Global/local modeling for PWB mechanical loading

PWB assemblies are sometimes subjected to mechanical loading during their lifetime, which will cause PWB deflection and stress/strain in the assemblies. These mechanical loads may be either monotonic or cyclic. For example, assembling force may cause PWB deformation if the PWB has an initial warpage and key striking may apply a cyclic load to PWB. These mechanical loadings will increase stress/strain level inside PWB and may have an effect on interconnect, PWB, or package reliability. In order to evaluate the reliability of PWB assembly, a global/local modeling methodology was developed. In this method, a PWB with micro-scale BGAs is modeled as a global model with relatively coarse mesh, which is used to capture the deformation of PWB under mechanical loading. Both the critical package and critical solder joint can also be located based on the stress/strain distribution obtained from this global model prediction. Then the critical package is modeled as a local model with a fine mesh to capture the details of packages and interconnects. In this study, solder joints of micro-scale BGAs are modeled in detail in order to capture the detail stress/strain distribution. The deformation captured by the global model is transferred to the local model as boundary conditions. Finally, a strain energy based reliability model is proposed to estimate the life of solder joints under cyclic mechanical loading and this model was calibrated by the experimental data.