Influence of Surface Mount Lead End Geometry on Fatigue Life

The local coefficient of thermal expansion (CTE) mismatch between compliant surface mount component leads and the solder that is used to attach the components to a printed wiring board can dramatically influence the thermal fatigue life of the solder joint. To quantify the contribution of the local CTE mismatch to the overall thermal fatigue damage of the solder joint, a finite element thermal fatigue simulation using an energy partitioning technique is used to compare four different lead end shapes. The four lead configurations considered are J-lead, and three gullwings leads; one with the foot parallel to the board surface, one with the foot sloped slightly downward towards the board, and one with the foot sloped slightly upward. The dimensions of the leads are purposely chosen so that the in-plane compliance is equal for the different lead shapes, only the shape of the lead end varied. Comparisons are first made with equal solder joint heights and then the solder height of the gull-wing lead is varied between 0.05 to 0.23 mm (2 to 9 mils). The influence of the solder wetting angle is also investigated.