Effects of laser parameters on the characteristics of a Sn-3.5 wt.%Ag solder joint

Abstract

Microstructure and shear strength were assessed for a Sn-3.5 wt.%Ag solder joint formed using a laser heat source. A continuous wave Nd:YAG laser was used to make the Sn-3.5 wt.%Ag solder joint. Solder balls of 400μm diameter were used and the laser beam spot diameter at focus was approximately 120μm. The UBM (Under Bump Metallurgy) on a FR4-PCB consisted of Cu/Ni/Au from bottom to top with a thickness of 15μm/5μm/0.05μm, respectively. In order to position solder balls on the UBM, RMA (rosin mildly activated) type flux for a BGA (Ball Grid Array) was used. Selected optimal conditions were as follows: a laser power of 2W and heating time of 0.3 s, 0.5 s, and 0.7 s; a laser power of 3 W and heating time of 0.1 s and 0.3 s; and a laser power of 4 W and a heating time of 0.1 s. Under all conditions, the shear strengths of the solder joint of (CuNi)₃Sn₄ at the interface between the pad and solder were larger than 554.37 gf (i.e. the shear strength obtained from hot plate reflow). When the laser power was set at 2 W, the microstructure of IMC (intermetallic compound) was recrystallized regularly due to active convection, which was caused by increased heating time. Under a laser power of 4 W and heating time of 0.7 s, the microstructure was recrystallized irregularly due to violent convection caused by excessive energy input (=laser power (W) ×heating time (s)). The IMC layer increased in thickness as a result of increasing the energy input, and was affected by laser power more than by heating time.