Extending the fatigue life of Pb-free SAC solder joints under thermal cycling

This paper reports on the dependence of the thermal fatigue life of Pb-free solder (Sn–3.0Ag–0.5Cu) joints on the points at which the printed circuit board (PCB) is fixed to a rigid support. Elastic–plastic finite element analysis under various thermal cycling conditions was performed. For the analysis, the mechanical boundary conditions around a solder joint bonded silicon chip were varied by changing the points at which the PCB was attached to its support. From the results of the analysis, it was found that the accumulated strain in the solder joints decreased when the PCB was fixed at points placed diagonally with respect to the silicon chip. The thermal fatigue life could therefore be expected to be extended by the choice of fixing points. Using a special support with a thermal expansion coefficient of less than 1.5 × 10−6 K−1, we demonstrated, experimentally, that the accumulated strain and the failure site of the solder joints could indeed be controlled by the positions at which the printed circuit board was fixed.