| Thermally-induced failure of printed wiring boards (PWBs) has become a primary reliability concern during the component assembly process and/or during subsequent field usage in which PWBs are subjected to thermal excursions. In general, failures of plated through holes (PTHs) may occur by overstress fracture due to a thermal shock and/or may occur by initiation and growth of a strain driven fatigue crack over the course of a number of cycles. The mismatch of the coefficient of thermal expansion (CTE) between PWB and PTH constituent materials is considered to be the major driving force for thermally-induced PTH failures. Complex states of stress caused by the thermal expansion mismatch tend to concentrate at very high levels at small, critical regions around a PTH, such as the middle of the copper barrel, the PTH knee or the PTH corner and the copper innerplane.;During the PWB wave soldering process, when PTHs are often filled with Pb-Sn solder or epoxy resin as a part of the process, the state of stress/strain in the vicinity of the PTH/PWB interface becomes even more complicated from due to thermomechanical loading conditions, giving rise to a transient nature of the heat transfer problem, solidification analysis and temperature dependence of the elasto-(visco)plastic properties of the board material, copper plating and solder. Ultimately, the effects of thermomechanical processing-induced residual stresses on fatigue cracking in the copper plating just after the soldering and/or after subsequent thermal service cycles are important for selecting the wave soldering process parameters. Few comprehensive analyses of this process have been reported in the literature, and fewer yet that consider realistic cyclic thermomechanical material behavior and (multiaxial) fatigue predictions. |
