A new index S for evaluating solder joint thermal fatigue strength

An index named S is proposed for the quick evaluation of the thermal fatigue strength of solder joints in electronic equipment. If a joint is designed so that its S value is less than one under a given thermal cycle condition, the inelastic strain range in the joint solder, including plastic and creep components, is less than 0.2%, which is small enough for most electronic equipment applications. In calculating S, the effects of the three-dimensional force and the moment acting on the joint, the dimensions of the joint, including the solder-filled height, and the temperature and hold time in the thermal cycling condition are considered. To enable quick calculation of S, simple formulas are derived for various types of joints including lap, butt, J-lead, leadless type, and lead-insertion type joints. Good correlation is obtained between calculated S values and experimentally obtained thermal fatigue lifetime values