Solid-State Interface Reactions between Silver and 95.5Sn-3.9Ag-0.6Cu and 63Sn-37Pb Solders

The microstructure and rate kinetics were investigated for solid-state intermetallic compound (IMC) layer growth in Sn-Ag-Cu/Ag and Sn-Pb/Ag couples. The aging temperatures were 55 °C to 205 °C and 70 °C to 170 °C, respectively. The aging times were 1 day to 400 days. The Sn-Ag-Cu/Ag IMC layer included the phases: Ag₃Sn (dominant), the ζ phase, and a solid-solution phase, xAg-ySn, (94 to 96 at. pct Ag). Void formation was observed at the solder/IMC (Ag₃Sn) interface, which was attributed to the Kirkendall effect. The rate kinetics of IMC layer growth were expressed through t ⁿ exp(−ΔH/RT). The values of n and ΔH for the IMC layer phases were: (1) ζ + xAg-ySn, n = 0.68 ± 0.23, ΔH = 59 ± 15 kJ/mol; (2) Ag₃Sn, n = 0.22 ± 0.05, ΔH = 28 ± 2 kJ/mol; and (3) total layer, n = 0.34 ± 0.06, ΔH = 45 ± 3 kJ/mol. The very low ΔH value for Ag₃Sn suggested a very fast, fast-diffusion mechanism. The IMC layer of Sn-Pb/Ag couples was predominantly the Ag₃Sn stoichiometry. The IMC layer growth was accompanied by the development of a Pb-rich phase layer at the solder/IMC interface. There was insignificant void development in these couples. The rate kinetics parameters, n and ΔH, were 0.44 ± 0.06 and 51 ± 4 kJ/mol, respectively, indicating a traditional fast-diffusion mechanism.