Kirkendall Voids in Soft Soldered Power Packages

Packages using Pb-based high temperature solders are commonly understood as reliable and well controlled, with known solder joint aging during thermal cycling. For certain package configurations the long-term robustness of the leadframe-solder interface was found to be sensitive to the formation of Kirkendall voids. Under thermomechanic stress, these voids cause a circular-shaped delamination of the solder layer from the underlying leadframe directly under the die. The formation of the Kirkendall voids can be controlled by the selection of solder material, temperature load at die attach, and Cu leadframe alloy composition to achieve long-term product robustness.