Low Cycle Fatigue Behavior of SnAgCu Solder Joints

Low cycle mechanical fatigue tests on 96.5Sn-3Ag-0.5Cu lead-free solder joints were carried out using a micro-uniaxial fatigue testing system at 25 °C with different frequencies (1∼10 Hz) and a wide range of strain (2%∼8%). The results show that the low cycle fatigue (LCF) life of the solder joint follows the Coffin-Manson equation in different strain ranges. Frequency-modified Coffin-Manson equation can describe the frequency effect on the fatigue life of lead-free solder joints. The analysis of failure process show that the fatigue cracks initiate at the interfaces between the solder and inter-metallic compound (IMC) around the edge of the joints, and then propagate within the solder along the solder/IMC interfaces proximately. Fracture morphologies of solder joints under different frequencies mainly consist of an initiated region, a propagation region and a final fracture region. The fracture mechanism of the final fracture region transforms from inter-granular fracture to trans-granular fracture with increasing of frequency.