Dissolution and Interface Reactions between Palladium and Tin (Sn)-Based Solders: Part I. 95.5Sn-3.9Ag-0.6Cu Alloy

The interface microstructures and dissolution behavior were studied, which occur between 99.9 pct Pd substrates and molten 95.5Sn-3.9Ag-0.6Cu (wt pct, Sn-Ag-Cu) solder. The solder bath temperatures were 513 K to 623 K (240 A degrees C to 350 A degrees C). The immersion times were 5 to 240 seconds. The IMC layer composition exhibited the (Pd, Cu)Sn-4 (Cu, 0 to 2 at. pct) and (Pd, Sn) solid-solution phases for all test conditions. The phases PdSn and PdSn2 were observed only for the 623 K (350 A degrees C), 60 seconds test conditions. The metastable phase, Pd11Sn9, occurred consistently for the 623 K (350 A degrees C), 240 seconds conditions. Palladium-tin needles appeared in the Sn-Ag-Cu solder, but only at temperatures of 563 K (290 A degrees C ) or higher, and had a (Pd, Cu)Sn-4 stoichiometry. Palladium dissolution increased monotonically with both solder bath temperature and exposure time. The rate kinetics of dissolution were represented by the expression At (n) exp(a dagger H/RT), where the time exponent (n) was 0.52 +/- A 0.10 and the apparent activation energy (a dagger H) was 44 +/- A 9 kJ/mol. The IMC layer thickness increased between 513 K and 563 K (240 A degrees C and 290 A degrees C) to approximately 3 to 5 A mu m, but then was less than 3 A mu m at 593 K and 623 K (320 A degrees C and 350 A degrees C). The thickness values exhibited no significant time dependence. As a protective finish in electronics assembly applications, Pd would be relatively slow to dissolve into molten Sn-Ag-Cu solder. The Pd-Sn IMC layer would remain sufficiently thin and adherent to a residual Pd layer so as to pose a minimal reliability concern for Sn-Ag-Cu solder interconnections.