This paper introduces a high performance thermal interface material (TIM) with vertically aligned graphite. The main structure of the TIM is a vertically laminated structure, in which thin solder layers are laminated with aligned graphite layers. Unlike traditional TIMs infiltrated with randomly oriented high conductive fillers, the laminated TIM with vertically aligned graphite provides extraordinarily high z-axis thermal conductivity and controllable stiffness by simply setting the thickness of each component layer to match different surfaces. Thus, this design greatly improves the overall heat transfer performance. In addition, using metallic-graphite composites greatly improves the bonding between the graphite and the metallic host compared to nonmetallic materials, and thus the thermal boundary resistance can be significantly reduced. Moreover, compared to organic hosts, solders have much smaller phonon spectra mismatch with graphite nanoplatelets (GNPs), and thus offer significantly higher interface conductance. Furthermore, vertically connected solder layers can also lock the graphite layers in place and reinforce the strength of the entire package. A series of experimental tests was conducted to evaluate the effects of processing pressure and surface roughness on the overall thermal performance of the graphite TIMs. The results indicated that the overall thermal resistance of two smooth surfaces soldered by a 200 μm-thick graphite TIM was reduced from 0.12 to 0.03 cm2•K/W when the compression pressure applied during the soldering process was increased from 7 to 68 psi. Increased surface roughness appeared to improve heat transfer across the interface by enlarging the contact areas between the surface and the graphite TIMs. A preliminary numerical simulation verified this trend.

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.

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