GB2316804A

GB2316804A - A high aspect ratio heat sink

Info

Publication number: GB2316804A
Application number: GB9716001A
Authority: GB
Inventors: Christian Belady
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1996-08-28
Filing date: 1997-07-29
Publication date: 1998-03-04
Also published as: DE19723085A1; JPH1092986A; GB9716001D0

Abstract

An air cooled heat sink 10 comprises a high aspect ratio single sheet folded into fins 12 with a base plate 16 and a top plate 18 attached to opposing sides of the fins. The folded sheet, base plate and top plate are preferably copper and they are preferably attached together by brazing. The provision of the top plate stabilises the fins which have a tendency to bend. Channels formed between the fins preferable have an aspect ratio greater than or equal to twenty. The heat sink may be attached to a substrate by any known means, possibly using the attachment holes 20, 22 and 24.

Description

2316804 IfiGH PERFORMANCE, IRGH ASPECT RATIO COPPER HEAT SINK FOR AIR

COOLING

FIELD OF T INVENTIO

The present invention relates generally to the field of semiconductor heat sinks and more particularly to a method of pushing the performance envelope of an air cooled heat sink.

BACKGROIJTM OF 3CHE INVENIffiN Generally, heat sinks are mounted to an outer surface of an integrated circuit (I.C.) package to facilitate the removal of heat from the integrated circuit contained therein. Most heat sinks are thermally conductive and have a plurality of fins to provide a large surface area, which allows heat to be more efficiently dissipated by natural or forced air flow. It is, generally desirable to increase the heat dissipating performance of a heat sink, while maintaining a relatively low cost. Typically, it has been difficult to cool a high powered semiconductor device, such as a 35 Watt semiconductor device, at a relatively low cost. It is generally advantageous to cool devices utilizing forced air, rather than exotic cooling methods such as heat pipes and liquid cooling, which are expensive in comparison. However, it is also advantageous to have a minimal impact on the flow rate (i.e., the pressure drop is such that the air flow through the heat sink is kept at a maximum), because of limitations of the fans or blowers in the system. Furthermore, the real estate on boards is very limited. Accordingly, it is preferable that heat sinks do not take up additional board space. In order to achieve these objectives, it would require a heat sink with a high height to channel aspect ratio, for example, a height to channel ratio of 20 or more.

One conventional heat sink is a stamped aluminum heat sink, which is fairly low cost.

However, stamped heat sinks are typically not high performance heat sinks, and therefore, are -I- typically only used in low power applications and are not applicable for high density, high power semiconductors.

Another conventional heat sink is an extruded aluminum heat sink, which is also relatively inexpensive, but has several performance limitations due to the manufacturing process. First, aluminum has a 50% lower conductivity than copper, and thus, is less efficient, which results in higher resistance and higher semiconductor temperatures. Second, the height to channel aspect ratio of extrusion is typically limited to 10, due to the limitations associated with the extrusion die during manufacturing. Thus, high density or "tall" heat sinks (i.e., aspect ratios greater than 10) are not feasible for extruded heat sinks. Third, the spreading resistance of the base is relatively high for concentrated heat sources, which could cause still higher semiconductor temperatures. Fourth, copper is difficult to extrude and is, thus, not considered for extruded heat sinks. Accordingly, extruded aluminum heat sinks will not generally be used in the future for high powerlhigh density cooling of semiconductors.

Yet another conventional heat sink is an aluminum bonded fin heat sink, which are is much costlier than extruded heat sinks, because each fin is individually epoxied into the base.

Although this technology allows the use of high aspect ratio fins, which generally means higher heat dissipation, it suffers three major limitations. First, the lower conductivity of the aluminum provides lower overall efficiency. Second, the bond between the fin and the base decreases the overall efficiency of the heat sink due to the relatively low conductivity of the epoxy. Third, the spreading resistance in the base is relatively high for concentrated heat sources.

With regards to the copper bonded fin heat sinks, typically this type of heat sink cost much more than the previous heat sinks because each fin is individually epoxied into the base and copper is much more expensive than aluminum. Although this technology also allows the use of high aspect ratio fins, which generally means greater heat dissipation, it suffers two major limitations. First, the cost of the heat sink is about twice that of an aluminum bonded flin heat sink. And second, the bond between the fin and the base decreases the overall efficiency of the heat sink due to the relatively low conductivity of the epoxy. This heat sink is, however, the best performing heat sink of the four conventional heat sinks described above.

Another conventional heat sink is a machined aluminum heat sink, which is more expensive than extrusions, but less expensive than bonded heat sinks, and also has several performance limitations due to the manufacturing process. First, aluminum has. a 50% lower conductivity than copper, and thus, is less efficient, which results in higher resistance and higher semiconductor temperatures. Second, the height to channel aspect ratio of machined heat sinks is typically limited to 20, due to the limitations associated with the sawing process during manufacturing. Thus, high density or "tall" heat sinks (i.e., aspect ratios greater than 20) are not feasible for machined heat sinks. Third, the spreading resistance of the base is relatively high for concentrated heat sources, which could cause still higher semiconductor temperatures. Fourth, copper is difficult to machine and is, thus, not considered for machined heat sinks. Accordingly, machined aluminum heat sinks will not generally be used in the future for high powerlhigh density cooling of semiconductors.

There is a need in the field of semiconductor heat sinks for an air cooled, high efficiency heat sink with a high performance to cost ratio that overcomes the limitations of the heat sinks of the prior art.

SUMMARY 01E THE INVENTION

It is an aspect of the present invention to provide an air cooled, high efficiency heat sink with a high performance to cost ratio that overcomes the limitations of the heat sinks of the prior art. Such a heat sink would have a low thermal resistance, capable of handling the cooling needs for high power semiconductors, and would be inexpensive. The above and other aspects of the present invention are accomplished in an air cooled, high aspect ratio, folded fin, copper heat sink, which allows a thermal performance to be achieved that is unmatched by aluminum extrusion or bonded fin heat sinks. Also, the present invention is easily and inexpensively manufactured. In addition, since copper fins are used, fin thickness can be decreased to minimize pressure drop, such that air flow through the heat sink can be maximized. Still further, a top plate may be mounted to the top of the heat sink to stabilize the copper fins, since the fins are prone to bending otherwise.

The present invention comprises an air flow, high aspect ratio heat sink for dissipating heat from an integrated circuit device within a package, said heat sink comprising of only three parts: a copper base plate having a low thermal resistance; a copper folded fin having a low thermal resistance mounted to said base plate, wherein said folded fin is a single sheet that is folded in an accordion fashion; and a top plate mounted to said folded fin such that said base plate and said top plate are on opposing sides of said folded fin, wherein air is capable of flowing through said folded fin between said base plate and said top plate. It should be noted that the top plate is only necessary if bending or damage to the fins are an issue, as the top plate is mainly used to stabilize the fins against bending.

BRIEF DE-SCRIP TION OF THE DRAWINGS 7he above and other objects, features and advantages of the present invention will be better understood by reading the following more particular description of the invention, presented in conjunction with the following drawings, wherein:

Figure 1 shows a top perspective view of the air cooled, folded fin copper heat sink according to the present invention; Figure 2 shows a side view of the air cooled, folded fin copper heat sink according to the present invention; and Figure 3 shows a top view of the air cooled, folded fin copper heat sink according to the present invention.

DETAILED DE-SCRIPHON OF THE PREFERRED EMBODIMENT Figure 1 shows a top perspective view of the air cooled, folded fin copper heat sink 10 according to the present invention. The heat sink 10 of the present invention may include folded fins 12 mounted on a base plate 16 and stabilized by a top plate 18. Heat sink 10 may be constructed of a sheet of copper that is folded accordion style at top surface 14 and bottom surface 28 to create folded, accordion type fins 12. Once the heat sink is cut and folded, it may then be mounted on the base plate 16, which may be of any high thermal performance material, but preferably of copper. The folded fin heat sink may be mounted to the base plate 16 by any known means, including epoxy or other adhesive, but preferably by means of brazing for the best thermal performance. This interface is critical for performance; thus, copper folded fins brazed to a copper base plate was found to be superior. Base plate 16 may also include a corner cut-out 26 to assist with alignment and orientation when mounting the heat sink to a semiconductor device (not shown).

Also, a top plate 18 may be mounted on top of the folded fins in order to stabilize the fins, which may tend to bend, especially if they are very thin. Top plate 18 may be of any material, but preferably of a material with low thermal resistance, and more preferably of copper. Top plate 18 may be mounted to the folded fins by any known means, since this interface has relatively little impact on the thermal performance of the heat sink. Although brazing is preferred, adhesives and epoxies are also feasible.

The heat sink 10 may be mounted to the package of a (not shown) device by any known means including any known adhesive, thermal grease or epoxy; solder; attachment by means of screws, rivets or the like through holes 20, 22, 24 and 30 in the base plate 16; attachment by means of a spring clip; or attachment by means of a heat spreader or thermal pad as taught in U.S. Patent Application Number 081617,002, entitled METHOD AND APPARATUS FOR ATTACHING A HEAT SINK AND A FAN TO AN INTEGRATED CIRCUIT PACKAGE by Timothy Schwegler, incorporated herein for all that it teaches.

Referring now to Figures 2 and 3, the dimensions of one embodiment of the present invention will be discussed. However, it should be noted that many of the dimensions may vary with the size of the semiconductor device and the specific heat dissipation needs of the semiconductor device. Base plate is approximately 1.670 inches (A) by 1. 670 inches (B) and inches thick (C). Top plate 18 is approximately 1.70 inches (D) by 1.000 inches (E) and inches thick (F).

The overall height of the heat sink 10 is approximately 1.62 inches tall (G).

Accordingly, folded fins 12 are approximately 1.50 inches tall [G - (C + F)]. Folded fins 12 are approximately.020 inches thick (H) and 1.000 inches wide (E). The beginning of one fin to the beginning of the next fin is approximately.22 inches (I). The width of one fin is approximately.13 inches (J).The last fin is cut at.07 inches (K) past the bend and then bent past 90 and mounted to the base plate at 32. Holes 20, 22, 24 and 30 in base plate 16 are approximately.125 inches in diameter, are spaced approximately 1.400 inches center to center apart (L), and are spaced approximately.135 inches (M) from the edge of base plate 16.

It should be noted that because the folded fins 12 are copper, they can be much taller than conventional heat sinks, and thus, significantly increasing the surface area, without dramatically decreasing the thermal efficiency of the heat sink. Thus, performance is significantly enhanced. The taller fins allows for a channel aspect ratio of over 20, which allows more air to move across the fins 12 for cooling, while at the same time keeping the channels between the fins wide so that the air flow rate through the fins 12 is maximized.

While the height to channel aspect ratio may be less than 20, the inventor has found that the thermal benefits start to diminish as the aspect ratio goes below 20. Also, because the fins 12 are thin and tall, they have a tendency to bend, so it improves the stability of the heat sink to attach a top plate 18 to the fins. The inventor has found that during brazing the copper annealed or softened, making the fins 12 relatively prone to bending and other damage.

Accordingly, the top plate 18 is used to stabilize the folded fins 12.

Furthermore, if the base plate 16, the folded fins 12 and the top plate are copper, thermal efficiency is maximized. The heat sink 10 is mounted to the semiconductor package (not shown) by means of solder, thermal grease or a thermal pad, the thermal resistance at this interface can be kept to a minimum. Likewise, if the copper fins 12 are attached to the base plate 16 by means of brazing, then thermal resistance at this interface is minimized. And if the top plate 18 is attached to the folded fins 12 by means of brazing, then thermal resistance at this interface is also kept to a minimum, although this interface is generally far enough away from the semiconductor device (not shown) that thermal resistance is not as big of an issue, such that other means of attachment (i.e., adhesives, epoxies, etc.) may be more practical from a cost/benefit perspective.

And finally, because the folded fins 12 are copper, they may be thinner and taller than if they were of aluminum without dramatically decreasing the overall thermal efficiency of the heat sink. Also, folded fins allow for a height to channel aspect ratio of over 20, which allows for more surface area to keep the thermal performance high, but at the same time allows for wider channels, which maximizes the flow rate through the fins or alternatively minimizes the impact to the flow rate through the fins. Accordingly, the present invention sets forth an improved air flow heat sink (i.e., lower flow resistance) that has a high thermal efficiency with a relatively low cost.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. For example, the dimensions of the folded fins or the heat sink could be modified without deviating from the concepts of the invention. Also, the base plate does not have to be oversized as shown in the figures. Moreover, different means of mounting the heat sink 10 to the package or the folded fins 12 could be utilized, which might impact the thermal resistance at that junction, but would not substantially deviate from the overall concepts of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.

Claims

CLAIM What is claimed is:

1. An air flow, high aspect ratio heat sink 10 for dissipating heat from an integrated circuit 2 device within a package, said heat sink 10 comprising:

a base plate 16 having a low thermal resistance; 4 a folded fin 12 having a low thermal resistance mounted to said base plate 16, wherein said folded fin 12 is a single sheet that is folded in an accordion fashion; and 6 a top plate 18 mounted to said folded fin 12 such that said base plate 16 and said top plate 18 are on opposing sides of said folded fin 12, wherein air is capable of flowing through 8 said folded fin 12 between said base plate 16 and said top plate 18.

2. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said base plate 2 16 is substantially copper.

3. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said folded fin 2 12 is substantially copper.

4. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said folded fin 12 2 is mounted to said base plate 16 by solder.

5. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said folded fin 2 12 is mounted to said base plate 16 by brazing.

11

6. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said top plate 18 2 is mounted to said folded fin 12 by brazing.

7. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein said base plate 2 16 and said folded fin 12 are substantially copper.

8. The air flow, high aspect ratio heat sink 10 according to claim 7 wherein said folded fin 2 12 is mounted to said base plate 16 by brazing.

9. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein the height to 2 channel aspect ratio of the folded fin 12 is approximately 20.

10. The air flow, high aspect ratio heat sink 10 according to claim 1 wherein the height to 2 channel aspect ratio of the folded fin 12 is greater than 20.