Prediction of temperature cycling life for SMT solder joints on TCE-mismatched substrates

A method for the prediction of solder-joint cycle life in surface-mount assemblies is presented. It is based on the conversion of plastic solder shear strain into cycle life with an equation derived by W. Engelmaier (1983). The shear strain is normally calculated from temperature and TCE (thermal coefficient of expansion) differentials between the package and interconnect board without consideration of elastic deformations. The suggested method derives the average plastic shear strain of the solder joint at the maximum temperature excursion from the finite-element analysis of a simple model that consists of the interconnect board, a solder joint, and the package. All materials in the model have linear (elastic) properties, except solder, which has nonlinear (elastic/plastic) characteristics. The solder joint is modeled as single finite element so that only one value is computed for the plastic shear strain in the solder joint. This value represents the average shear strain, which is converted into solder-joint cycle life. The cycle life predictions with the finite-element method are confirmed by cycling results obtained on actual hardware.<
>