Effect of Joint Size on Creep Behavior of Sn-Ag-Cu Solder-Cu Joints

This study investigates the effect of the joint length scale on the creep behavior of Pb-free Sn-rich Sn-3.0 wt.% Ag-0.5 wt.%Cu (SAC 305) solder-Cu joints. SAC 305-Cu solder joints having SAC solder layers of different thickness ranging from 1 mm to 50 mu m were fabricated by diffusion bonding and examined using a scanning electron microscope to ascertain the role of length scale on the microstructure of the joints. Subsequently, tensile creep tests were performed at 85 degrees C. At given nominal stress, a systematic reduction in the creep rate occurred with a reduction in joint thickness, which can be expressed by a power law. A comparison of the creep stress exponent of bulk SAC solder and the solder-Cu joints revealed that the creep mechanism was dislocation climb-controlled by core or pipe diffusion in both types of samples. The overall size-dependent creep strengthening of SAC 305-Cu joints can be attributed to the geometrical effects, which affect the macroscopic stress state, and the microstructural effect arising due to a change in the statistical distribution of Sn grains in the solder and the constraints on dislocation motion due to elastic Cu substrates. The additional strengthening imparted by precipitates in SAC solder was delineated by comparing the creep behavior of Sn-Cu joints with the SAC-Cu joints.

Automated summary

This study investigates the effect of joint length scale on the creep behavior of Pb-free Sn-rich SAC 305 solder-Cu joints. The results show a systematic reduction in creep rate with a decrease in joint thickness. The creep mechanism and microstructural effects on the creep strengthening of the joints are discussed.