Formation and Distribution of Sn-Cu IMC in Lead-Free Soldering Process Induced by Laser Heating

The thickness evolution of Sn-Cu intermetallic compounds (IMCs) in laser reflowed solder joints was investigated. We applied a Q-switched, frequency tripled Nd:YAG laser to precisely control the exposing energy. Sn96.5Ag3.0Cu0.5 lead free solder alloy was heated up by a two-phase, pre-heat and reflow, soldering technique. The microstructure of the solder joint was analyzed with SEM on cross-section samples. We have identified the composing elements by SEM-EDS. The results showed that the mass fraction of the IMC inside the solder bulk is significantly high, which might be a reason of the completely different mechanical properties of the laser reflowed solder joint. At the beginning of the dissolution only small sized particles were observable inside the solder bulk. With the increasing of the soldering time, the growth rate of the IMC decreased.