Experimental and numerical analyses of laser remelted Sn–0.7 wt%Cu solder surfaces

Abstract

Numerous studies on the effects of solidification thermal parameters, microstructure and reliability of Sn–Cu solder alloys can be found in the literature, however only a very limited number of investigations deal with the application of laser surface remelting (LSR) in soldering processes. One of the key advantages of the LSR rapid solidification conditions relies on the reduction in size and the more homogeneous distribution of intermetallic particles. This study aims to analyze the effect of LSR process parameters on microstructural changes and hardness evolution in the remelted region of a Sn–0.7 wt%Cu alloy. Optical and SEM microstructures were used for determination of geometrical and dimensional aspects of the remelted pool profile such as depth (d) and width (w) as a function of the laser beam speed (Vb). Both dimensions are shown to vary linearly with Vb. Hardness varied noticeably with Vb (from 11.5 to 19.5 HV for Vb varying in the range 2.4–9.0 m/min) caused by the fineness of the eutectic mixture and the corresponding more homogeneous distribution of intermetallic Cu6Sn5 particles. The theoretical predictions provided by a heat transfer numerical model are shown to represent the dimensions of the laser-remelted Sn–0.7 wt%Cu alloy pool. A growth law is proposed relating the microstructural interphase spacing to the cooling rate, encompassing both directional solidification and LSR.