Thermal expansion-enhanced heat sink for an electronic assembly

CROSS-REFERENCE TO RELATED APPLICATION

0001This application is a continuation of U.S. Ser. No. 13/271,268, entitled “Thermal Expansion-Enhanced Heat Sink for an Electronic Assembly,” filed Oct. 12, 2011, and which is hereby incorporated herein by reference in its entirety.

BACKGROUND

0002Operating electronic components or devices produce heat. This heat should be removed from the devices in order to maintain device junction temperatures within desirable limits, with failure to remove produced heat resulting in increased device temperature, potentially leading to thermal runaway conditions. Several trends in the electronics industry have combined to increase the importance of thermal management, including heat removal from electronic components, including technologies where thermal management has traditionally been less of a concern, such as CMOS. In particular, the need for faster and more densely packed circuits has had a direct impact on the importance of thermal management. For example, power dissipation, and therefore heat production, increases as device operating frequencies increase. Also, increased operating frequencies may be possible at lower device junction temperatures. In addition, as more and more devices are packed onto a single chip, power density (Watts/cm²) increases, resulting in the need to remove more heat from a given size chip or module.

0003Existing cooling technologies typically utilize air or water to carry heat away from an electronic component, and reject the heat. Heat sinks with heat pipes or vapor chambers are commonly used in air-cooled devices, while cold plates are most prominent in water-cooled structures. However, with both types of cooling assemblies, it is necessary to attach the cooling assembly to the heat-generating electronic component or device. This attachment results in a thermal interface resistance between the cooling structure or assembly and the electronic component. One aspect limiting the capability to cool a given electronic component is the thermal interface between the component to be cooled and the cooling structure.

BRIEF SUMMARY

0004In one aspect, a method of fabricating a thermal expansion-enhanced heat sink is provided. The method includes: providing a heat sink base comprising a base surface configured to couple to a surface of a heat-generating electronic component to facilitate removal of heat from the heat-generating electronic component, the heat sink base having a first coefficient of thermal expansion; and providing a frame configured to couple to the heat-generating electronic component and constrain the base surface of the heat sink base in opposing relation to the surface of the heat-generating electronic component, the frame having a second coefficient of thermal expansion, wherein the first coefficient of thermal expansion of the heat sink base is greater than the second coefficient of thermal expansion of the frame, and wherein at least one of the heat sink base or the frame is configured so that heating of the heat sink base expands the heat sink base relative to the frame and results in a force component at the base surface of the heat sink base towards the surface of the heat-generating electronic component that facilitates heat transfer from the surface of the heat-generating electronic component to the heat sink base.

0005Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

0006One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

0007<figref idref="DRAWINGS">FIG. 1A</figref> depicts one embodiment of an electronic assembly wherein thermal grease is employed to couple an electronic component to a cooling assembly;

0008<figref idref="DRAWINGS">FIG. 1B</figref> is a partial embodiment of another electronic assembly wherein an advanced epoxy interface is employed to couple an electronic component to a lid of the electronic assembly;

0009<figref idref="DRAWINGS">FIG. 2</figref> is a cross-sectional elevational view of one embodiment of an electronic assembly comprising a thermal expansion-enhanced heat sink coupled to a heat-generating electronic component, in accordance with one or more aspects of the present invention;

0010<figref idref="DRAWINGS">FIG. 3</figref> is a cross-sectional elevational view of the electronic assembly of <figref idref="DRAWINGS">FIG. 2</figref>, with the higher coefficient of thermal expansion (CTE) heat sink base shown expanded relative to the frame to apply a compressive loading force (F) to the thermal interface material between the base surface of the heat sink base and the electronic component to be cooled, in accordance with one or more aspects of the present invention;

0011<figref idref="DRAWINGS">FIG. 4</figref> is a cross-sectional elevational view of an alternate embodiment of an electronic assembly comprising a thermal expansion-enhanced heat sink coupled to a heat-generating electronic component, in accordance with one or more aspects of the present invention; and

0012<figref idref="DRAWINGS">FIG. 5</figref> is a cross-sectional elevational view of another embodiment of an electronic assembly comprising a thermal expansion-enhanced heat sink coupled to a heat-generating electronic component, in accordance with one or more aspects of the present invention.

DETAILED DESCRIPTION

0013As used herein, “electronic component” comprises any heat-generating component of, for example, a computer system or other electronic unit requiring cooling. The term includes, for example, one or more integrated circuit chips, devices or modules, either with or without a thermal module cap or thermal spreader.

0014Reference is made below to the drawings, which are not drawn to scale for ease of understanding, wherein the same reference numbers used throughout different figures designate the same or similar components.

0015A thermal grease or an epoxy may be employed as a thermal interface material within an electronic assembly. <figref idref="DRAWINGS">FIG. 1A</figref> depicts one embodiment of an electronic assembly, generally denoted <b>100</b>, wherein a cooling assembly <b>120</b>, such as a heat sink with a heat pipe or vapor chamber, is coupled to an electronic module <b>110</b>. Module <b>110</b> includes a printed circuit board <b>112</b>, side walls <b>114</b>, and lid or cap <b>116</b>, within which an integrated circuit chip <b>124</b> is packaged. Integrated circuit chip <b>124</b> connects (in one example) via a flip chip interface <b>126</b> with a substrate <b>122</b>, and a Ball Grid Array (BGA) <b>121</b> is employed to electrically connect substrate <b>122</b> to printed circuit board <b>112</b>. A thermal grease <b>130</b> is employed to thermally couple integrated circuit chip <b>124</b> to lid <b>116</b>, and hence to cooling assembly <b>120</b>.

0016<figref idref="DRAWINGS">FIG. 1B</figref> depicts a partial embodiment of an electronic assembly wherein an advanced epoxy interface <b>140</b> is used to couple an integrated circuit chip <b>134</b> to a lid <b>142</b>, which comprises part of or couples to a cooling assembly (not shown). The integrated circuit chip <b>134</b> can electrically connect to a substrate <b>132</b> via conventional flip chip interface technology <b>136</b>.

0017The heat transfer performance of the thermal grease of <figref idref="DRAWINGS">FIG. 1A</figref> and the epoxy of <figref idref="DRAWINGS">FIG. 1B</figref> is directly proportional to the thermal conductivity of the material employed, and is inversely proportional to the interface thickness, which is also known as the bond line thickness. Thermal grease interfaces typically have higher thermal conductivity, but also exhibits a thicker bond line, while epoxies typically have a lower thermal conductivity, but the joints can be manufactured with thinner bond lines. In general, thermal greases show relatively low thermal performance, but yield more under stress, while these characteristics are reversed for an epoxy interface. Also, to achieve very small interface thicknesses using an epoxy, while minimizing stresses, it is typically necessary to bond the electrical component (e.g., integrated circuit chip) to a material that has a thermal expansion coefficient very similar to that of silicon.

0018Thermal performance of the electronic assembly can be further enhanced by providing an interface between the electronic component and the cooling structure, alternatively referred to herein as a heat sink. Thermal performance of such an interface can be enhanced by introducing pressure to couple the cooling structure and the electronic component to be cooled, for example, using a clamping mechanism or fasteners, as well as an intermediary substance, generally referred to herein as a thermal interface material (TIM) to fill the resulting gaps created by fine machining marks and the material's crystal structure. Further advantage can be obtained by utilizing the inherent differences in thermal expansion of different materials with different coefficients of thermal expansion (CTE), and the fact that in operation, there will exist a significant temperature gradient from the electronic component to be cooled to the cooling structure (or assembly).

0019Advantageously, disclosed herein are heat sinks or cooling structures that take advantage of a thermal expansion coefficient mismatch between different materials of the heat sink to increase the pressure of contact between the heat sink and the surface of the electronic component to be cooled, and therefore reduce the thermal resistance of the interface as the temperature of the electronic component increases during operation.

0020<figref idref="DRAWINGS">FIGS. 2 &amp; 3</figref> illustrate one embodiment of an electronic assembly <b>200</b> comprising one or more heat-generating electronic components <b>220</b>, and a thermal expansion-enhanced heat sink <b>210</b> coupled to a surface <b>221</b> of, in this embodiment, electronic component <b>220</b>. Electronic component <b>220</b> is, by way of example, an electronic module comprising an integrated circuit chip <b>222</b> which is electrically connected <b>223</b> to a substrate <b>224</b>, which itself may be connected to a printed circuit board (not shown). The electronic module further includes a cap (or lid) <b>225</b> which is coupled to substrate <b>224</b> and encloses integrated circuit chip <b>222</b>. Thermal expansion-enhanced heat sink <b>210</b> is shown coupled via securing mechanisms <b>218</b> to cap <b>225</b> of electronic component <b>220</b>, with a base surface <b>211</b> of heat sink base <b>212</b> of thermal expansion-enhanced heat sink <b>210</b> disposed in opposing relation to surface <b>221</b> of electronic component <b>220</b> to be cooled. As shown, a thermal interface material <b>230</b> is provided between base surface <b>211</b> of heat sink base <b>212</b> and surface <b>221</b> of electronic component <b>220</b> to enhance heat transfer performance from the electronic component <b>220</b> to the heat sink base <b>212</b>. Heat sink base <b>212</b> further includes an upper surface <b>213</b> disposed parallel to base surface <b>211</b>, from which a plurality of air-cooled fins <b>214</b> extend (in one embodiment) as shown. In this embodiment, heat sink base <b>212</b> also comprises an angled side surface <b>215</b> sloping inward from base surface <b>211</b> to upper surface <b>213</b>.

0021The thermal expansion-enhanced heat sink <b>210</b> further includes a frame <b>217</b>, which is configured to the heat-generating electronic component <b>220</b>, and which partially encircles and constrains the heat sink base to maintain base surface <b>211</b> of heat sink base <b>212</b> in opposing relation to surface <b>221</b> of heat-generating electronic component <b>220</b>. Frame <b>217</b> further includes an angled inner surface <b>216</b> in opposing relation to and (in one embodiment) in contact with angled side surface <b>215</b> of heat sink base <b>212</b>.

0022In accordance with one or more aspects of the present invention, heat sink base <b>212</b> and frame <b>217</b> are fabricated of different materials with different coefficients of thermal expansion (CTE), wherein heat sink base <b>212</b> has a higher CTE than the material of frame <b>217</b>. In operation, heat is transferred across thermal interface <b>230</b> from electronic component <b>220</b> into heat sink base <b>212</b>. With heating of heat sink base <b>212</b>, the heat sink base expands, forcing angled side surface <b>215</b> of heat sink base <b>212</b> against angled inner surface <b>216</b> of frame <b>217</b>. This in turn results in a force component (F) as shown in <figref idref="DRAWINGS">FIG. 3</figref>, which forces base surface <b>211</b> of heat sink base <b>212</b> towards surface <b>221</b> of heat-generating electronic component <b>220</b>, to facilitate heat transfer from the surface <b>221</b> of heat-generating electronic component <b>220</b> to heat sink base <b>212</b>. For example, heating of heat sink base <b>212</b> increases compressive loading on thermal interface material <b>230</b> disposed between base surface <b>211</b> of heat sink base <b>212</b> and surface <b>221</b> of electronic component <b>220</b>, which facilitates heat transfer from the surface of the heat-generating electronic component to the heat sink base through the thermal interface material. In <figref idref="DRAWINGS">FIG. 3</figref>, heat sink base <b>212</b> is shown shifted slightly towards surface <b>221</b> of electronic component <b>220</b> to illustrate this compressive force (F) being applied to thermal interface material <b>230</b> between surfaces <b>211</b>, <b>221</b>.

0023As one specific example, thermal expansion-enhanced heat sink <b>210</b> is a bi-metallic, air-cooled fin heat sink. Heat sink base <b>212</b> may be fabricated of a conductive metal with a high thermal coefficient of expansion, such as copper, while frame <b>217</b> may be fabricated of a lower CTE alloy, such as steel. In one embodiment, in the as-fabricated condition, the base surface of the heat sink base is co-planar with the corresponding surface of the frame <b>217</b>, as shown in <figref idref="DRAWINGS">FIG. 2</figref>. In operation, the electronic component produces heat, and the heat sink temperature rises, with the heat sink base material expanding more than the frame material. The heat sink base <b>212</b> wants to expand more at the base surface than at the upper surface because the heat sink base dimensions are larger at the base surface than at the upper surface. The differential expansion of the heat sink base results in a tendency for the heat sink base thermal interface to become convex. However, since the heat sink base is coupled through the frame to the cap of the electronic component, the tendency of the heat sink base to become convex translates into an increased pressure between the heat sink base and the cap of the electronic component. Increased contact pressure results in a decreased interface thermal resistance across thermal interface material <b>230</b>. Thus, the thermal performance of the thermal expansion-enhanced heat sink is improved by fitting, for example, a heat sink base <b>212</b> fabricated of copper into a metal frame <b>217</b> fabricated of steel. The shape of the steel ring is such that the heat sink base does not slip out of the ring, and provides enough differential expansion for heat sink base <b>212</b> to tend to become convex when heated.

0024Note that if the heat sink base were fabricated of multiple layers of metals of differing thermal coefficients of expansion, the tendency to become convex when heated can still be achieved, but at an expense of reduced thermal conductivity through the heat sink base. The reason for this is that the metal mated to, for example, copper, will have a lower thermal coefficient of expansion than copper only. Additional materials for frame <b>217</b> include molybdenum and aluminum-silicon carbide. Example thermal interface materials include compliant-cured materials (such as certain pastes available from Shin Etsu Chemical, Co., Ltd., of Tokyo, Japan), an interface pad (such as available from Parker Hannifin Corporation, of Woburn, Mass., USA, or GrafTech International, of Lakewood, Ohio, USA), or indium-based structures, (including indium variants, such as patterned indium, aluminized indium, etc.).

0025<figref idref="DRAWINGS">FIG. 4</figref> depicts an alternate embodiment of an electronic assembly <b>400</b>, in accordance with one or more aspects of the present invention. Electronic assembly <b>400</b> is similar to electronic assembly <b>200</b> described above in connection with <figref idref="DRAWINGS">FIGS. 2 &amp; 3</figref>. However, in the embodiment of <figref idref="DRAWINGS">FIG. 4</figref>, electronic assembly <b>400</b> includes a thermal expansion-enhanced heat sink <b>410</b> which comprises a heat sink base <b>212</b>, such as described above, as well as a liquid-cooled cold plate <b>420</b> integrated with heat sink base <b>212</b>. The liquid-cooled cold plate <b>420</b> facilitates removal of heat from heat sink base <b>212</b> to liquid coolant passing through liquid-cooled cold plate <b>420</b>. In the embodiment illustrated, liquid-cooled cold plate <b>420</b> includes a coolant inlet <b>421</b> and a coolant outlet <b>422</b> in fluid communication with one or more coolant-carrying channels <b>423</b>. Otherwise, the operation of thermal expansion-enhanced heat sink <b>410</b>, and in particular heat sink base <b>212</b>, is similar to that described above in connection with thermal expansion-enhanced heat sink <b>210</b> of electronic assembly <b>200</b> depicted in <figref idref="DRAWINGS">FIGS. 2 &amp; 3</figref>.

0026As illustrated, thermal expansion-enhanced heat sink <b>410</b> is coupled via securing means <b>218</b> to, for example, cap <b>225</b> of electronic component <b>220</b>, which is shown to include integrated circuit chip <b>222</b> in electrical contact <b>223</b> with a substrate <b>224</b> (by way of example only). A compressive force (F) is produced (with heating of heat sink base <b>212</b> from operation of electronic component <b>220</b>) between base surface <b>211</b> and surface <b>221</b> of electronic component <b>220</b>. This compressive force (F) is a loading force on the thermal interface which enhances the heat transfer performance of thermal interface material <b>220</b> in the region between base surface <b>211</b> and surface <b>221</b> of electronic component <b>220</b>, as explained above.

0027<figref idref="DRAWINGS">FIG. 5</figref> depicts another embodiment of an electronic assembly <b>500</b>, in accordance with one or more aspects of the present invention. In this embodiment, electronic assembly <b>500</b> is shown to comprise a thermal expansion-enhanced heat sink <b>510</b> and an electronic component <b>220</b>. Electronic component <b>220</b> includes integrated circuit chip <b>222</b> electrically connected <b>223</b> to a substrate <b>224</b>. The integrated circuit chip <b>222</b> is enclosed via a cap <b>225</b> secured (in this example) to substrate <b>224</b>. Thermal expansion-enhanced heat sink <b>510</b> is secured to the cap <b>225</b> (in one embodiment) via securing mechanisms <b>518</b>.

0028In the embodiment depicted, thermal expansion-enhanced heat sink <b>510</b> comprises a heat sink base <b>512</b>, which comprises a cylindrical side surface <b>515</b>, a base surface <b>511</b> and an upper surface <b>513</b>. A plurality of air-cooled, thermally conductive fins <b>514</b> extend from upper surface <b>513</b>, and base surface <b>511</b> is shown disposed in opposing relation to surface <b>221</b> of electronic component <b>220</b> to be cooled. Thermal interface material <b>230</b> is provided between thermal expansion-enhanced heat sink <b>510</b> and surface <b>221</b> of electronic component <b>220</b>. In the illustrated embodiment, frame <b>517</b> comprises a cylindrical sheath with a top edge or lip <b>519</b> that engages upper surface <b>513</b> of heat sink base <b>512</b> and holds heat sink base <b>512</b> in position within the thermal expansion-enhanced heat sink at the upper surface thereof. Frame <b>517</b> is constructed, in one embodiment, such that there is a gap between heat sink base <b>512</b> and the cylindrical sheath of the frame <b>517</b> when the heat sink base is unheated. Heating of heat sink base <b>512</b> causes the heat sink base to expand uniformly in the Z-direction, thereby forcing (F) base surface <b>511</b> of heat sink base <b>512</b> towards surface <b>221</b> of heat-generating electronic component <b>220</b> to facilitate heat transfer from surface <b>221</b> to heat sink base <b>512</b> through the compressively loaded thermal interface <b>230</b> disposed between base surface <b>511</b> and surface <b>221</b>. Note in this regard that top lip <b>519</b> of frame <b>517</b> prevents heat sink base <b>512</b> from expanding in the Z-direction outward away from electronic component <b>220</b>.

0029In this alternate design, the heat sink base and frame comprise straight wall, cylindrical surfaces disposed in opposing relation. Heat sink base <b>512</b> is fabricated, in one embodiment, as a thermally conductive, metal-cylindrical core, with a high CTE, which makes contact with the surface <b>221</b> of electronic component <b>220</b> at its base surface (e.g., across the thermal interface material <b>230</b>), and includes air-cooled, thermally conductive fins at its upper surface. The structure is surrounded by a cylindrical sheath, fabricated of a different material, which has a lower CTE, and which includes a lip (or partial cover) on the top edge that holds heat sink base <b>512</b> in place within the thermal expansion-enhanced heat sink. The edges of the core and the sheath are vertical, with a small gap allowing for free expansion in the radial direction. When the heat sink structure is cold, there is no gap between the upper surface <b>513</b> of heat sink base <b>512</b> and the lip <b>519</b> of frame <b>517</b>. As the integrated circuit chip <b>222</b> operates and heats electronic component <b>220</b>, the heat sink base <b>512</b> heats and expands. Since the vertical height is uniform, it expands uniformly in the Z-direction, but because it is bound at the upper surface <b>513</b>, the heat sink base <b>512</b> instead compresses uniformly on the thermal interface material <b>230</b> between base surface <b>511</b> and surface <b>221</b> of electronic component <b>220</b>. This in turn reduces the bond line thickness and reduces the thermal interface resistance. Due to the uniform compression this implementation generates, it uses (in one embodiment) a softer (and more flowable) thermal interface material which can easily flow in and out of the space between base surface <b>511</b> and surface <b>221</b>.

0030The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed.

0031The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention.