Novel Liquid Phase Sintered Sn-In Solders with Tailorable Properties for Thermal Interface Material and Interconnect Applications

At present, there is significant interest in low-temperature, indium-based materials for both thermal interface material (TIM) applications as well as interconnect applications for packaging thermally sensitive next-generation devices. The attractive properties of these solders include: (i) high shear compliance under low strain rate conditions, and (ii) high electrical/thermal conductivity, both of which are critical for TIMs and interconnects. However, currently used indium-based solders suffer from 2 serious shortcomings: (i) high cost due to high indium content, and (ii) very low compressive strength and creep resistance which may lead to structural instability following heat-sink attachment. In order to circumvent these problems, and also introduce a built-in melting point hierarchy following initial reflow, a radically different approach for producing microelectronic solder joints based on liquid phase sintering (LPS) is being developed. In this paper, we report on the processing and characterization of LPS Sn-In solders, the microstructure of which consists predominantly of particles of the high melting Sn and a smaller amount of particles of low melting In. By optimizing the In content, highly compliant LPS solders with flow stresses close to that of pure In were obtained. The electrical and thermal conductivity of the LPS solder was found to be about half that of pure In. Following LPS, the melting point of the solder was found to have increased by 30-40degC, thereby enabling this material to be resistant to melting during subsequent high temperature packaging steps. Finally, it is demonstrated that metallurgically good joints can be produced between this new solder and Cu substrates during a single step which combined LPS with joining.