A NEW INDEX-S FOR EVALUATING SOLDER JOINT THERMAL FATIGUE-STRENGTH

A new index named S is proposed for quick evaluation of 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 following factors are taken into account: 1) three-dimensional force and the moment acting on the joint, 2) dimensions of the joint, including the solder fillet height, and 3) temperature and hold time in the thermal cycle condition. To enable quick calculation of S, simple formulas of various types of joints, including lap, butt, J-lead, leadless type, and lead-insertion type joints, are derived. Good correlation is obtained between calculated S values and experimentally obtained thermal fatigue lifetime values. © 1990 IEEE