IBM engineers recently conducted a study to better understand and control the reliability of copper pillar solder joints in 2.5-D packages. Here they describe their approach and the results they obtained. They explain how they created test samples to evaluate different solder compositions, pillar geometries, and thermal histories and assess their effect on microstructure, precipitate morphology, intermetallic layer thickness, and shear strength. They also present thermal cycling test results comparing the performance of silicon and glass interposers.

This article presents two case histories that shed light on the role of gold in lead-free solder joint failures and the damage mechanisms involved. One of the failures, a brittle fracture of the solder joint, is attributed to the synergistic effects of voids, intermetallic compounds, and CTE mismatch. The investigation of the other failure revealed evidence of tin-whisker formation. As the author explains, the growth of tin whiskers is due to compressive stress in the tin solder material caused by diffusion of end-cap metals (Ni and Cu) and the formation of Sn-Ni-Au intermetallics. In both cases, the failures can be prevented by limiting the thickness of gold on all components.

This article presents a method for determining the integrity of solder joints made from mixed and lead-free solders. It discusses the procedures involved in sample preparation and testing and explains how to interpret the results, particularly the effect intermetallic formation, cracking, and voiding.