When an electronic package is subjected to thermal cycling, the solder joint interconnects are subjected to a complex stress system. If the stress is sufficiently large, the solder joint will show evidence of plastic flow along with microstructure coarsening and possible fatigue crack initiation and propagation. Plastic flow has not been studied as thoroughly as the later two phenomena although it is often observed at surface mount or through-hole solder joints. The thermal expansion mismatch between different materials in the package is responsible for the plastic deformation which accumulates with thermal cycling. In this study, the accumulated plastic deformation process is modelled with finite element (FE) methods and compared with experimental results. Lead-frame solder joints have been analyzed with a nonlinear FE program using temperature and time-dependent properties. Steady-state creep is considered using data for eutectic lead/tin solder which is described by a hyperbolic sine creep law: ε = A(sinh Bσ)ndmexp(−Q/RT). The analysis correctly simulates the large plastic flow found experimentally in a lead-frame solder joint. The resulting stress and strain distributions indicate possible failure modes which are not anticipated on the basis of uniform shear assumptions or predictable from an FE analysis of the initial geometry.

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