Formation of a Metastable Phase at the Interface Between Sn and Ag–Pd Substrates During Liquid-State Reaction

Ag–Pd alloys are widely used as thick-film conductors and are potential alternatives to the expensive Au bump. In this work, because Sn is the primary element in solders, we investigated Sn/Ag–Pd interfacial reactions at 250°C as a means of assessing the reliability and evaluating reflow reactions at joints between solder and Ag–Pd conductor contacts, and in the Ag bump combined with the solder cap. The experimental results showed that Sn/Ag–Pd interfacial reactions at 250°C are different from those of Sn/Ag and Sn/Pd. A metastable Sn–Ag–Pd ternary phase is formed when the amount of Pd added is 20–40 at.%. Because, in commercial applications, at least 20 wt.% Pd (~20 at.% Pd) is used in Ag–Pd alloys to eliminate the silver-migration phenomenon, assessment of the reliability of Ag bumps and the soldered joints of Ag–Pd thick film hybrid circuits must be based on Sn/Ag–Pd interfacial reactions, not those of Sn/Pd and Sn/Ag.