Effects of Cooling Rate on the Microstructure and Morphology of Sn-3.0Ag-0.5Cu Solder

This study explored the effect of the cooling rate on the microstructure and morphology of Sn-3.0Ag-0.5Cu (SAC305) lead-free solder. In the experiments, rapid cooling (P1: 63.17°C/s) of SAC305 solder resulted in high tensile strength (60.8 MPa) with no significant loss in ductility (strain >40%) due to the formation of fine-grained primary β-Sn (average size ∼14 μm) surrounded by a network-like fine eutectic structure consisting of β-Sn and particle-like Ag₃Sn compound. As the cooling rate was reduced, the morphology of the Ag₃Sn compound evolved progressively from a particle- to a needle-like form and finally to a leaf- or plate-like form. The cooling rate significantly affected the β-Sn grain size and the morphology of the Ag₃Sn compound. Water cooling (at the fastest cooling rate of 100°C/s) of a solder sample resulted in a microstructure consisting of the finest structure of Ag₃Sn and β-Sn with no Cu₆Sn₅, consequently exhibiting the highest hardness of the various specimens. By contrast, after cooling at the slowest rate of 0.008°C/s, the sample exhibited a coarse eutectic structure consisting of large plate-like Ag₃Sn compound and isolated long rod-like Cu₆Sn₅ precipitates. This coarse structure resulted in both lower hardness and poorer tensile strength.