JPH0337309B2 - - Google Patents

Description

BACKGROUND OF THE INVENTION This invention relates to integrated circuit packaging and cooling technology.

One commonly used prior art integrated circuit package is illustrated in cross section designated by the reference numeral 10 in FIG. This package 1
Some of the main components of 0 are a substrate 11, a lid 12, a plurality of conductive leads 13, an integrated circuit chip 14, etc.

As shown in FIG. 1, the substrate 11 has a rectangular cross section. It also has an upper major surface 11a and a lower major surface 11b. The surface 11a has a depression in its center, and the depression is formed to receive the chip 14. Surface 11
b is just a plane in comparison.

The plurality of conductors 11d are arranged within the substrate 11 between the periphery of the recess 11c and the periphery of the surface 11a. An electrical connection is formed between the conductor 11d and the chip 14 by a plurality of bonding wires 15 around the recess 11c. At the periphery of surface 11a, electrical conductors 11d connect directly to leads 13, which in turn extend from surface 11a to form electrical connections to external systems (not shown).

The lid 12 covers the recess 11c and is fixedly attached to the surface 11a by a lid attachment material 16 around the recess. This capping material 16 forms a hermetic seal for the chip 14 with the cap 12 and substrate 11.

A specific example of the materials and dimensions of the circuit package described above is as follows: the substrate 11 is made of ceramic;
0.950 inch length, 0.950 inch width, and
It has a height of 0.060 inches. Chip 14 is primarily formed from silicon;
0.300" length, 0.300" width, and
It has a thickness of 0.020 inches. Lid 12 is made of ceramic; and it has a length of 0.580 inches, a width of 0.580 inches, and a thickness of 0.030 inches. Additionally, the lid attachment material 16 is formed from a layer of glass having a thickness of 0.002 inches.

When the chips 14 in the package 10 are of the type that use relatively little power (eg, less than 1 watt), there is no need to install a heat sink on the package. However, as the amount of power used by the chip 14 increases, a point will eventually be reached where a heat sink must be attached to the package to ensure that the chip 14 does not overheat.

Conventionally, the heat sink is formed of a metal such as copper or aluminum; and it is fixedly attached to the underlying surface 11b of the chip 14 by epoxy or solder. During the attachment process, the epoxy or solder is heated to a fluid state and spread into a thin smooth layer between surface 11b and the heat sink. The epoxy or solder is then cooled and solidified.

However, this cooling and hardening step induces stresses in the package, particularly in the lid attachment material 16. The magnitude of these stresses will vary depending on the overall shape of the particular heat sink being installed. Also, depending on the shape of the heat sink, the stress may be large enough to cause cracks in the lid attachment material 16. When this occurs, the hermetic seal for chip 14 is broken and the package is rendered inoperable.

To further understand how the heat sink induces stresses in the lid attachment material 16, reference should now be made to FIG. The figure shows a graph in which the temperature of the integrated circuit package is plotted on the horizontal axis and the stress within its lidding material 16 is plotted on the vertical axis. In this graph, curve 21 shows how the stress in the lid attachment material 16 varies as a function of temperature under the condition that no heat sink is attached to the package 10;
Another curve 22 shows how the stress in the lid attachment material 16 changes under the conditions that the heat sink is attached to the package 10.

Curves 21 and 22 begin at temperature T _LID , which is the temperature at which the lid fitting material solidifies. For example, when the lid mounting material is glass, the temperature T _LID is about 320°C. In order to attach the lid 12 to the surface 11a, the lid attachment material must be heated above a temperature T _LID , typically it is heated to 420°C. The package is then cooled to room temperature T _RT .

During this cooling, both substrate 11 and lid 12 adhere. However, this assembly requires mounting material 1
6 only induces relatively small stresses within.
This is because lid 12 and substrate 11 are both made of essentially the same material and therefore they will shrink at approximately equal rates.

Package 10 is then reheated to attach the heat sink to surface 11b. In this step, the temperature at which the package is heated is equal to the freezing temperature of its heat sink mounting material, T _HS
Must be above. For example, the temperature for solder is about 183°C and for epoxy it is about 150°C.

As long as the heat sink attachment material remains liquid, the stresses induced in the lid attachment material 16 remain relatively small. However, once the heat sink mounting material solidifies at temperature T _HS ,
The stress within the lid attachment material 16 increases rapidly.
This rapid increase in stress is due to the fact that the heat sink shrinks much faster than the ceramic substrate. For example, the coefficients of thermal expansion for copper and aluminum are approximately 2.6 and 3.6 times that of ceramic, respectively.

As the heat sink contracts, it shrinks from the surface 11b to which it is attached.
tends to compress that part of the This then bends the substrate 11 into an arc as shown in FIG. In this figure, the amount of bending is
It is shown greatly exaggerated only to illustrate that such bending actually occurs.
This bending then causes a rapid increase in stress within the lid attachment material 16.

After the heat sink is attached and the integrated circuit is placed in an operating environment, the package is exposed to a predetermined range of operating temperatures. A typical maximum operating temperature is, for example, 125°C; and a typical minimum operating temperature is -55°C. Such maximum and minimum operating temperatures are indicated in FIG. 2 as T _MAX and T _MIN , respectively.

Curve 22 shows the lid mounting material 1 at temperature T _MAX
6 is shown to be at a minimum level S _MIN ; while at temperature T _MIN the stress in the lid attachment material 16 is shown to be at a maximum level S _MAX . In such an operating environment, the stress within the lid attachment material varies between S _MAX and S _MIN . Moreover, the magnitude of the maximum stress S _MAX and the frequent repetition between S _MAX and S _MIN cause cracks in the lid attachment material.

Furthermore, as the heat sink mounting material solidifies at temperature T _HS , stresses begin to develop at the interface between the heat sink and the substrate 11. This stress, similar to the stress in the lid mounting material,
This is due to the fact that it contracts much more rapidly than the substrate 11.

When the temperature decreases from T _HS to T _RT , the stress at the substrate-heat sink interface increases rapidly. The heat sink and integrated circuit package are then placed in an operating environment and at a temperature
When cycled between T _MIN and T _MAX ,
The stress at the interface between the substrate and the heat sink is approximately cycled to its maximum value. Both the magnitude of its maximum value and the degree of cycling therefrom can cause cracks between the heat sink and the substrate 11.

SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide an improved heat sink for integrated circuit packages.

Another object of the invention is to provide a heat sink that reduces the magnitude of thermomechanical stresses induced within an integrated circuit package when the package and heat sink are bonded and subjected to thermal cycling. It is to provide a sink. Yet another object of the invention is to provide a heat sink that not only reduces the occurrence of cracks in integrated circuit packages to which the heat sink is attached, but also has superior cooling properties and is easy to manufacture. That's true.

In one embodiment of the invention, these and other objects are achieved by a heat sink formed from a single thin sheet of material having two opposing major surfaces. These major surfaces have a common perimeter; the common perimeter forms spaced apart fingers of the heat sink that extend radially from the central portion. There is. The central portion is flat and attached to the integrated circuit package; while the fingers are bent out of the plane of the central portion to form cooling fins for the central portion.

Another embodiment of the invention is a sheet sink comprised of a single thin sheet of material having two opposing major surfaces with a common perimeter. However, this embodiment also has a plurality of radially oriented elongated holes with end walls at a predetermined distance from a central point within the periphery. The end walls near the center point form the central part of the heat sink; the end walls far from the center point, together with their common perimeter, form a ring-shaped part around the center part; The side walls of the hole form a plurality of fingers connecting the ring-shaped portions from the central portion. The central portion is flat and attached to the integrated circuit package; while the fingers are bent in one direction from the plane of the central portion to provide a means for cooling the central portion together with the ring portion. .

Various features and advantages of the invention are set forth in the detailed description in conjunction with the accompanying drawings.

A further embodiment of the invention is obtained by modifying both of the embodiments described above, such that the central portion of the heat sink is of convex shape.
With this modification, the adhesive that attaches the central portion of the heat sink to the integrated circuit package must be thinner in the center and gradually thicker toward the periphery of the central portion. As a result, thermally induced stresses at the interface between the integrated circuit package and the heat sink are substantially reduced, heat transfer between the integrated circuit package and the heat sink remains high, and the adhesive has a convex shape. Since it is fixed by shape, no fittings are required to set the adhesive thickness.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGS. 4 and 5, one preferred embodiment of the invention will now be described in detail. This particular embodiment is designated by the reference numeral 30 in FIGS. 4 and 5. FIG. 4 depicts a heat sink 30 shown attached to surface 11b of integrated circuit package 10 of FIG. On the other hand, the fifth
The figure is a top view of the heat sink 30, showing an intermediate stage in the manufacturing process.

Heat sink 30 is comprised of a single thin sheet of material having two opposing major surfaces 31a and 31b. Surfaces 31a and 31b have a common perimeter 32. The periphery 32 is the central part 33
forming a plurality of spaced apart fingers 34 of the heat sink extending radially from the heat sink.

In the preferred embodiment shown in FIGS. 4 and 5, there are eight fingers, numbered 34-1 through 34-8; they are equally spaced around the central portion. However, alternatively, the number of fingers may be any other number greater than or equal to 8, preferably in the range 4 to 18.

Heat sink 30 is manufactured from a single rectangular thin flat sheet of material. The rectangular material is punched out by a cutting device having a perimeter 32 as described above. FIG. 5 shows the flat sheet material after this punching.

Thereafter, all the fingers are formed into a cylindrical shape, with the central portion 33 at one end of the cylinder; and the tips of the fingers are bent outward like a fan. , in one or more planes parallel to the central portion 33.
Preferably, these steps are on each finger.
This is done by making two bends of 60° to 90°. Due to these bends, the fingers 34-1 to 34-8 form cooling fins for the central portion 33. This is shown in FIG.

Preferably, the distance between two bends of adjacent fingers varies periodically around the central portion 33. In other words, preferably the heights of the open ends of the cooling fins are staggered periodically around the central portion. This height change creates a disturbance in the airflow through the cooling fins, which increases the cooling capacity of the heat sink.

More preferably, a plurality of holes 35 are provided in the central portion. These holes strengthen the bond between the heat sink and the package to which it is attached. In particular, the holes provide a means by which gas can escape between the heat sink mounting material and the mounting surface of the integrated circuit package, and reduce voids within the heat sink mounting material. Additionally, the holes provide a means for heat sink attachment material to seep out of the central portion and adhere to the surface 31a surrounding the holes.

One specific example of materials and dimensions for the above-described embodiments of the invention is as follows. Heat sink 30 is formed from a metal sheet, such as copper or aluminum, that is 0.015 to 0.045 inches thick. From such a sheet a heat sink is stamped in the circumferential shape of FIG. Preferably, the radius of the central portion is 0.200 inches;
Finn 34-1, 34-3, 34-5, and 3
The distance between the two bends in 4-7 is 0.060 inch,
On the other hand, the length of the rest of those fins is
0.220 inch; fins 34-2, 34-4,
The distance between the two bends 34-6 and 34-8 is
0.150 inch while the length of the rest of their fins is 0.130 inch;
5 are evenly spaced and have a radius of 0.020 inch.

Reference should now be made to FIGS. 6A, 6B, and 6C, in which various test results for the above-described heat sinks of FIGS. 4 and 5 are given in three tables. There is. These tables also include other test results for prior art heat sinks for comparison. Figure 6A shows the results of the temperature cycle test; Figure 6B shows the results of the thermal shock test; and Figure 6C shows the results of the cooling test.

Referring first to the table of FIG. 6A, the table has seven columns 40-46 and three rows. The rows represent the heat sinks in the tests of "Embodiment of FIG. 4,""Prior Art Embodiment of FIG. 3," and "Embodiment of FIG. 7C," while the columns represent the various heat sinks. test data is shown. In the following discussion, the embodiment of FIG. 4 is compared to the prior art. The embodiment of FIG. 7 (another embodiment of the invention) is then compared with the prior art.

In performing the temperature cycling test of Figure 6A, 20 heat sinks of the type of Figure 4 were
Attached to each integrated circuit package of the type shown; twenty prior art heat sinks of the type of FIG. 3 were attached to similar integrated circuit packages. A heat sink manufactured and sold by Thermalloy Corporation was selected as the prior art heat sink of FIG. Because it has very interesting cooling properties with omnidirectional airflow. Columns 42-46 in FIG. 6 show all physical parameters for the heat sink of FIG. 4 and the heat sink of FIG. 3 that are relevant to temperature cycling tests.

Two independent chambers are prepared for temperature cycling testing. One of these chambers is filled with air at a temperature of 125°C;
The other chamber is filled with air at -55°C. The integrated circuit packages with their respective attached heat sinks are transferred cyclically between these two chambers; It is left in the chamber for 15 minutes.

Looking at column 41, after 200 cycles,
It can be seen that none of the twenty heat sinks of FIG. 4 caused any cracks in the integrated circuit packages to which they were attached. In contrast, after only 80 cycles, all 20 of the prior art heat sinks of FIG. 3 developed cracks in the package lid mounting material to which they were attached.

To detect the presence or absence of cracks in the lid fitting material, the tested parts are immersed in radioactive gas. If a crack is present, some of the radioactive gas will be trapped within the depression 11c. The portion is then removed from the gas and the presence or absence of any captured molecules is detected by a Geiger counter. Therefore,
The above test is very accurate.

Next, consider the results of the thermal shock test in the table of Figure 6B. In that table, the rows as before identify the heat sinks tested and the columns show specific test data. It should be appreciated that in the thermal shock test of Figure 6B, all physical parameters of the heat sink tested are the same as those shown in columns 42-46 in Figure 6A for the temperature cycling test. .

To perform the thermal shock test, as before,
Two chambers are prepared; but this time they are filled with liquid. In one of the chambers, the liquid is at a temperature of 125°C; in the other chamber, the liquid is at a temperature of -55°C.
As before, integrated circuit packages, each with an attached heat sink, are periodically transferred from chamber to chamber. Placed in one chamber for 5 minutes, then placed in the other chamber for 5 minutes.

Column 50 in FIG. 6B indicates that 20 heat sinks of the FIG. 4 embodiment were subjected to a thermal shock test. These heat sinks are
As before, it was attached to an integrated circuit package of the type shown in FIG. Column 51 shows that after 15 cycles none of the heat sinks of the type of FIG. 4 caused any cracks in the lids of the integrated circuit packages to which they were attached.

This thermal shock test is a much more severe test than the temperature cycling test described above. Therefore, no heat sink of the type shown in FIG. 3 has been subjected to thermal shock testing. Because they couldn't even pass the temperature cycle test.

Let us now consider the results of the cooling test shown in the table of Figure 6C. In this table, as before, the particular heat sink tested is shown in its row; the various test data are shown in columns 60-69.

Columns 64 and 65 indicate that the heat sinks being tested have the same overall height and width. This is necessary to make a fair comparison because the cooling capacity of a heat sink generally increases as its size increases. Additionally, rows 68 and 69 have their respective heat sinks formed of the same material and via the same mounting material.
Please note that it shows that it is attached to the IC package cage.

Ten heat sinks of the type shown in Figure 4 are placed in the third
Tested with 10 heat sinks of the prior art type shown in the figure. Column 61 shows the average thermal resistance for all ten tested heat sinks of the same type, along with the corresponding standard deviation from that average thermal resistance. Looking at column 61, we see that the average thermal resistance of the type of heat sink of FIG. 4 is greater than that of the prior art type of heat sink of FIG.
It can be seen that the heat sink of FIG. 4 has a small standard deviation, which is within 0.8° C./W of the average thermal resistance of the sink. This means that both heat sinks have approximately the same cooling capacity.

Next, in summary, the test results of Figures 6A, 6B, and 6C tend to significantly reduce thermally induced stress in integrated circuit packages without sacrificing cooling capacity. This shows the superiority of the heat sink of the type shown in FIG. Furthermore, the heat sink of FIG. 4 cools in all directions, which is of course important. This is because it allows the integrated circuit packages to which the heat sinks are attached to be positioned and mounted on the printed circuit board in several different orientations without interfering with their cooling. Furthermore, the heat in Figure 4
The sink is easy to manufacture as described in connection with FIG. Their manufacturing steps only include stamping and folding operations; in contrast, the prior art heat sink of FIG. 3 must be cut from a rotating base material on a rotating device.

Turning now to FIGS. 7A-7D, various alternative preferred embodiments of the invention will be described. Starting with FIG. 7A, the embodiment shown therein is similar to the previously described embodiment of FIG. have a common periphery forming a . However, the embodiment of FIG. 7A has a total of twelve fingers (versus eight fingers in the embodiment of FIG. 4). moreover,
In the embodiment of Figure 7A, all of the fingers are bent so that their open ends are at the same height above the central part. Next, consider the embodiment of FIG. 7B. It is also similar to the previously described embodiment of FIG. However, it differs in the embodiment of FIG. 4 in that the open ends of the fingers are all twisted about their axes at a predetermined angle. Preferably this angle is in the range 0-45°. The twisted open ends of these fingers create turbulence in the air passing through them, thus increasing the cooling capacity of the heat sink.

Next, consider the embodiment of FIG. 7C. It has a total of twelve fingers around its central portion, and in addition, the fingers are bent in an arc from the plane of its central portion. This is different from the example.

Finally, the embodiment of FIG. 7D is fourth in that the common periphery of its heat sink does not form either a central portion or a finger portion.
This is different from the embodiment shown in the figure. Instead, the embodiment of FIG. 7D has a plurality of radial elongated holes, each of which has a pair of holes at two predetermined distances from a central point within a common perimeter. It has an end wall. These end walls closer to the center point are
forming the central part of the heat sink;
Those end walls remote from the central point form a ring-shaped section around the central part with a common perimeter. Furthermore, the side walls of the hole connecting the end walls form a plurality of fingers connecting the central part and the ring-shaped part. The central portion is flat and attached to the integrated circuit package; while the fingers are bent out of the plane of the central portion and together with the ring portion they provide a means for cooling the central portion.

The embodiments of FIGS. 7A-7D described above all involve preparing a thin rectangular metal sheet; and punching a heat sink from the sheet by a cutting device having a perimeter of the desired heat sink shape. Cut out the punched part;
This is done by bending as shown in Figure D.

The test results for the embodiment of FIG. 7C are shown in FIG. 6A.
6B and 6C. Row 41 of FIG. 6A shows 30 heat sinks of the type of FIG. 7C that have been temperature cycled 200 times;
They show that no cracks occurred in the IC package. Column 51 of FIG. 6B is
Thirty heat sinks of the type of Figure 7C were subjected to 15 thermal shocks; as before, none of them showed any cracks in the IC package. Column 60 in Figure 6C is 7C.
Ten heat sinks of the type shown have been tested for their thermal cooling capacity and are shown to have a lower average thermal resistance than the prior art heat sink of FIG.

Referring to FIG. 8, yet another embodiment of the invention will be described. This embodiment is shown in cross-section in FIG. 8 and is designated by the reference numeral 80.
Heat sink 80 has cooling fins shaped similarly to the cooling fins of heat sink 30 of FIGS. 4 and 5. However, an improved feature of the heat sink 80 is that its central portion has a convex shape, as indicated by the reference numeral 81. Appropriately, this convex shape is
It is obtained by squeezing and placing the punched part of FIG. 5 between a convex member and a concave member.

Because of the convex shape 81, the adhesive 82 attaching the heat sink 80 to the substrate 11 is thin at the axis 83 of the heat sink and becomes progressively thinner as the distance from the axis 83 increases. Preferably, the adhesive 82 is 0.5 to 2.0 mil thick at the shaft 83 and the convex surface 81 is such that it extends along the shaft 83 by at least one-twentieth the length of half its cross-section.
So much so that it swells outward. For example, adhesive 8
If the edge of 2 is 125 mils from axis 83, its thickness must be at least 6.0 mils. This may be achieved by making the cross-section of surface 81 part of a circle, ellipse, or parabola.

One feature of the convex shape 81 is that thermal stresses are reduced between the substrate 11 and the heat sink 80, while at the same time good heat transfer between those components is maintained. This is clear from the equation in FIG. Equation 1
represents the distance that point A on surface 81 moves when subjected to a temperature difference. In this equation, △L _HS is the distance that point A moves and K _HS is the distance that the heat sink 80
is the coefficient of thermal expansion of , L ₀ is the distance of point A from axis 83 before the temperature change, and ΔT is the temperature change.

Similarly, Equation 2 represents the distance that point B moves on substrate 11 when undergoing a temperature change.
In that equation, L _SUB is the distance that point B moves, K _SUB is the coefficient of thermal expansion of the substrate, and L ₀ and ΔT are as defined above.

Subtracting Equation 2 from Equation 1 yields Equation 3, which gives the distance D that point A moves relative to point B. This relative movement causes members 80, 82, and 1
1 deforms and also generates shear stress. Equation 4
is a concept that expresses Hook's law, which states that shear stress (SS) is equal to the shear coefficient (G) times the angle at which the material is deformed (○-).

The angle ◯ is expressed in terms of distance D and thickness t of adhesive 82 between points A and B via Equation 5. Substituting Equation 3 and Equation 5 into Equation 4 yields Equation 6. Inspection of Equation 6 shows that it involves two variables, L ₀ and t. All other terms in Equation 6 are constants. And so it can be simplified and rewritten as Equation 7.

Equation 7 shows that the shear stress between the heat sink 80 and the substrate 11 is proportional to the distance L ₀ ,
This shows that the thickness of the adhesive at that distance is inversely proportional to t. Therefore, by making the adhesive 82 progressively thicker towards the periphery of the central section, the stresses will be reduced if the central section is flat and the adhesive between the central section and the substrate is uniformly thin. , smaller throughout the central section.

On the other hand, the central part of the heat sink is flat;
And if the adhesive 82 were made too thick everywhere below the central portion to reduce thermal stress, the cooling ability of the heat sink would be substantially compromised. This is clear in view of the fact that the rate at which heat is transferred from one surface to another is inversely proportional to the thickness of the material between those surfaces. In FIG. 8, the adhesive 82 is very thin below the center of the heat sink and is therefore
The good cooling properties described above in connection with diagram C are
This is maintained by placing the shaft 83 directly on the integrated circuit 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 the process, adhesive solder or epoxy is heated above its liquefaction temperature T _HS . Gas bubbles therefore form in the adhesive and they rise upward until they hit the surface 81. Then, because of the convex shape 81, the gas bubble moves towards the periphery of the central part where it leaks into the air.

By comparison, when the central portion of the heat sink is flat, the gas bubbles do not move laterally and instead remain below the heat sink. This also creates voids between the substrate and the heat sink after the temperature is lowered below T _HS . Such voids have poor thermal conductivity properties and also cause microcracks in the adhesive during temperature cycling.

Yet another feature of the convex surface 81 is that it allows the adhesive thickness to be reproduced with a high degree of accuracy in a mass production environment. This means that the adhesive at the central axis can easily become very thin (i.e. 0.5 to 2.0 mils) by allowing the weight of the heat sink or external weight to simply collapse the adhesive in its liquid state. within range)
can be made into The thickness of the adhesive from the axis 83 to the periphery is then simply determined by the convex shape 81. This thickness is made symmetrical about axis 83 by drilling holes 84 in some of the cooling fins and placing vertical rods in the holes to hold the heat sink straight during the installation process. be made into

By comparison, when the central portion of the heatsink is flat, the thickness of the adhesive between the heatsink and the substrate is very precise to hold the heatsink at the desired height above the substrate by its cooling fins. Accurate control can only be achieved by providing suitable fittings. Also, for this fixture to work, the cooling fins that the fixture holds must be bent very precisely, as if they are too high or too low, the precision of the fixture itself will be negated. .

Various preferred embodiments of the invention have been described in detail. However, in addition, many changes and modifications may be made to these embodiments without departing from the nature and spirit of the invention.
For example, the cross-sectional view of FIG. 8 is shown as a modification of the embodiment of FIG. 5, but the embodiment of FIGS. A similar modification can also be made by forming it to have a convex shape. Holes 84 can also be provided in some of the cooling fins so that the heat sink can be easily stood upright during the installation process.
Additionally, other heat sinks of the prior art (such as the heat sink of FIG. 3) may be modified in accordance with the present invention such that those portions that attach to the integrated circuit package have the convex shape 81 of FIG. can.

It is therefore to be understood that the invention is not limited to the detailed embodiments described above, but is instead defined by the claims appended hereto.

[Brief explanation of the drawing]

FIG. 1 is an enlarged cross-sectional view of a prior art integrated circuit package suitable for use with the heat sink of the present invention. FIG. 2 is a graph illustrating how a prior art heat sink induces stresses in the lid mount material of the integrated circuit package of FIG. FIG. 3 is a schematic diagram showing how a prior art heat sink bends the integrated circuit package of FIG. FIG. 4 is a pictorial diagram of a preferred embodiment of a heat sink formed in accordance with the present invention. FIG. 5 is a plan view of the heat sink of FIG. 4, showing an intermediate stage in its manufacture. 6A-6C are tables containing test data demonstrating several features of the present invention over the prior art. 7th A
Figures 7D are pictorial illustrations of additional preferred embodiments of the invention. FIG. 8 shows the cooling fins 34 of FIG. 4 showing a modification of the heat sink of FIG.
It is a sectional view through -6 and 34-1. 9th
Figure 1 explains the features of the heat sink in Figure 8.
It is a figure which shows a set of equations. In the figure, 10 is an integrated circuit package; 11 is an integrated circuit package;
is the substrate, 11a and 11b are the surfaces,
11c is a hollow, 11d is a conductor, 12 is a lid, 13
14 is a conductive lead, 14 is an integrated circuit chip, 15 is a bonding wire, 16 is a lid mounting material, 30 and 80 are a heat sink, 31a and 31b are a surface, 32
is the periphery, 33 is the central portion, 34 is the finger-like portion, 35
84 is a hole, 81 is a convex surface, 82 is an adhesive, and 83 is an axis of the heat sink.

Claims (1)

Claims: 1. A heat sink for cooling an integrated circuit package, comprising a thin sheet of material having two oppositely oriented major surfaces, the major surfaces extending from a central portion and from the central portion. having a common periphery defining a plurality of radially extending spaced fingers, the fingers being configured to extend outwardly from the central portion to provide cooling fins for the central portion; and wherein the central portion has a convex shape for attachment to a flat surface on the integrated circuit package with an adhesive of non-uniform thickness. 2. The heat sink of claim 1, wherein the convex shape of the central portion bulges outward by at least one-twentieth of the length of half its cross section. 3. The convex shape of the central portion has a cross section that is curved like a circular, elliptical, or parabolic section;
A heat sink according to claim 1. 4. The heat sink of claim 1, wherein the cooling fins have a set of holes for receiving posts to prevent the heat sink from tipping over when it is on the convex central portion. 5. The heat sink of claim 1, wherein the fingers bend outward from the central portion at an angle of 60°-90° and then turn in the opposite direction to be parallel to the central portion. 6. The heat sink of claim 1, wherein the finger-like portions gradually curve outward in an arc from the central portion. 7. The heat sink of claim 1, wherein all of the fingers terminate at the same distance from the central portion. 8. said finger portions periodically terminate at relatively long and relatively short distances from said central portion;
A heat sink according to claim 1. 9. The heat sink of claim 1, wherein the fingers are twisted along their respective axes at a predetermined angle. 10 A flat substrate having flat top and bottom surfaces, the top surface having a recess in which a semiconductor chip is disposed, covering the recess and the semiconductor chip, and covering the recess and the semiconductor chip by an encapsulant. further comprising a lid sealed to a top surface and a heat sink attached to the bottom surface by adhesive, the heat sink having a central portion and a plurality of spaced fingers extending radially from the central portion. the central portion is a small portion of the heat sink that is attached to the bottom surface by the adhesive, and the finger portions are formed to have a curved outwardly from the bottom surface to provide cooling fins for
The integrated circuit package wherein the central portion has a convex shape and the adhesive is gradually thicker toward the periphery of the central portion. 11. An integrated circuit package comprising a substrate, the heat sink having a central portion attached to said package by adhesive, said portion having a length of at least 20 mm over half its cross-section.
An integrated circuit package having a convex shape that bulges outward by a factor of 1, and wherein the adhesive is thicker around the central portion than at the center.