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Elimination Of Thermal Face Warpage Of Soldered Fin-in-groove Heat Sinks

IP.com Disclosure Number: IPCOM000012647D
Original Publication Date: 2003-May-19
Included in the Prior Art Database: 2003-May-19
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Abstract

Large copper heat sinks for cooling electronics hardware, such as the multi-chip module (MCM), are a challenge to manufacture. The dense fin geometry allows few, cost effective, manufacturing options but to solder the copper fins into grooves cut in a copper heat sink base. The problem with the soldered fin-in-groove design is that it is not possible to maintain a flat thermal face required for good thermal interface between the heat-sink base and the MCM cover. This invention overcomes the heat-sink thermal face warpage problem by using a solder that has a thermal coefficient of expansion (TCE) that is slightly lower than the TCE of copper (=17 ppm per degree C). A simple change of the solder material from the usual Pb-Sn solder (TCE = 25 ppm per degree C) to one with a TCE of 12-15 ppm per degree C changes the thermal face from a thermally undesirable concave shape to a much more desirable convex shape.

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Elimination Of Thermal Face Warpage Of Soldered Fin-in-groove Heat Sinks

  Large copper heat sinks with closely spaced fins are a challenge to manufacture. The dense fin geometry allows few, cost effective, manufacturing options but to solder the copper fins to the copper base. One such design, the soldered fin-in-groove design, shown below, involves cutting grooves in the copper base plate and soldering fins in the base plate, followed by machining flat the thermal face of the copper heat sink base. .

    The problem with the soldered fin-in-groove design is that it is not possible to maintain a flat thermal face. Stress relaxation in the heat sink assembly keeps warping the thermal face. The extent of warpage as measured by the line, with arrows, shown in the figure below, can be as high as 25 micrometers. Stress relaxation heat treatments and modifications thereof and various changes in the manufacturing process details have not produced heat sinks with stable thermal faces.

    This invention solves the problem of thermal face warpage by choosing the right solder for this application. In addition the proposed invention allows a controlled convex curvature to the thermal face that can be thermally superior to a flat thermal face.

     To explain how the proposed invention works, it is instructive to show why the present state-of-the-art fails to produce a stable and flat or slightly convex thermal face. oIn the present art, lead-tin solder is used which has a TCE of 25 ppm/ C. Copper has a TCE of 17 ppm/ oC. When the assembly is cooled from the soldering temperature, the osolder becomes fully solidified at 183 C. As the assembly cools below 183 oC, the solder shrinks more than the copper. The solder goes into tension and the copper, near the thermal face, in compression. The thermal face becomes conc...