The Characteristics of Cu/Sn/Cu and Ni/Sn/Ni Sandwich Solder Systems for Gold-free Wafer Bonding Technology

摘要

Two sandwich-structured solder systems (Cu/Sn/Cu, Ni/Sn/Ni) are suggested for gold-free wafer bonding with a thermally stable joint. The influence of the thickness ratio of the metal layers on the microstructure and mechanical bonding strength of the solder joints was investigated by varying the thicknesses of the Cu or Ni side-layer. During the bonding process, the side-layer metal (Cu or Ni) reacts so rapidly with Sn that it is completely transformed into high melting point IMCs (Cu/Sn/Cu: Cu_6Sn_5, Cu_3Sn or Ni/Sn/Ni: Ni_3Sn_4, Ni_3Sn). Although the microstructures of sandwich joint samples are very similar to each other, the bond strength of the Cu-Sn samples is notably higher than that of Ni-Sn samples. It is inferred that both the irregular morphology and brittleness of the Ni-Sn IMCs could strongly affect the strength of Ni-Sn sample, resulting in a reduced bond strength compared to Cu-Sn.