Abstract
This study aims to improve the impact performance via modified reflow conditions. A high speed shear tester was used to evaluate the impact reliability of solder joints. During solidification step of reflow process, the solder joints were annealed under various annealing temperature and heating time. The microstructure, β-Sn structure, micro-hardness of Sn-3.0Ag-0.5Cu/Cu (wt. %, SAC/Cu) and Sn-3.0Ag-0.5Cu/Ni (wt. %, SAC/Ni) solder joints were investigated. To evaluate the effect of annealing temperature on impact performance, the SAC/Cu solder joints were annealed at 180 oC and 210 oC, respectively, during the solidification step. In the annealed solder, the precipitations in the limited region near the joint interface varied from the dot type toward the network type. Additionally, electron backscatter diffraction (EBSD) analysis indicated that the β-Sn grain structure depended on the distribution of precipitations. The β-Sn grain structure was altered from interlaced grains to larger grains due to the redistribution of Cu in the solder. Noteworthily, the annealing at higher temperature (210 oC) can accelerated the variation of above-mentioned microstructure, leading to enhancement of the impact reliability. The effect of annealing process on various UBMs was investigated. The SAC/Cu and SAC/Ni solder joints were annealed to 210 oC for 50 s and 100 s, respectively. In the rapid-cooled solder joints, the network type precipitations were distributed in all the solder volume of SAC/Cu joint, while the dot type precipitations in the SAC/Ni joint. With increasing annealing time, these precipitations grew larger, the interfacial intermetallic compounds (IMCs) became slightly thicker, and the hardness of solder alloys gradually decreased. After annealing for 50 s, the impact toughness of both SAC/Cu and SAC/Ni solder joints was enhanced, and the fraction of ductile fracture in these solder joints increased. However, the growth of (Cu,Ni)6Sn5 at the SAC/Ni interface degraded the impact toughness as SAC/Ni was annealed for 100 s. The difference of impact toughness in SAC/Cu and SAC/Ni was correlated to the variation of microstructure and hardness in solder joints.