. As the heat sink contracts, it tends to compress that portion of surface 11b to which the heat sink is attached. This in turn causes substrate 11 to bend in an arc-shaped fashion as illustrated in Figure 3. In this Figure, the amount of bending is greatly exaggerated merely to illustrate the point that such bending actually does occur. This bending, in turn, causes the rapid increase in stress in the lid attach material 16. After the heat sink is attached and the integrated circuit package is placed in an operating environment, the package is subjected to some predetermined range of operating temperatures. A typical maximum operating temperature, for example, is 125°C; and a typical minimum operating temperature is -55°C. Such maximum and minimum operating temperatures are indicated in Figure 2 as T max and T min respectively. Curve 22 shows that at temperature T max , the stress in lid attach material 16 is at a minimal level S min ; whereas at temperature T min , the stress in lid attach material 16 is at a maximum level S max . So in the operating environment, stress in the lid attach material varies between S max and S min . And the magnitude of the maximum stress S max as well as any cycling between S max and S min frequently cause lid attach material to crack. In addition, when the heat sink attach material solidifies at temperature T HS , stress begins to occur at the interface between the heat sink and substrate 11. This stress, as with the stress in the lid attach material, is due to the fact that the heat sink contracts much more rapidly than substrate 11. As the temperature decreases from T HS to T RT , the stress at the substrate-heat sink interface rapidly increases. Thereafter, when the heat sink and integrated circuit package are placed in an operating environment and cycled between the temperatures T min and T max , the stress at the substrate-heat sink interface cycles about a maximum value. Both the magnitude of that maximum value and the degree of cycling from it can produce cracks between the heat sink and substrate 11. In US-A-3 212 569 there is disclosed a heat sink for cooling individual transistors. This heat sink has a planar bottom portion which includes three holes. The transistor lies on the bottom portion such that the transistor leads pass through the holes. A plurality of fingers extend upwardly from the bottom portion in a direction opposite to the direction of the transistor leads so that the transistor is enclosed within the fingers. With this design the area of the bottom portion of the heat sink is forced to be larger than the area of the transistor that is being cooled, and in such an arrangement cracks will occur in the surface of the transistor in much the same way as previously described above in connection with Figures 1-3. In DD-A-79783 there is disclosed a heat sink for an integrated circuit package having a cup-shaped housing. The heat sink embraces the cup-shaped housing, and a plurality of fingers extend radially from the housing. The problem of cracking referred to above is not relevant to this type of housing and heat sink. EP-A-0 094 200 discloses a heat sink for cooling an integrated circuit package, said heat sink comprising a single thin sheet of material having two oppositely facing major surfaces. These two surfaces have a common perimeter defining a plurality of spaced-apart finger-shaped radially extending portions of the thin sheet of material. The central portion is flat for attachment to the integrated circuit package and the finger-shaped portions extend out of the plane of the central portion to become cooling fins. EP-A-0 071 748 discloses a disc-shaped thermal bridge element for use in a semiconductor package to conduct heat from a semiconductor device to a cold plate. The bridge element has a bulged shape with a first set of inwardly extending slots and a second set of outwardly extending slots emanating from the centre of the disc. The central section of the bridge element is flat to contact a flat surface of the semiconductor device. The present invention relates to an integrated circuit package having a flat surface, and a heat sink which is attached to said package solely by an adhesive that is disposed between a central section of said heat sink and said flat surface. The invention as claimed is characterised in that said central section of said heat sink is convex shaped, and said adhesive is progressively thicker starting from the centre of the central section and continuing to its perimeter. The convex shape of the central section of a head sink according to the invention forces the adhesive which attaches the central portion of the heat sink to the integrated circuit package, to be thin at the centre and progressively thicker towards the periphery of the central portion. As a result, thermally induced stress at the interface between the integrated circuit package and heat sink is substantially reduced, heat transfer between the integrated circuit package and heat sink remains high, and no fixtures are required to set the thickness of the adhesive since it is fixed by the convex shape. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an enlarged cross-sectional view of a prior art integrated circuit package that is suitable for use with a heat sink of the present invention; Figure 2 is a graph illustrating the manner in which a heat sink of the prior art induces stress in the lid attach material of the Figure 1 integrated circuit package; Figure 3 is a schematic diagram illustrating how a heat sink of the prior art bends the Figure 1 integrated circuit package; Figure 4 is a pictorial view of a heat sink constructed according to EP-A-0 094 200; Figure 5 is a planar view of the heat sink of Figure 4 at an intermediate stage of its fabrication; Figures 6A-6D are pictorial views of further heat sinks according to EP-A-0 094 200; Figure 7 is a cross-sectional drawing taken through cooling fins 34-6 and 34-1 of Figure 4 which shows an embodiment of the present invention; and Figure 8 is a set of equations which explain a feature of the Figure 7 heat sink. Referring now to Figures 4 and 5, one heat sink according to EP-A-0 094 200 will be described in detail. This particular embodiment is identified in Figures 4 and 5 by reference numeral 30. Figure 4 illustrates heat sink 30 in a pictorial view which also shows the heat sink attached to surface 11b of the integrated circuit package 10 of Figure 1 Figure 5, by comparison, is a planar view of the heat sink 30 at an intermediate stage of its fabrication process. Heat sink 30 consists of a single thin sheet of material having two oppositely facing major surfaces 31a and 31b. Surfaces 31a and 31b have a common perimeter 32. That perimeter 32 defines a plurality of spaced-apart finger-shaped portions 34 of the heat sink that extend radially from a central portion 33. In the illustrated embodiment of Figures 4 and 5, there are eight finger-shaped portions 32-1 through 34-8; and they are equally spaced around the central portion. As an alternative, however, the number of finger-shaped portions can be other than eight and preferably it is in the range of four to 18. Heat sink 30 is fabricated from a single thin flat sheet of material that is rectangular-shaped. That rectangular-shaped material is stamped with a cutting tool having the above-described perimeter 32. Figure 5 shows the flat sheet of material after this stamping step. Thereafter, all of the finger-shaped portions are formed into a cylindrical shape such that the central portion 33 is at one end of the cylinder; and, the extremities of the finger-shaped portions are formed outward in a fan-like manner so as to reside in one or more planes parallel with the central portion 33. Suitably, these steps are performed by forming two bends of 60°-90° in each finger-shaped portion. Due to these bends, the finger-shaped portions 34-1 through 34-8 form cooling fins for the central portion 33. This is illustrated in Figure 4. Preferably, the distance between the two bends in adjacent finger-shaped portions varies in a cyclic fashion around the central portion 33. In other words, preferably the height of the open ends of the cooling fins alternates in a cyclic fashion around the central portion. This variance in height produces a turbulence in any airflow that passes through the cooling fins which enhances the heat sink's cooling capability. Also preferably, a plurality of holes 35 are included in the central portion. These holes enhance the strength of the bond between the heat sink and package to which the heat sink is attached. In particular, the holes provide a means whereby any gas can escape from between the heat sink attach material and the integrated circuit package attach surface, and thus reduce voids in the heat sink attach material. Also, the holes provide a means for the heat sink attach material to seep through the central portion and bond to surface 31a around the holes' perimeter. One specific example of the materials and dimensions in the above-described embodiment of the invention is as follows. Heat sink 30 is formed of a sheet of metal, such as copper or aluminum, which is 0.038 cm. (0.015 inches)-0.114 cm. (0.045 inches) thick. From such a sheet, the heat sink is stamped with the perimeter figure 5. Suitably, the radius for the central portion is 0.508 cm. (0.200 inches); the distance between the bends of fins 34-1, 34-3, 34-5, and 34-7 is 0.152 cm. (0.060 inches) while the remaining length of those fins is 0.559 cm. (0.220 inches); the distance between the bends of fins 34-2, 34-4, 34-6, and 34-8 is 0.381 cm. (0.150 inches) while the remaining length of those fins is 0.330 cm. (0.130 inches); and the holes 35 are equally spaced and have a radius of 0.051 cm. (0.020 inches). Turning now to Figures 6A-6D, various alternate preferred embodiments of EP-A-0 094 200 will be described. Beginning with Figure 6A, the embodiment there illustrated is similar to the previously described embodiment of Figure 4 in that it also is made of a single thin sheet of material having a common perimeter that defines a plurality of finger-shaped portions around a central portion. However, the Figure 6A embodiment has a total of 12 finger-shaped portions (as opposed to eight finger-shaped portions in the Figure 4 embodiment). Also, all of the finger-shaped portions in the Figure 6A embodiment are bent such that their open ends lie at the same height above the central portion. Next, consider the embodiment of Figure 6B. It also is similar to the previously described embodiment of Figure 4. However, it differs from the Figure 4 embodiment in that the open ends of the finger-shaped portions are all twisted on their axis at a predetermined angle. Preferably, this angle is in the range of 0-45°. These twisted open ends of the finger-shaped portions cause a turbulence in the air that passes through them which in turn enhances the heat sink's cooling action. Consider next the embodiment of Figure 6C. It differs from the Figure 4 embodiment in that it has a total of 12 finger-shaped portions around the central portion; and in addition, the finger-shaped portions are bent in an arc-shaped fashion out of the plane of the central portion. Finally, the Figure 6D embodiment differs from the Figure 4 embodiment in that the common perimeter of the heat sink does not define any central portion or finger-shaped portions at all. Instead, the Figure 6D embodiment has a plurality of radially-oriented elongated slots each of which have a pair of end-walls at two predetermined distances from a point that is centrally located within the common perimeter. Those end-walls nearest the central point define the central portion of the heat sink; while those end-walls furthest from the central point together with the common perimeter define a ring-shaped portion around the central portion. Also, the side-walls of the slots which join the end-walls define a plurality of finger-shaped portions that connect the central portion to the ring-shaped portion. The central portion is flat and attaches to the integrated circuit package; whereas the fingers are bent out of the plane of the central portion, and together with the ring-shaped portion they provide a means for cooling the central portion. All of the above-described Figure 6A-6D embodiments are made by providing a thin rectangular-shaped sheet of material; stamping the heat sink from that sheet with a cutting tool having a perimeter the desired heat sink shape; and bending the stamped part in the manner illustrated in Figures 6A-6D. Referring now to Figure 7, an embodiment of the present invention will be described. This embodiment, which is shown in cross section in Figure 7, is indicated by reference numeral 80. Heat sink 80 has cooling fins which are shaped the same as the cooling fins of heat sink 30 of Figures 4 and 5. However, the improved feature of heat sink 80 is that its central section has a convex shape as indicated by reference numeral 81. Suitably, this convex shape is obtained by placing the stamped part of Figure 5 in a press between one convex member and one concave member. Due to the convex shape 81, the adhesive 82 which attaches heat sink 80 to substrate 11 is thin at the heat sink's axis 83 and is progressively thicker as the distance from axis 83 increases. Preferably, the adhesive 82 is 12.7 µm to 50.8 µm (0.5 to 2.0 mils) thick at the axis 83, and the convex surface 81 is such that it bulges outward at axis 83 by at least one-twentieth of the length of half its cross section. For example, if the edge of the adhesive 82 is 3.175 mm (125 mils) from axis 83, its thickness should be at least 0.152 mm (6.0 mils). This may be achieved by making the cross section of surface 81 a part of a circle, an ellipse, or a parabola. One feature of the convex shape 81 is that thermal stress is reduced between substrate 11 and heat sink 80 while at the same time, good heat transfer between those components is maintained. This is evident from the equations of Figure 8. Equation 1 expresses the distance by which a point A on surface 81 will move when it is subjected to a temperature difference. In that equation, Δ L HS is the distance by which point A moves, K HS is the thermal expansion coefficient of heat sink 80, L O is the distance of point A from axis 83 before the temperature change, and Δ T is the temperature change. Similarly, equation 2 expresses the distance by which a point B will move on substrate 11 when it is subjected to a temperature change. In that equation, L SUB is the distance by which point B moves, K SUB is the thermal expansion coefficient of the substrate, and L O and Δ T are as defined above. Substracting equation 2 from equation 1 yields equation 3 which gives the distance D that points A moves relative to point B. This relative movement causes members 80,82, and 11 to deform which in turn produces shear stress. Equation 4 is a statement of Hook's law which says that the shear stress (SS) is equal to the shear stress modulus (G) times an angle (ϑ) by which the material is deformed. Angle ϑ is expressed via equation 5 in terms of the distance D and the thickness t of the adhesive 82 between points A and B. Substituting equation 3 and equation 5 into equation 4 yields equation 6. Inspection of equation 6 shows that it contains two variables, L O and t. All other terms in equation 6 are constant; and so it can be rewritten in simplified form as equation 7. Equation 7 states that the shear stress between heat sink 80 and substrate 11 is proportional to the distance L O and inversely proportional to the thickness t of the adhesive at that distance. Therefore, by making the adhesive 82 progressively thicker towards the perimeter of the central section, the stress is smaller throughout the central portion than it would otherwise be if the central section was flat and the adhesive between it and the substrate was uniformly thin. On the other hand, if the central portion of the heat sink was flat and the adhesive 82 was made very thick elsewhere below the central section in order to relieve thermal stresses, then the cooling ability of the heat sink would be substantially impaired. This is evident by considering the fact that the rate at which the heat is transferred from one surface to another is inversely proportional to the thickness of the material between those surfaces. In Figure 8, the adhesive 82 is very thin below the centre of the heat sink, and so the good cooling property that was previously described in conjunction with Figure 6C is maintained by placing axis 83 directly over the integrated circuit which is to be cooled. Yet another feature of the convex surface 81 is that it prevents voids from forming in the adhesive 82 during the attachment process. In that process, the adhesive solder or epoxy is heated past the liquefying temperature T HS . This causes gas bubbles to form in the adhesive, and they rise upward until they hit surface 81. Then, due to the convex shape 81, the gas bubbles move toward the periphery of the central portion where they escape into the air. By comparison, when the central portion of a heat sink is flat, the gas bubbles do not move laterally but instead they stay beneath the heat sink. This in turn produces voids between the substrate and the heat sink after the temperature is lowered below T HS . Any such void has poor heat-conducting properties, and it also generates micro cracks in the adhesive during temperature cycling. Still another feature of the convex surface 81 is that it enables the thickness of the adhesive to be reproduced in a mass production environment with a high degree of accuracy. This is because the adhesive at the central axis can easily be made very thin (i.e., within the 0.5 to 2.0 mil range) by simply having the weight of the heat sink or an external weight squash the adhesive in its liquid state. Then, the thickness of the adhesive from axis 83 to the periphery is determined solely by the convex shape 81. This thickness is made symmetrical about the axis 83 by having holes 84 punched in several of the cooling fins, and placing vertical rods in the holes to hold the heat sink upright during the attachment process. By comparison, when the central section of the heat sink is flat, the thickness of the adhesive between the heat sink and the substrate can be accurately controlled only by providing a very accurate fixture which holds the heat sink by its cooling fins above the substrate at the desired height. Also, for this fixture to work, the cooling fins which the fixture holds must be very accurately bent since if they are too high or too low, the accuracy of the fixture itself will be nullified. While the cross section of Figure 7 is shown as a modification to the Figure 5 embodiment, similar modifications can also be made to the embodiments of Figures 6A, 6B, 6C and 6D by forming the central section of those embodiments with a convex shape. Holes 84 can also be provided in some of their cooling fins so the heat sinks can readily be stood up vertically during the attachment process. In addition other heat sinks of the prior art (e.g., the heat sink of Figure 3) can be modified in accordance with this invention such that their section which attaches to the integrated circuit package has the convex shape 81 of Figure 8. Accordingly, it is to be understood that the invention is not limited to the above described detailed embodiments but is defined by the appended claims.

Claims: An integrated circuit package (11) having a flat surface and a heat sink (80) which is attached to said package solely by an adhesive (82) that is disposed between a central section of said heat sink and said flat surface, characterised in that said central section of said heat sink is convex shaped with no flat portion lying parallel to said surface, and said adhesive has a non-zero minimum thickness at a central axis of the central section and becomes progressively thicker with distance therefrom in order that thermal stress between said integrated circuit and said heat sink is reduced. An integrated circuit package according to Claim 1, characterised in that the convex portion of said central section extends outwardly by at least one twentieth of the length of half its cross section. An integrated circuit package according to Claim 1, characterised in that the convex portion of said central section has a cross section which has the curvature of a circle, and ellipse or a parbola. An integrated circuit package according to Claim 1, characterised in that said heat sink has cooling fins with a set of holes for receiving posts to prevent said heat sink from tipping as it rests on said convex central section. An integrated circuit package according to Claim 1, characterised in that said heat sink has finger-shaped portions that bend away from said central section at an angle of 60° to 90°, and thereafter bend in the opposite direction to lie parallel to said central section. An integrated circuit package according to Claim 1, characterised in that said heat sink has finger-shaped portions that bend away from said central section in a gradual, arc-like fashion. An integrated circuit package according to Claim 1, characterised in that said heat sink has finger-shaped portions, all of which terminate at the same distance from said central section. An integrated circuit package according to Claim 1, characterised in that said heat sink has finger-shaped portions that terminate in a cyclical fashion at relatively large and relatively small distances from said central section. An integrated circuit package according to Claim 1, characterised in that said heat sink has finger-shaped portions that twist along respective axes at a predetermined angle.

Inventors: LEWIS, TERRENCE EVAN, RICE, REX, SMILEY, STEPHEN ALAN
Applicants: UNISYS CORPORATION
Application year: 1986
Main classification: H01L23
IPC codes: H01L23