Effect of Silver Content on Thermal Fatigue Life of Sn-xAg-0.5Cu Flip-Chip Interconnects

The thermal fatigue properties of Sn-xAg-0.5Cu (x = 1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver content (x = 1 and 2) had a greater failure rate compared to those with higher silver content (x = 3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x = 3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag₃Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.