FATIGUE-CREEP CRACK-PROPAGATION PATH IN SOLDER JOINTS UNDER THERMAL CYCLING

The present study investigates the thermal fatigue crack propagation p ath in a eutectic solder joint between a 2512 leadless chip resistor a nd a printed wiring board which had experienced thermal cycling betwee n -55 and 125 degrees C. This was achieved through the microstructural examination of fractured surfaces of the joints. Patches of finely sp aced striations were observed in a predominant shear strain field in t he joints. These striations were attributed to the tensile strain comp onents in the field and used, to add the identification of the fatigue crack propagation direction. It was observed that cracks did not simp ly propagate across the depth of the joint from the inner end (the hee l) to the outer end (the toe) in the longitudinal direction, but from a corner point on the free edge of the heel to the center across the j oint depth, making an angle of about 70 degrees with respect to the lo ngitudinal direction.