Solder-graphite composites as high-performance thermal interface materials
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A high-performance thermal interface material in the form of solder-graphite composite sheet has been developed with solder content of 55 vol.%. Solders Sn63-Pb 37 (type A) and Sn95.5-Ag4-Cu0.4 (type B) in powder form (<10 μm) are separately used. A compressed mixture of exfoliated graphite (made by heating intercalated graphite flakes) and solder powder is heated above the melting point of the solder and is followed by hot pressing at a pressure of 11.2 MPa. Due to the cellular structure of exfoliated graphite, the pressing causes mechanical interlocking between the pieces of exfoliated graphite and hence a graphite network, which coexists with the distributed solder. The high thermal contact conductance, high thermal conductivity and low CTE make the developed composite superior to all previously reported thermal interface materials. Both solder composites show similar properties, with type A performing slightly better, due to the superior solder properties. At the same contact pressure, the thermal contact conductance of the composite increases with decreasing thickness for both types of solder composite. With a contact pressure of 0.92 MPa and a thickness of 40-50 μm, thermal contact conductance 26 x 104 W/m2.K has been achieved with copper sandwiching surfaces of 15 μm roughness for both type A and type B solder composites. The thermal contact conductance increases monotonically as the thickness is reduced, such that it abruptly increases at a thickness of 100 μm, due to an abrupt increase in the through-thickness solder connectivity. Above 100 μm, the total thermal resistivity is dominated by the composite resistivity; below 100 μm, it is dominated by the composite-copper interfacial resistivity. At the same thickness, an increase in the contact pressure from 0.46 to 0.92 MPa decreases the total thermal resistivity, due to enhanced conformability, as indicated by the decrease of the interfacial thermal resistivity from 2.6 x 10-6 to 1.4 x 10-6 m2.K/W. Coefficient of thermal expansion (CTE) values of 12.7 x 10-6/°C and 10.2 x 10-6/°C have been achieved for the type A and type B solder composites respectively. The thermal conductivity increases abruptly from 2 to 52 W/m.K and the total thermal resistivity decreases abruptly upon decreasing the thickness below 100 μm. The contact pressure has negligible effect on the thermal conductivity of the composites. The composite is anisotropic, with the solder connectivity being much lower in the through-thickness direction than the in-plane direction and the electrical conductivity anisotropy 7.3. Compared to the corresponding graphite without solder, the composite with thickness below 100 μm gives higher thermal contact conductance, higher thermal conductivity and lower interfacial thermal resistivity. Compared to solder in the absence of graphite, the composite with thickness below 100 μm gives higher thermal contact conductance and lower CTE.