Surface activation process of lead-free solder bumps for low temperature bonding

Focused on the problems of the lead-free alloys bonding at high melting temperature, high density, low cost and low temperature lead-free flip chip bonding process was developed by the surface activated bonding (SAB) method. Sn-3.0Ag-0.5Cu (wt %) alloy, with better reliability and solderability than other alternatives for electronic packaging, were chose for the experiments to be bonded with typical electrodes, SnAg, Cu and Au film. The feasibility of Sn-3.0Ag-0.5Cu SAB bonding at room temperature was confirmed. The bonding strength in vacuum and N₂ was high. In air, it was found that the bonding strength was highly depended on exposure time. Combined with low temperature (150°C), SAB bonding process was developed in air by using 30 μm pitch Au/Sn flip chip samples for the first time. Resistance and tensile test showed good electrical and mechanical properties of the bonded Au/Sn samples. Bonding interfaces were observed by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA).