Experimental Study of Coefficient of Thermal Expansion of Aligned Graphite Thermal Interface Materials

Abstract

Carbon-based materials draw more and more attention from both academia and industry: its allotropes, including graphene nanoplatlets, graphite nanoplatlets and carbon nanotubes, can readily enhance thermal conductivity of thermal interface products when served as fillers. Structuraloptimization in micro/nano-scale has been investigated and expected to finely tune the coefficient of thermal expansion (CTE) of thermal interface materials (TIMs). The capability of adjusting CTE of materials greatly benefits the design of interface materials as CTE mismatch between materials may result in serious fatigue at the interface region that goes through thermal cycles. Recently, a novel nano-thermal-interface material has been developed, which is composed of tin (Sn) solder and graphite nanoplatlets. CTE of such sort of TIMs can be adjusted to match well with the substrate materials. A customized, optical CTE measuring system was built to measure CTEs of these thin and flexible samples. The averaged CTEs of samples made by this new approach range from -0.267×10 -6 /°C to 5×10 -6/°C between 25°C and 137°C, which matches CTEs of typical semiconductor materials (the CTE of silicon is ~3×10 -6 /°C in the same temperature range). This unique CTE-matching feature of a bonding material will have great potential to impact future development of high power microelectronics devices.

Solder–Graphite Network Composite Sheets as High-Performance Thermal Interface Materials

“…At a thickness of 0.26 mm, the thermal contact conductance reported is up to 17.0 9 10 4 W/(m 2 K), 34 which is lower than the highest value obtained in this work. A laminate of multiple flexible graphite sheets (50 lm to 200 lm thick per sheet) bonded together by soldering gives in-plane thermal contact conductance up to (29 ± 4) 9 10 4 W/(m 2 K), 42,43 which is essentially equal to the highest value of (26.2 ± 0.6) 9 10 4 W/(m 2 K) obtained in this work by using a solder-graphite network composite. Both the laminate of Refs.…”

“…In contrast to the laminate structure of prior work, 42,43 this work combines solder with exfoliated graphite, such that the resulting structure is a solder-graphite composite with both solder and graphite exhibiting a degree of networking and with the two networks interpenetrating. Due to the compression of the exfoliated graphite during composite fabrication, flexible graphite is formed within the graphite network.…”

“…The graphite-solder laminate of prior work 42,43 contains around 10 vol.% solder, whereas the solder-graphite composites of this work contain around 55 vol.% solder. Reference 42 mentions that the conformability of the TIM rather than the thermal conductivity of the TIM governs the TIM performance and that the conformability of the TIM stems from that of the graphite part of the laminate.…”

“…42,43 The essentially zero value of the CTE is too low for matching with the CTE of ceramics or semiconductors encountered in electronic packages.…”

“…However, the composite fabrication process is more complicated for the laminate of Refs. 42, 43, since it involves electroplating of solder on flexible graphite 42 prior to lamination and cutting of the laminate in the through-thickness direction with subsequent lapping to form slices of thickness around 200 lm to 250 lm to serve as TIMs with the heat flow direction along the thickness of a slice. 43 The difficulty of fabricating TIMs of thickness below 200 lm is another drawback of this laminate route.…”