Influence of thermal cycling on fatigue strength of Cu/Sn/Cu solder joints

Abstract

The influence of thermal cycling on the fatigue life of Cu/Sn/Cu solder joints has been examined. Copper plates were bonded with tin foil (with a solder thickness of 60 µm) and suffered thermal cycling in a temperature range of 55 or 125 K. Then they were subjected to fatigue testing at a shear stress amplitude of 2 MPa and a frequency of 3.6 Hz. With the increasing number of the thermal cycles, the fatigue life decreased from 3.0×10⁵ to 5.0×10⁴ at thermal cycle 6000. However, the fatigue life did not change so much during thermal cycling in different temperature ranges. When the solder joints suffered the thermal cycling, the η phase at the bonding interface coarsened and elongated, and its arrangement became irregular. After larger numbers of thermal cycles, fine cracks appeared in the η phase parallel to the interface. After fatigue testing, circular patterns were observed inside the bonded region on a fracture surface, and their shape and size became irregular and larger with the increasing number of thermal cycles, respectively. These showed that the reduction in fatigue life was caused by improved propagation of the fatigue crack following changes in the morphology and arrangement of the η phase during thermal cycling.