Direct robust bonding between Sn-based solder and Si substrate

The robust bonding between silicon and low melting point solders composed of Sn–Ag solder with addition of Ce and Nd rare earth (RE) element is obtained. The content of the RE element is 2, 5 wt.%, respectively. The RE elements are stored as island compounds in the solder matrix and their addition has little effect on the melting point of the Sn–Ag solder. When soldering at 250 °C, the intermetallic compounds (IMC) layer is discontinuous and the bonding strength is lower. While soldering at 300 °C, the IMC layer become continuous and stronger bonding strength is obtained. The addition of 5 wt.% RE can result in higher bonding strength than that of 2 wt.% RE. In the present research work, we find that in order to get strong bonding between the Si substrate and the composite solders, suitable pressure should be employed to overcome the oxide skin of the liquid solders formed during soldering. The interfacial structure and fracture analysis of the solder joints are investigated to reveal the bonding nature. Complex IMC are found in the residual IMC layer after shear test. Therefore it is clear that RE elements diffuse to the interface and react with thin SiO₂ layer that exists on the Si surface to form IMC during soldering.