Local CTE mismatch in SM leaded packages: a potential reliability concern

Thermal expansion mismatches across solder-joint interfaces, referred to as local mismatches, create thermomechanical strains and thus fatigue damage in solder joints of leaded devices. The authors report on an experiment and a modeling program that were conducted to investigate local mismatch effects in SM (surface mount) leaded packages. It was found that, when leads are compliant enough, plastic strains in solder are due mainly to local effects. The models and failure analysis show that the impact of local effects is larger with increasing CTE (coefficient of thermal expansion) mismatch at the lead/solder interface. This is more of a concern with Alloy 42 than with copper-alloy lead frames on FR-4 substrates. Simple mechanical models are also presented to derive acceleration factors for the assessment of global and local effects in the field