(Drs. Charles Ume and David McDowell, co-advisors)
"Application of ISV Models to Thermal Processing and Reliability of PTHs"
Wave soldering of lead attachments to printed wiring boards (PWBs) through plated through holes (PTHs) is a mainstay packaging process. The process is extremely complex from a thermomechanical standpoint, given the transient heat transfer problem, solidification analysis and temperature dependence of the elasto-viscoplastic properties of the board material, copper plating and solder. Ultimately, the effects of thermomechanical processing-induced residual stresses on cracking in the copper plating just after the soldering or after subsequent thermal service cycles are important for selecting the wave soldering process parameters, for example. Few comprehensive analyses of this process have been reported in the literature, and fewer yet that consider realistic cyclic thermomechanical material behavior. Existing analyses have predominately employed decoupled secondary creep and power law isotropic hardening plasticity.
In this research, the cyclic thermoplastic and thermoviscoplastic internal state variable (ISV) models of McDowell are implemented within ABAQUS User Material Subroutine (UMAT) using a semi-implicit integration scheme with automatic time step controls. We report analyses of several coupled thermomechanical wave soldering processes for both empty and filled PTHs in PWBs. The dependence of the residual stress systems are discussed as a function of process route, and the influence of residual stress in the copper barrel and corner of the PTH on multiaxial fatigue is examined during subsequent military cycles.