Fluxless Tin and␣Silver-Indium Bonding Processes for␣Silicon onto␣Aluminum

Abstract

The high thermal conductivity, light weight, and low cost of aluminum (Al) make it a promising material for use in high-power electronic packaging. The challenges are its high coefficient of thermal expansion (CTE) of 23 × 10⁻⁶/°C and difficulty in soldering. In this research, we surmounted these challenges by bonding large Si chips to Al boards using fluxless Sn and Ag-In processes, respectively. Despite the large CTE mismatch, the bonded structures were strong as determined by fracture force measured by shear test machine. The reference is the fracture force specified in MIL-STD-883H method 2019.8. The microstructure and composition of the joints were examined using scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis. The resulting Sn joint is almost pure Sn with thin intermetallic layer. The Ag-In joint consists of Ag/(Ag)/Ag₂In/(Ag)/Ag that has a melting temperature higher than 695°C even though the bonding process was performed at 180°C. These bonding processes are entirely fluxless. The fluxless feature greatly helps reduce voids in the joints, which in turn increases the joint strength. These preliminary but encouraging results should open up new applications of Al boards in electronic packaging where Al was avoided because of its high CTE and difficulty in bonding.