Effects of spreading behaviors on dynamic reflectivity in laser soldering

Abstract

The random occurrence of component burn damage is a major challenge for laser soldering, but the mechanism has not been clear. In order to find the reason, a new integrating sphere platform was developed to simulate the actual process of laser soldering, and observe the real-time variation of laser reflectivity (LR) from initial fusion to fully spreading out of the solder. The results showed that the whole process is divided into 5 stages, which are initial low LR stage, rapidly rising moment, decreasing stage, re-increasing stage, and stable stage with high LR (around 64%). By analyzing the spreading behaviors and cross microstructures of solder joints, it was found that LR has no direct correlation with the spreading shape or temperature variation of solder paste, but is dependent on the interaction of laser multi-reflections within solder powders, one-time reflection of melt surface, and heat conduction from upper melted liquid to lower unmelted powders. The findings indicated that the burn damage is attributed to the combined effect of high LR and the concentrated reflection caused by crescent solder at the final stage.