Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability

TLDR

The industry uses electroless Ni(P)/Au pads, but persistent black pad defects from surface cracks have prompted exploration of Cu‑based alternatives, which risk rapid solder–Cu reactions during solid‑state aging and threaten joint reliability at high temperatures. This study investigates the reaction between eutectic SnPb solder and electrodeposited laminate Cu. It focuses on microstructural evolution at the Cu₃Sn/Cu interface during solid‑state aging and its effect on joint reliability. Kirkendall voids at the Cu₃Sn/Cu interface weaken bonding, causing brittle fracture in shear and pull tests, and demonstrate that a Cu diffusion barrier is required for high‑temperature (>100 °C) and high‑stress applications.

Abstract

The electronic packaging industry has been using electroless Ni(P)∕immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni(P) plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu3Sn and Cu. The void formation resulted in weak bonding between solder and Cu and led to brittle fracture at the interface in the ball shear and pull tests. The experimental results indicate that a barrier for Cu diffusion may be needed between the solder and the type of Cu used in the test vehicle for the packages that will experience high temperature (>100°C) and high stress.