Evaluation of Fatigue Damage in Surface Mount Solder Joints

Study of Evaluation Method

Abstract

Fatigue in solder joints in thermal cycling is one of the problems in surface mount technology. This paper proposes the evaluation method of fatigue damage (fatigue crack length) in solder joint by the measurement of residual strength (peeling strength) of solder joint. An attempt was made to evaluate quantitatively the fatigue damage in solder joint, paying attention to the residual strength of cracked solder joint. The fatigue damage in solder joints was given by a four-point bending mechanical fatigue test of the substrate, on which some packages were mounted. After the fatigue test, the peeling test of each solder joint and the observation of each fracture surface by a scanning electron microscope were carried out to investigate the relationship between the fatigue damage and the residual strength. On the other hand, numerical analyses were carried out to estimate the residual strength of the cracked solder joint. The elasto-plastic finite element method was employed for these analyses. Furthermore, the effects of solder material, solder volume and package lead shape on the fatigue damage were investigated. Two kinds of solder materials (35Pb63Sn2Ag and 96.5Sn3.5Ag) were used in these solder joints.
It was found that the fatigue damage can be evaluated quantitatively by the residual strength of solder joint, and the residual strength can be estimated by the finite element analysis.