Solder Creep - Fatigue Interactions With Flexible Leaded Parts

With flexible leaded parts, the solder-joint failure process involves a complex interplay of creep and fatigue mechanisms. To better understand the role of creep in typical multi-hour cyclic loading conditions, a specialized non-linear finite-element creep simulation computer program has been formulated. The numerical algorithm includes the complete part-lead-solder-P WB system, accounting for strain-rate dependence of creep on applied stress and temperature, and the role of the part-lead dimensions and fi’exibihty that determine the total creep deflection (solder strain range) during stress relaxation. The computer program has been used to explore the effects of various solder creepfatigue parameters such as lead height and stiffness, thermal-cycle test profile, and part/board differential thermal expansion prop erties. One of the most interesting findings is the strong presence of unidirectional creep-ratcheting that occurs during thermal cycling due to temperature dominated strain-rate effects. To corroborate the solder fatigue model predictions, a number of carefully controlled thermal-cycle tests have been conducted using special bimetallic test boards.