Characterization of Structurally-Enhanced Solder Joints Using Vertically-Aligned Carbon Nanotubes

Abstract

Since the demand for more efficient means of electronics cooling has grown significantly over recent years, the need for more advanced thermal interface materials has also increased. Thermal interface materials (TIMs) are a vital part of electronics cooling. TIMs are materials that are used to fill in the microscopic spaces that exist between electronic circuits and heat sink surfaces due to surface roughness or other surface imperfections. Additionally, TIMs are used to promote effective thermal transport across interfaces. In general, materials that have high thermal conductivities and low thermal resistances are ideal for use as TIMs. The thermal performance of a TIM is typically characterized by its thermal conductivity and thermal resistance, which are both measurements of how well heat travels through a medium. Carbon nanotubes (CNTs) have been reported to possess very good thermal and mechanical properties. For example, the theoretical thermal conductivity of an individual single-walled CNT has been calculated to be 37,000 W/m/K. Experimental thermal conductivity values have been found in the range of 25 to 267 W/m/K. These values are influenced by parameters such as the alignment and quality of CNTs. Furthermore, CNTs have been found to have good compliancy and resiliency, which are also ideal traits for TIM applications. Not only do these characteristics make CNTs excellent candidates for TIMs, but they also make them promising for use in solder joint enhancement. This is due to the fact that a common drawback concerning solder joints in electronics packaging is coefficient of thermal expansion (CTE) mismatches, which frequently result in solder joint failure. Incorporating CNTs into a solder interface could potentially extend the life of the solder joint by making it stronger and more pliable. In this study, a solder-CNT composite TIM was fabricated using vertically-aligned carbon nanotubes (VACNTs) due to the intrinsically high thermal conductivity of carbon nanotubes (CNTs). The constructed TIM consisted of vertically-aligned carbon nanotube arrays (VACNAs), which were grown by chemical vapor deposition (CVD), soldered to copper disks on both sides via a bismuth/tin/silver solder. Soldering the free ends of the CNTs overcame one of the major contributors to thermal resistance, which is the contact resistance between the CNT free ends and an opposing substrate. The thermal performance potential of the produced sample as a TIM was measured using an Analysis Tech ASTM 5470 Thermal Interface Material Tester. This apparatus measured the thermal resistance and thermal conductivity of the constructed TIM, and it also analyzed the pressure dependence of the thermal properties. The interfacial thermal resistance of the CNT-solder composite was examined and reported for pressures of 20 psi and 50 psi. The average thermal resistance value recorded for the constructed sample was 0.458 (cm^2-K)/W at a pressure of 20 psi and 0.435 (cm^2-K)/W at a pressure of 50 psi.