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

Low-cost solder-graphite composite sheets (a parts per thousand yen55 vol.% solder), with solder and graphite forming interpenetrating networks to a degree, are excellent thermal interface materials (TIMs). Solders 63Sn-37Pb and 95.5Sn-4Ag-0.5Cu are separately used, with the latter performing better. In composite fabrication, a mixture of micrometer-size solder powder and ozone-treated exfoliated graphite is compressed to form a graphite network, followed by fluxless solder reflow and subsequent hot pressing to form the solder network. The network connectivity (enhanced by ozone treatment) is lower in the through-thickness direction. The electrical conductivity obeys the rule of mixtures (parallel model in-plane and series model through-thickness), with anisotropy 7. Thermal contact conductance a parts per thousand currency sign26 x 10(4) W/(m(2) K) (with 15-mu m-roughness copper sandwiching surfaces), through-thickness thermal conductivity a parts per thousand currency sign52 W/(m K), and in-plane thermal expansion coefficient 1 x 10(-5)/A degrees C are obtained. The contact conductance exceeds or is comparable to that of all other TIMs, provided that solder reflow has occurred and the composite thickness is a parts per thousand currency sign100 mu m. Upon decreasing the thickness below 100 mu m, the sandwich thermal resistivity decreases abruptly, the composite through-thickness thermal conductivity increases abruptly to values comparable to the calculated values based on the rule of mixtures (parallel model), and the composite-copper interfacial thermal resistivity (rather than the composite resistivity) becomes dominant.