Intermetallic Kinetic Growth and Latent Solder Joint Shear Failure Modeling
Latent solder joint failure stems from thermally activated intermetallic growth and Kirkendall voiding, requiring high-speed shear testing to detect interface embrittlement.
Kirkendall Void Formation
Vacancy Aggregation
Microscopic cavities appear at the interface of two metals when the rates of atomic diffusion in opposite directions are unequal. The occurrence of Kirkendall void formation typically happens between the copper pad and the tin-rich solder in electronic assemblies. These holes coalesce along the intermetallic boundaries, creating a perforated line that weakens the structural attachment.
Such defects are often invisible to standard X-ray inspection because of their extremely small size.
Interface Degradation
Diffusion imbalances lead to a net flux of vacancies that accumulate at the grain boundaries. In the context of Kirkendall void formation, the copper atoms migrate into the solder faster than the tin atoms can move into the copper. This disparity leaves behind empty lattice sites that eventually merge into larger voids.
Failure Mechanism
Mechanical stress triggers the propagation of cracks along these weakened interfaces. If Kirkendall void formation is present, a solder joint that appears electrically sound might fail suddenly when subjected to thermal cycling or physical shock. High-temperature storage testing is the primary method used to evaluate the susceptibility of a specific plating and solder combination to this defect.
Surface finishes like Electroless Nickel Immersion Gold help mitigate the problem by providing a barrier layer that prevents direct copper-tin contact.