Formation of Ag₃Sn plates in Sn-Ag-Cu alloys and optimization of their alloy composition

Abstract

Owing to active R & D efforts, substantial progress toward a full transition to Pb-free soldering technology has been made recently. At present, the leading candidate solders are near-ternary eutectic Sn-Ag-Cu alloys for various soldering applications. The near-eutectic ternary Sn-Ag-Cu alloys yield three phases upon solidification, β-Sn, Ag3Sn and Cu6Sn5. During solidification, the equilibrium eutectic transformation is kinetically inhibited. While the Ag3Sn phase nucleates with minimal undercooling, the β-Sn phase requires a typical undercooling of 15 to 30°C for nucleation. As a consequence of this disparity in the required undercooling, large, plate-like Ag3Sn structures can grow rapidly within the liquid phase, during cooling, before the final solidification of solder joints. When large Ag3Sn plates are present in solder joints, they may adversely affect the mechanical behavior and possibly reduce the fatigue life of solder joints by providing a preferential crack propagation path along the interface between a large Ag3Sn plate and the β-Sn phase. In the present paper, an indepth study to understand the formation of large Ag3Sn plates in Sn-Ag-Cu solder balls and joints is reported. It is also demonstrated how large Ag3Sn plate formation can be minimized in solder joints through optimizing their alloy compositions or controlling cooling rates.