Bonding Silicon Chips to Aluminum Substrates Using Ag–In System Without Flux

A fluxless bonding process was developed between silicon chips and aluminum substrates using Ag-In binary system. Results indicate that the Ag-In system can manage the large mismatch between the coefficient of thermal expansion (CTE) of Si chips (2.7×10^-6/°C) and Al substrates (23×10^-6/°C). The bonding structures are Si/Cr/Au/Ag and Al/Cr/Cu/Ag/In/Ag. Cross-section SEM images exhibit nearly perfect joints between the Si chips and Al substrates. Energy dispersive X-ray analysis shows that the joint consists of Ag/(Ag)/Ag2In/(Ag)/Ag, where (Ag) is a solid solution phase. The joint can achieve a melting temperature of 650°C at least, even though the bonding is performed at 180°C. Six samples are shear tested. The shear strengths obtained far exceed the requirement specified in MIL-STD-883H standards. Al is not considered as a favorable substrate material because it is not solderable and has a high CTE. The new method presented in this paper seems to have surmounted these two challenges.