A Nonlinear Multi-Domain Thermomechanical Stress Analysis Method for Surface-Mount Solder Joints—Part II: Viscoplastic Analysis

This is part II of a two-part paper presented by the authors for thermomechanical stress analysis of surface mount interconnects. A generalized multi-domain Rayleigh Ritz (MDRR) stress analysis technique has been developed to obtain the stress and strain fields in surface-mount solder joints under cyclic thermal loading conditions. The methodology was first proposed in Part I by the authors and results were presented for elastic-plastic loading (Ling et al., 1996). This paper extends the analysis for viscoplastic material properties. The solder joint domain is discretized selectively into colonies of nested sub-domains at locations where high stress concentrations are expected. Potential energy stored in the solder domain and in the attached lead and Printed Wiring Board (PWB) is calculated based on an assumed displacement field. Minimization of this potential energy provides a unique solution for the displacement field, consequently, stress and strain distribution. The MDRR technique was demonstrated to provide reasonable accuracy for elastic deformation (Ling and Dasgupta, 1995) and for time-independent elastic-plastic deformation (Ling and Dasgupta, 1996) for solder joints under cyclic thermal loading conditions. A piecewise linear incremental loading technique is used to solve the nonlinear elastic-plastic problem. The focus in the current paper is primarily on time-dependent viscoplastic deformation of the solder joints. Full field elastic, plastic, and viscoplastic analyses are performed, and the stress, strain hysteresis loops are obtained. Results are presented for a J-lead solder joint as an illustrative example.