Experimental Study of a High Performance Aligned Graphite Thermal Interface Material

Abstract

Thermal interface materials (TIMs) play a critical role in microelectronics packaging. In this paper, a novel aligned-graphite/solder TIM is described. Unlike traditional TIMs infiltrated with randomly-oriented high-conductivity fillers, the aligned-graphite/solder TIMs provide both extraordinarily high thermal conductivity along the heat transport direction, and controllable stiffness to conform to surfaces with different roughness and hardness, greatly improving the overall heat transfer performance. In addition, vertically connected solder layers can lock the graphite layers in place and reinforce the strength of the entire package. Thermal performance of the graphite TIMs was determined experimentally based on the ASTM-D5470 method with comparison to two commercially available TIMs. The graphite TIMs also experienced a thermal cycling test and a high temperature stability test to establish its performance merit in practical applications. Experiments showed that the overall thermal resistivity of a 150-to-200-μm-thick graphite TIM film was less than 0.035 °C/(W/cm2) when bonding two smooth copper surfaces together at a processing pressure of 30 psi, which corresponds to an approximately 2–3X improvement over a Ag-Sn solder alloy (Indalloy 121). Preliminary thermal cycling and high temperature stability tests showed that the thermal performance of the graphite TIM was very stable, and did not degrade during these tests. The tests also indicated that the presence of surface roughness of 10 μm on one of the copper surfaces reduced the overall thermal resistivity by approximately 30%. A numerical simulation verified this trend.