Dynamic wetting and spreading mechanisms of SAC305 with added Au elements in the laser jet weld ball bonding process

The lead-free solder SAC305 has inferior wetting properties compared to conventional leaded solders, which deteriorates the weld quality. In this study, Au coatings with varying thickness were applied to improve its wettability and the influencing mechanism was investigated. The results indicated that the wetting angle (WA) and spreading area (SA) increased and then decreased with increasing thickness of the Au coating. The minimum value of WA and maximum value of SA were 25.4° and 8.7 mm2, respectively, when Au coating thickness was 50 nm. The analysis of the interfacial microstructure revealed that the generation of AuSn4 in the triple line region would significantly improve the wettability of Sn alloys. However, when the Au coating thickness reach up to 75 nm, the dense intermetallic compound (IMC) (Cu, Ni)6Sn5 was generated and the wettability of Sn alloys was reduced. The effect of the interfacial compounds on wettability depended on the change of energy consumption mechanism during wetting. The main energy consumption mechanism changed from adsorption of elements to a reaction-controlled, which reaction wetting dominated when the Au coating thickness was greater than 50 nm. This study will provide a deep understanding of wetting behavior and a valuable insight for optimizing solder wetting processes.