The effect of residual stress on the flexing strength of PCB assembly

In surface mount technology (SMT), during the assembly process such as reflow and bake, the residual stress, due to CTE (coefficient of thermal expansion) mismatch, will be existed in the surface and interface between different packaging layers, especially in the root of solder joints. The residual stress will affect the interconnect strength of solder joints and the whole assembly structure. In this study, the PCB assembly test vehicles with six solder balls experienced different assembly conditions are subjected to standard three-point bending test and the finite element analysis (FEA) is also carried out to characterize the process-induced residual stresses in the solder joints. The simulation shows the same trend as failure analysis after bending test. Testing results indicate that critical bending force will decline after long time baking.