Thermodynamic understanding of Sn whisker growth on the Cu surface in Cu(top)-Sn(bottom) bilayer system upon room temperature aging

Sn whiskers are observed by scanning electron microscope on the Cu surface in Cu(top)-Sn(bottom) bilayer system upon room temperature aging. Only Cu6Sn5 phase appears in the X-ray diffraction patterns and no Sn element is detected in the Cu sublayer by scanning transmission electron microscopy. Based on the interfacial thermodynamics, the intermetallic Cu6Sn5 compound phase may form directly at the Sn grain boundary. Driven by the stress gradient during the formation of Cu6Sn5 compound at Sn grain boundaries, Sn atoms segregate onto the Cu surface and accumulate to form Sn whisker.