Effect of laser input energy on wetting areas of solder and formation of intermetallic compounds at Sn-3.5Ag-0.75Cu/Au right-angled joint interface

This paper presents the effect of laser input energy on the wetting areas of solder on pads after first laser reflow, and the formation of IMC (intermetallic compounds) at Sn3.5Ag-0.75Cu/Au right-angled joint interface during second laser reflow. The results showed that the wetting areas of solder on pads were getting larger with the increase of laser input energy. AuSnx IMC were observed at the interface of solder and the pad finished with 4 μm Au/0.1 μm Ta after second laser reflow, the amount and distribution of IMC was strongly affected by laser input energy. In addition, when the laser input energy was increased, the diffusion of copper at the interface of solder and the pad made of Cu plated with 3 μm Au was getting larger, and the AuSnx IMC layers could not be well distinguished as the interface between solder and the pad finished with 4 μm Au/0.1 μm Ta, which may be caused by the increased diffusion of Cu and formation of Cu_xSn_y or Cu_xAu_ySn_z IMC in the condition of high energy input.