Fluxless bonding of Si chips to aluminum boards using electroplated Sn solder

The high thermal conductivity and light weight properties of aluminum (Al) make it a promising material in high power device packaging and automotive electronics. 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) solder. The Si–Al pair probably has the largest CTE mismatch among all bonded structures in electronic packaging. In experiments, 0.1 μm Cr layer and 0.2 μm Cu layer were deposited on Al substrates, followed by an electroplated 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 joint thickness was controlled either by bonding pressure or by incorporating Cu spacers. Microstructure and composition of the joints were studied with scanning electron microscopy and energy dispersive X-ray spectroscopy. Despite the large CTE mismatch, the bonded structures did not break. The minimum fracture force measured among six samples by shear test is higher than three times of what specified in MIL-STD-883H, method 2019.8. This preliminary result corroborates potential adaption of Al substrates and boards in electronic packaging where lightweight is essential.