Tin, the major constituent in Sn-Ag-Cu (SAC) lead-free solders often has difficulty in nucleating during solidification. This often results in the formation of large, brittle pro-eutectic intermetallic (IMC) phases, particularly Ag3Sn, in addition to reduced coupled eutectic volume. This results in weaker, less reliable joints. This research seeks to catalyze tin nucleation at lower undercooling, thereby maximizing the eutectic volume while preventing large pro-eutectic phase formation to promote SAC solder joint reliability.
To accomplish this, a near eutectic (NE) SAC alloy is modified with a fourth element (X) selected to favor substitution for Cu in Cu6Sn5. While unadulterated Cu6Sn5 is a poor catalyst for β-Sn, a Cu6Sn5 lattice strained by X may improve this. X candidates (Co, Ni, Fe, Mn, Zn, and Al) were selected based on Darken-Gurry criteria for having a similar atomic radius and electronegativity relative to copper.
Undercooling was measured using DSC with fluxed Cu pans at cooling rates common in industry, thereby mimicking real processing conditions. These calorimetric solder joints were then cross-sectioned and analyzed. It was discovered that some X additions increase the undercooling relative to the base SAC alloy while others decrease it. Of the selected X elements, Zn and Al, both with larger atomic radii than Cu, substitute into Cu6Sn5 and result in significantly reduced undercooling. Their corresponding microstructures are favorable and include a high eutectic volume and no embrittling Ag3Sn precipitates
