Characterizations of CW Nd:YAG laser soldering of lead free solder

A continuous wave Nd:YAG laser was used to investigate the characteristics of Sn-3.5wt%Ag solder Joint. 400 um diameter solder balls were used and the laser beam diameter at focus was around 120. Experimental conditions were as follows: a laser power of 2W and heating times of 0.3, 0.5, and 0.7 second; a laser power of 3W and heating times of 0.1 and 0.3 second; and a laser power of 4W and a heating time of 0.1 second. Under all conditions, the shear strengths of IMC of (CuNi)₃Sn₄ at the solder-pad interface were larger than 554.37 gf (obtained from hot plate reflow). When the laser power was set at 2W, the microstructure of IMC was recrystallized regularly due to the active convection which was caused by increased heating times. Under the condition of a laser power of 4W and a heating time of 0.7 second, the microstructure was recrystallized irregularly due to the violent convection caused by excessive energy input (= laser power (W) x heating time (sec)). The LMC layer growth was increased by increasing the energy input; and particularly was attributable to laser power rather than heating time.