Interfacial reactions and mechanism properties between SAC 405 and SACNG lead-free solders with Au/Ni(P)/Cu substrates reflowed by CO2 laser

This study investigate the interfacial reactions and mechanical property between Sn-4.0Ag-0.5Cu (SAC, in wt%) and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge (SACNG, in wt%) lead free solders with Au/Ni(P)/Cu substrate by CO₂ laser reflow. These results compare with the same systems under a conventional hot-air reflowing method in the oven. After reflowing, all samples were then taken heat-treatment at 150°C for 1000 hours. Under CO₂ laser reflowing, mixtures of the (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ phases and (Ni,P,Cu) solid solution were formed at the interface. After taking a long period of heat-treatment, the (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄ were still observed at the interface. But, the Ni(P)+Cu solid solution disappeared as increasing reaction times. The experimental results compared with the same couples by conventional hot-air reflowing at 240°C, three layer structures, (Cu,Ni)₆Sn₅, (Ni,Cu)₃Sn₄ and Ni₃P phases, were formed at the interface. The spalling intermetallic compounds could be observed in the SACNG/Au/Ni/Cu couple. The mechanical strength of solder joints by the laser reflowing technique is very similar to that by the hot-air reflowing method.