WO1998007303A1

WO1998007303A1 - Heat sink

Info

Publication number: WO1998007303A1
Application number: PCT/US1996/013037
Authority: WO
Inventors: Craig N. Johnston
Original assignee: Aavid Thermal Technologies, Inc.
Priority date: 1996-08-09
Filing date: 1996-08-09
Publication date: 1998-02-19

Abstract

A heat sink (10) comprising a sheet of thermally conducting material with a substantially uniform thickness that is bent back on itself to form two spaced-apart legs (22, 24) coupled together by a surface (26), and a pair of heat-convecting fins (28, 30), wherein the effective thickness of each of the legs is about twice the thickness of the thermally conducting material. Each of the legs (22, 24) provides a larger effective thermal flow path than that provided by the fins (28, 30). Also disclosed is a heat sink (10) that includes a pair of heat-convecting fins (28, 30) with one or more apertures (32) formed therein. A method for making the heat sink is also disclosed.

Description

HEAT SINK Background of the Invention This invention relates to heat sinks. Various heat sink designs have been proposed.

McGaha (U.S. Patent No. 5,311,395) discloses a heat sink that includes two side members, a connecting bridge, a foot on each side member for soldering to a mounting pad, a locating pin extending from the foot of each side member, and heat-dissipating fingers on each side member. Harmon (U.S. Patent No. Des. 361,317) shows a heat sink design formed from two side members joined by a center region, and having a plurality of heat-dissipating fingers that extend from the center region along edges that are orthogonal to the edges to which the side members are coupled.

Summary of the Invention In one aspect, the invention features a heat sink comprising thermally conductive legs for coupling to a heat pad, the legs being coupled together by a surface, and heat-convecting fins that are thermally coupled to the legs, wherein each of the legs provides a larger effective thermal flow path than provided by the fins.

In another aspect, the invention features a heat sink comprising a sheet of thermally conducting material with a substantially uniform thickness that is bent back on itself to form two spaced-apart legs coupled together by a surface and a pair of heat-convecting fins, wherein the effective thickness of each of the legs is about twice the thickness of thermally conducting material.

In another aspect, the invention features a heat sink comprising thermally conductive legs for coupling to a heat pad, the legs being coupled together by a surface, and heat-convecting fins thermally coupled to the legs, the fins being formed from thermally conductive material with one or more apertures formed therein.

Embodiments may include one or more of the following features. The fins preferably extend directly from the legs. The surface is preferably coupled orthogonally to the legs. Each of the fins is preferably bent so as to lie in a respective plane that is substantially parallel to the heat pad when the legs of the heat sink are coupled thereto. The legs are preferably coupled together in a manner defining sufficient space to receive a heat-generating component without direct contact.

In another aspect, the invention features a method for making a heat sink comprising the steps of: providing a sheet of thermally conductive material; bending the sheet along two parallel lines to form a surface and respective portions of a pair of legs; bending the sheet back on itself again along two parallel lines to complete formation of the pair of legs; and extending the bent back portions of the thermally conductive sheet above the surface to form a pair of heat convecting fins that provide a direct thermal path from the legs.

Embodiments may include one or more of the following features. One or more apertures are preferably provided in each of the fins. Each of the fins is preferably bent at an angle with respect to the legs. Each of the fins is preferably bent so that they each lie in a respective plane that is substantially parallel to the heat pad when the heat sink is coupled thereto.

Aspects of the invention concern surface mountable heat sinks for convecting heat from a heat-generating component that is thermally coupled to a thermally conductive heat pad. Among the advantages of the invention are the following. By providing a larger thermal flow path through the legs of heat sink, the invention increases the heat-dissipating performance of the device for a given material thickness. Apertures in the fins increase the effective surface area of the fins and thereby increase the heat-convecting performance of the fins. Furthermore, forming the fins with apertures, as opposed to finger-like structures, the invention prevents heat sinks that come into contact with each other during, e.g., a SMT manufacturing process, from becoming interlocked. The provision of the pick-and-place surface allows the heat sink to be manipulated by automated SMT equipment. Because the invention may be soldered directly to the heat pad, the same surface mount technology (SMT) used to mount electronic components on a circuit board may be used to mount the invention. The extra steps used to mount standard clip attach or epoxy attach heat sinks are not needed for mounting the invention. Thus, relative to such heat sink mounting techniques, the invention saves process time, set-up time, material costs, and labor costs. Because invention is surface mountable, it may be advantageously used where geometric tolerances of a heat generating component hinder or inhibit direct attachment of the heat sink to the component. The invention is particularly advantageous when used with heat generating components that have a shorter thermal path into the heat pad on the circuit board. Other features and advantages will become apparent from the following description and from the claims. Brief Description of the Drawings Fig. 1 is a diagrammatic exploded view of a surface mountable heat sink, a heat generating component, a thermally conductive heat pad, and a circuit board. Fig. IA is a diagrammatic side view of the surface mountable heat sink of Fig. 1.

Fig. IB is a diagrammatic top view of a surface mountable heat sink of Fig. l.

Description of the Preferred Embodiments As shown in Fig. 1, a surface mountable heat sink 10 fits over a heat generating component 12 (e.g. , a D2PAK power transistor or a D3PAK power transistor) , both of which are mounted onto a thermally conductive heat pad 14 on a circuit board 16. Heat sink 10 defines sufficient space 18 to receive component 12 so that the heat sink does not directly contact the component. Component 12 has several leads 20 that extend beyond the ends of heat sink 10 and couple to respective circuit pads 21 on circuit board 16. Referring to Fig. IA, heat sink 10 includes a pair of thermally conductive legs 22, 24 that couple to heat pad 14 (Fig. 1) and extend away from the heat pad when the heat sink is soldered to the heat pad. Heat sink 10 also includes a pick-and-place surface 26 that is disposed between and coupled to each of the legs. As mentioned above, the two legs and the pick-and-place surface define sufficient space 18 so that the heat sink does not directly contact the heat generating component. A pair of heat-convecting fins 28, 30 are formed from extensions of legs 22, 24, respectively. The fins provide a thermal path from the legs to ambient atmosphere. The top portions of fins 28, 30 are bent at an angle of 90° with respect to the legs. As shown in Fig. IB, the fins include one or more apertures 32, which increase the effective heat-convecting surface area of the fins.

Referring back to Fig. 1, heat sink 10 is formed into a unitary structure from a single piece of thermally conductive material. The sheet is bent along two parallel lines 34, 36 to form pick-and-place surface 26 and the inner portions 38, 40 of legs 22, 24, respectively. The sheet is bent back on itself along parallel lines 42, 44 to complete the formation of the legs and to form fins 28, 30 by extending the outer portions 46, 48 of legs 22, 24 above pick-and-place surface 26. The sheet is bent along parallel lines 50, 52 at an angle of 90° to form the top portions of the fins. Apertures 32 are cut out of the sheet to increase the effective heat-convecting surface area of the fins. Thus, legs 22, 24 are formed from a single sheet of thermally conductive material that is bent back on itself. By this construction, each of the legs has an effective thickness (T') that provides a thermal flow path from heat pad 14 that is larger than that provided by a single thickness (T) of the material from which the heat sink is formed, increasing the effectiveness of the heat sink in convecting heat away from the head pad.

In a presently preferred embodiment, designed for D2PAK components, the spacing between inner portions 38, 40 of the legs is 0.5 inch and the height from the bottom of the legs to the underside of the pick-and-place surface is 0.29 inch. The distance from the outer edge of one fin to the outer edge of the other fin is 1.03 inch and the height of the underside of the fins above the heat pad is at least 0.36 inch. The distance of the end of the fin apertures to the outer edge of the fins is 0.05 inch. The copper stock is 0.025 inch thick and the tin/lead plating has a maximum thickness of 0.0001 inch. In a presently preferred embodiment, designed for D3PAK components, the spacing between inner portions 38, 40 of the legs is 0.69 inch and the height from the bottom of the legs to the underside of the pick-and-place surface is 0.30 inch. The distance from the outer edge of one fin to the outer edge of the other fin is 1.22 inch and the height of the underside of the fins above the heat pad is at least 0.36 inch. The distance of the end of the fin apertures to the outer edge of the fins is 0.05 inch. The copper stock is 0.025 inch thick and the tin/lead plating has a maximum thickness of 0.0001 inch.

The heat sinks of each of the above embodiments can be manufactured using a progressive stamping die in a discrete, individual format or in a continuous strip format which can be rolled up onto a reel. In a presently preferred embodiment, the heat sinks are formed from copper stock that has been tinned for solderability with 63/37 tin/lead, either before or after forming. This allows the heat sink to be attached to the heat pad in the same manner as electronic components in many SMT processes. The heat sinks can be plated by conventional techniques, e.g., by electrodeless tin plating in continuous strip format or by barrel plating in discrete format; the heat sink may also be manufactured from pre- tinned copper stock. The continuous strip format can be readily placed in tape-and-reel package format for delivery to automatic pick-and-place equipment, and can be easily integrated into an automated SMT line that uses such pick-and-place equipment. Other embodiments are within the scope of the claims.

What is claimed is:

Claims

CLAIMS: - 7 -

1. A heat sink comprising thermally conductive legs for coupling to a heat pad, the legs being coupled together by a surface, and heat-convecting fins thermally coupled to the legs, wherein each of the legs provides a larger effective thermal flow path than provided by the fins.

2. The heat sink of claim 1 wherein each of the legs is formed from thermally conductive material that is bent back on itself.

3. The heat sink of claim 1 wherein the fins extend directly from the legs.

4. The heat sink of claim 1 further comprising a surface coupled between the legs.

5. The heat sink of claim 4 wherein the legs, the surface, and the fins are formed into a unitary structure from the same piece of thermally conductive material.

6. The heat sink of claim l wherein each of the fins is bent so as to lie in a respective plane that is substantially parallel to the heat pad when the heat sink is coupled thereto.

7. The heat sink of claim 1 wherein the legs are coupled together in a manner defining sufficient space to receive a heat-generating component without direct contact.

8. A heat sink comprising a sheet of thermally conducting material with a substantially uniform thickness that is bent back on itself to form two spaced- apart legs coupled together by a surface, and a pair of heat-convecting fins, wherein the effective thickness of each of the legs is about twice the thickness of thermally conducting material.

9. The heat sink of claim 8 wherein each of the fins has one or more apertures formed therein.

10. The heat sink of claim 8 wherein each of the fins is bent at an angle of about 90° with respect to the legs.

11. A heat sink comprising thermally conductive legs for coupling to a heat pad, the legs being coupled together by a surface, and heat-convecting fins thermally coupled to the legs, the fins being formed from thermally conductive material with one or more apertures formed therein.

12. The heat sink of claim 11 wherein each of the fins is bent at an angle with respect to the legs.

13. The heat sink of claim 12 wherein each of the fins is bent so as to lie in a respective plane that is substantially parallel to the heat pad when the legs of the heat sink are coupled thereto.

14. The heat sink of claim 11 wherein each of the legs provides a larger effective thermal flow path than that provided by the fins.

15. The heat sink of claim 11 wherein each of the legs is formed from thermally conductive material that is bent back on itself.

16. The heat sink of claim 11 wherein the fins extend directly from the legs.

17. The heat sink of claim 11 wherein the surface is coupled orthogonally to the legs.

18. The heat sink of claim 17 wherein the legs, the surface, and the fins are formed into a unitary structure from the same piece of thermally conductive material.

19. The heat sink of claim 11 wherein the pair of legs are coupled together in a manner defining sufficient space to receive a heat-generating component without direct contact.

20. A method for making a heat sink comprising the steps of: providing a sheet of thermally conductive material; bending the sheet along two parallel lines to form a surface and respective portions of a pair of legs; bending the sheet back on itself again along two parallel lines to complete formation of the pair of legs; and extending the bent back portions of the thermally conductive sheet above the surface to form a pair of heat convecting fins that provide a direct thermal path from the legs.

21. The method of claim 20 further comprising the step of providing one or more apertures in each of the fins.

22. The method of claim 21 further comprising the step of bending each of the fins at an angle with respect to the legs.

23. The method of claim 15 further comprising the step of bending each of the fins so that they each lie in a respective plane that is substantially parallel to the heat pad when the heat sink is coupled thereto.