Fluxless tin bonding of silicon chips to aluminum substrates

The high thermal conductivity and light weight properties of aluminum (Al) make it a promising material in high power device packaging and automotive design applications. A primary challenge is its high coefficient of thermal expansion (CTE) of 23 ppm/°C. In this research, we investigated the possibility of surmounting this challenge by bonding large Si chips to Al substrates using fluxless tin (Sn). Si versus Al pair probably has the largest CTE mismatch among all bonded structures in electronic packaging. In experiments, 0.1μm Cr layer and 0.2 μm Cu layer were deposited on Al substrates, followed by an electroplated thicker 25 μm copper (Cu) layer. The Sn solder layer was then electroplated over the Cu followed immediately by thin (0.1 μm) silver (Ag) layer. The bonding process is entirely fluxless. The joint thickness was controlled either by bonding pressure or by Cu spacers. Microstructure and composition of the joints were studied under scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Despite the large CTE mismatch, the bonded structures did not break. This preliminary result suggests potential adaption of Al substrates in electronic packaging where Al is avoided because of its high CTE.