Browse Prior Art Database

Method for solder thermal interface materials with high bulk thermal conductivity and low interfacial resistance

IP.com Disclosure Number: IPCOM000018753D
Publication Date: 2003-Aug-06
Document File: 8 page(s) / 102K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for solder thermal interface materials with high bulk thermal conductivity and low interfacial resistance. Benefits include improved thermal performance, improved reliability, and improved design flexibility.

This text was extracted from a Microsoft Word document.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 20% of the total text.

Method for solder thermal interface materials with high bulk thermal conductivity and low interfacial resistance

Disclosed is a method for solder thermal interface materials with high bulk thermal conductivity and low interfacial resistance. Benefits include improved thermal performance, improved reliability, and improved design flexibility.

Background

        � � � � � As CPUs achieve improved processing performance, they generate more heat and require improved heat removal. The primary method for improving heat removal has been the innovation of improved thermal interface material (TIM) technologies. They have prevented the implementation of active cooling and radical changes to the form factor. However, further improvement to TIM technologies is required to meet the advanced performance targets.

        � � � � � Indium solder has not yet been demonstrated as a viable TIM technology for flip-chip ball grid array (FC-BGA) packages. Indium TIM tends to form voids during high-temperature reflow.

        � � � � � Solder TIM technology is successful with TIM2 and mobile applications. However, the materials used in these applications must be reworkable and easily assembled. The use of indium and tin solders with high liquidus temperatures is not acceptable because they are susceptible to hardening and voiding, as detected during reliability stress testing.

        � � � � � Conventional TIM has high bulk thermal conductivity and high interfacial resistance. An indium solder preform has a liquidus temperature of about 157°C, bulk thermal conductivity of about 84 W/mK, and thickness of about 200 microns. To use this material, the die and integrated heatsink (IHS) are coated with gold. When assembled under pressure at temperatures above about 160°C, intermetallic formation occurs. This assembly has higher than expected thermal resistance, which is believed to result from a combination of voiding and interfacial resistance.

        � � � � � Conventional TIM2 and mobile TIM has very low bulk thermal conductivity yet very low interfacial resistance. Phase change material (PCM) has a bulk thermal conductivity less than about 3 W/mK and a melting point below the application temperature, so that it is in the liquid state during operation. As a result, its contact resistance is essentially zero, which is why good thermal performance is achieved despite the low bulk thermal conductivity.

        � � � � � A stack up using indium has a bulk thermal conductivity of about 84 W/mK and liquidus temperature of about 157oC, between the die and the IHS. Packages prepared in this manner with a mean BLT of 194 microns provide a total resistance of about 0.120oCcm2/W. The bulk resistance of the die and IHS is measured at about 0.080oC cm2/W. The expected bulk resistance for the indium is about 0.023oCcm2/W, which means that the contact resistance is about 0.017oCcm2/W (see Figure 1).

        � � � � � Note: Early indium solder TIM data had a mean Rjc of 0.130oCcm2/W, and the decrease to 0.120oCcm2/W is primarily due t...