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Method for a foam-type tungsten/copper thermal interface material

IP.com Disclosure Number: IPCOM000132642D
Publication Date: 2005-Dec-28
Document File: 6 page(s) / 121K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a foam-type tungsten/copper thermal interface material. Benefits include improved functionality, improved thermal performance, improved performance, improved reliability, and improved cost effectiveness.

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Method for a foam-type tungsten/copper thermal interface material

Disclosed is a method for a foam-type tungsten/copper thermal interface material. Benefits include improved functionality, improved thermal performance, improved performance, improved reliability, and improved cost effectiveness.

Background

              The requirement for optimal cooling is driven by the high power consumption and associated heat dissipation of increasingly powerful processors, which must operate at their optimal device junction temperatures. Conventionally, a copper (Cu) heat spreader is integrated in the package to dissipate heat from the die while the thermal interface material (TIM) connects the heat spreader to the silicon die.

              The conventional solution to heat dissipation for a flip-chip package is the use of a heat spreader/lid and TIM. It is designed to eliminate air gaps, increasing heat flow through the die-to-lid interface. It thermally couples the silicon (Si) die and the Cu spreader. Additionally the interface mechanically decouples the coefficient of thermal expansion (CTE) mismatch between the Si and Cu.

              Conventional technology uses solder, such as indium (In), that demonstrates end-of-line (EOL) performance requirements due to the material’s high bulk thermal conductivity. However, the material’s thickness and material/process interactions contribute to the increase in thermal contact resistance of the package. The requirement for better thermal coupling between interfaces must be addressed. Furthermore, most solder materials are rigid and brittle. As a result, they are not good choices in terms of die warpage. The Si dice range from 750-775 µm in thickness with thermal conductivity of approximately 120 W/mK. A Cu heat spreader is typically 1.5-mm thick and 31 mm x 31 mm in dimension. Its thermal conductivity is at 390 W/mK. The In TIM thickness is approximately 200 µm with 88 W/mK thermal conductivity.

              With conventional solder (In) TIM, thermo-mechanical stresses can cause a soft metal to pump-out from the die top into the substrate surface. The integrity of TIM attachment on the die is directly affected by the amount of force applied by the clips that keep the In preform in place. Sometimes the force is not maintained. Voids and delamination can result due to the loss of material, which manifests as an increase in thermal resistance.

              Metal foams are produced from pure metals and composites/alloys by aeration (introduction of air or gas while the metal is liquid) or self-expansion (inclusion of a foaming agent in the metal that releases a gas at the melting point of the metal).

General description

              The disclosed method is a tungsten/copper alloy TIM in the form of W60/Cu40 foam. Boron nitride (BN) filled polydimethylsiloxane (PDMS) is embedded in the foam. The foam/polymer is attached to the Cu heat spreader/lid.

              The key elements of the disclosed method include:

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