The roles of dendritic spacings and Ag3Sn intermetallics on hardness of the SAC307 solder alloy

Sn-Ag-Cu alloys have emerged as the most promising lead-free solder series among a number of alternatives. These alloys generally present a dendritic Sn-rich matrix surrounded by a eutectic mixture (β + α), where β is the Sn-rich phase and α is the Ag3Sn intermetallic compound. The present study aims to investigate the effects of dendritic (λ2, λ3) and eutectic (λ) spacings and the morphology of Ag3Sn particles on hardness of the Sn-3.0 wt%Ag-0.7 wt%Cu alloy (SAC307). In order to establish correlations between λ2,3 and hardness, transient directional solidification (DS) experiments were performed permitting a wide range of different microstructures to be examined. The techniques used for microstructure characterization included dissolution of the Sn-rich matrix, optical/scanning electron microscopy. A dendritic microstructure prevailed in the entire DS casting. It is shown that the hardness tends to decrease with the increase in λ2, λ3 and λF (eutectic spacing for Ag3Sn having a fiber morphology). Experimental equations relating microstructural spacings to hardness are proposed.