Effect of composition on the low-cycle fatigue of Pb alloy solder joints

Low-cycle fatigue tests in shear at 300 K (total strain control, reversed ramp cycling at a frequency of 0.1 Hz) were performed on solder joints of Pb-Sn alloys with 10-75 wt.% Sn, a 36Pb-62Sn-2Ag alloy, and a 50Pb-50In alloy. The fatigue results (based on 95% lead-drop failure criterion) followed the Coffin-Manson (C-M) equation with a C-M exponent q≈0.4 which decreased slightly with volume fraction of the β-phase (f_β^v) and a C-M constant C≈1 which decreased slightly with F_β^v. Plots of the decrease in load and increase in plastic strain range with cycling exhibited stage-I behavior only for F_β^v>0.7 and total strain range <0.1. The number of cycles required for the termination of stage I increased significantly when F_β^v was greater than this amount. No significant effect of 2-wt.% Ag on the various mechanical properties could be detected other than would occur from the addition of an equivalent amount of Sn. The addition of 50-wt.% In to Pb had no appreciable effect on the monotonic or cyclic stress-strain behavior, nor on the C-M exponent; it, however, produced a three-fold increase in the C-M constant, thereby giving superior fatigue resistance over the Pb-Sn (Ag) alloys for the 95% load-drop criterion. However, no beneficial effect of the In occurred when the termination of stage I was taken as the failure criterion, suggesting that In retards crack growth but not initiation. The effects of alloy content on the fatigue behavior of the present alloys are in qualitative agreement with theoretical considerations