Meaning
A failure mechanism in semiconductor packaging where small vacancies in the solder interface migrate and join together to form a continuous gap. This planar void coalescence typically occurs at the boundary between the solder and the copper pad during thermal aging or power cycling. The process terminates when the electrical connection is severed or the mechanical strength of the joint falls below the operational requirement.
Growth Mechanism
Diffusion of atoms at high temperatures causes the expansion of individual micro-voids along the intermetallic compound layer. Vacancies created by the Kirkendall effect tend to align along a single horizontal axis under steady thermal load. This alignment creates a weak path that is susceptible to crack propagation when the assembly is handled or vibrated.
Reliability Assessment
Cross-sectional imaging using scanning electron microscopy is required to detect the early stages of a continuous gap forming. Engineers measure the percentage of the interface occupied by these gaps to predict the remaining useful life of the component. A coverage exceeding twenty-five percent of the total pad area usually indicates an imminent failure in high-reliability applications.
Material Influence
Solder paste chemistry and the surface finish of the printed circuit board significantly impact the rate of vacancy formation.
