In situ studies revealing dendrite and eutectic growth during the solidification of Sn-0.7Cu-0.5Ag Pb-free solder alloy

Low-Ag Sn-0.7Cu-0.5Ag Pb-free solder solidifies into βSn dendrite and coupled eutectics structures. In this study, a range of solidification events in Sn-0.7Cu-0.5Ag solder alloy, such as primary βSn dendrite growth, binary eutectics and ternary eutectics growth, have been studied using in situ time-resolved synchrotron X-ray radiography. It was found that primary βSn dendrite arms showed accelerated growth, whilst secondary dendrite arms competitive growth/coarsening was strongly influenced by neighboring arms. Axial remelting of tertiary arms of βSn dendrite also occurred during coarsening. Most importantly, simultaneous growth of invariant binary βSn-Ag3Sn eutectic/ternary βSn-Cu6Sn5-Ag3Sn eutectic were observed.