The degradation of solder joints under high current density and low-cycle fatigue

Solder joints subjected to current densities in the order of 10⁴ A/cm² will degrade due to the migration of atoms. In this paper, electromigration, in combination with low-cycle mechanical fatigue were investigated for lead free solder joints. Model solder joints were tested at 40°C, 125°C and in thermal cycles between 40 and 125°C by applying a current density of 10⁴ A/cm² and, at the same time, displacement cycling at ±7 ¿m corresponding to a shear strain of ±0.02 in the solder joint. The lifetime of these solder joints was compared to those tested without current. Both current density and ambient temperature of the joint were found to affect joint reliability with the joints mechanically failing earlier at high temperatures and with high current density. The analysis of the microstructure of the tested joints revealed that the structure changed depending on the testing conditions. In particular, the formation of large areas of Cu₆Sn₅ intermetallic compound was observed. These areas tended to grow perpendicular to the interface and often in star-shaped structures.