Effects of nitrogen on wettability and reliability of lead-free solder in reflow soldering

Electronic assembly technology is in the transition from traditional tin-lead to lead-free, which leading to challenge in many aspects. It is accepted that nitrogen can improve the wettability and reliability of lead-free solders; however, few systemic researches have been reported. In this work, the spreading ratio of Sn-Ag-Cu solder was measured through spreading tests under different atmosphere conditions. When oxygen concentration was 1000ppm and reflow temperature was 5degC lower than non-inerted reflow, the spreading ratio of Sn-Ag-Cu solder was comparable with the non-inerted one. PCBs and SMT components were assembled in nitrogen-inerted (oxygen concentration: 1000ppm) and non-inerted reflow. The nitrogen-inerted reflow temperature was 5degC lower. Pull and shear tests were employed as the evaluation of the soldering. Random vibration test and isothermal aging test were conducted as reliability evaluation. The morphology of the interfacial intermetallic compounds (IMCs) during wetting and reliability tests were studied as well.