Phase stability and thermal expansion behavior of Cu6Sn5 intermetallics doped with Zn, Au and In

Cu6Sn5 is a critical intermetallic compound in future three-dimensional integrated circuit (3D IC) in micro-electronic packaging and has different crystal structures in the solid state. This paper investigates the effect of Zn, Au and In on the phase stability and thermal expansion behavior of Cu6Sn5 intermetallics over the temperature range of −80 °C to 240 °C, using in-situ variable temperature synchrotron powder X-ray diffraction (PXRD), dilatometry and first principles calculations. The results show Zn, Au and In stabilize the equilibrium high temperature hexagonal Cu6Sn5 crystal variant in directly alloyed samples, over the entire range of temperatures investigated. When present in hexagonal Cu6Sn5, Zn, Au and In atoms preferentially occupied specific Cu or Sn sites as leading to a more thermodynamically stable phase. The stabilization effect of Zn, Au and In was confirmed using PXRD, dilatometry and first principles calculations. The magnitude of thermal expansion and coefficients of thermal expansion (CTEs) were characterized for each addition. The stabilization of the hexagonal Cu6Sn5 structure prevented the discontinuity in volume expansion that occurs with the polymorphic transformation.