COMBINATORIAL EFFECTS OF ELECTROMIGRATION AND LOW CYCLE FATIGUE
Authors: Christopher Hunt, Davide Di Maio, Charles Murdock, Owen Thomas Company: National Physical Laboratory Date Published: 10/24/2010
Abstract: Electromigration effects are well known above 104A/cm2, and induce excessive intermetallic growth due to enhanced diffusion processes driven by the electron current in the joint. This severe degradation of a joint will affect the mechanical integrity of the joint. In this work we have simultaneously applied mechanical fatigue in a shear mode, while applying current densities of 104A/cm2. This was done at two isothermal conditions, 40 and 125ºC, and under thermal mechanical fatigue conditions between these two temperatures. A shear strain of ±0.02 was applied to the 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.