Mechanical reliability of transient liquid phase bonding of Au-Sn solder with Ni(Cu) substrates

Mechanical reliability of transient liquid phase (TLP) bonded Au-Sn/Ni(Cu) joints was investigated in this study. The results show that the TLP bonded Au-Sn/Ni(Cu) joints have excellent mechanical performance, even after exposed at 400 or 450 A degrees C, especially in Au-Sn/Ni-30Cu and Au-Sn/Ni-40Cu systems. The shear strength could reach 75 MPa in TLP bonded Au-Sn/Ni-40Cu joint and increased to 87 and 97 MPa after exposed at 400 or 450 A degrees C for 24 h, respectively. It was found that during TLP bonding at 350 A degrees C, ordered AuCu, Ni3Sn2, and alpha(Au) phases formed in the Au-Sn/Ni-Cu joints, then the ordered AuCu phase transferred to disordered (Au,Cu) phase and a new Ni3Sn phase appeared between substrate (Au,Cu) and the Ni3Sn2 layers after exposure at 400 or 450 A degrees C. However, only Ni3Sn2 layer formed after TLP bonded and new Ni3Sn layer appeared near substrate after exposure at 400 or 450 A degrees C in the Au-Sn/Ni bonding system. The ductile Au-Cu [ordered AuCu or disordered (Au,Cu)] layer releases local stress concentration in joint under shear stress, which improves the mechanical reliability. Moreover, the maximum stress in Au-Sn/Ni(Cu) joints decreases with the increasing thickness of Au-Cu layer. As a result, the Au-Sn/Ni joint presents lower shear strength as brittle Ni3Sn or Ni3Sn2 layer formed near substrate, whereas the shear strength of Au-Sn/Ni-Cu joint increases with thicker Au-Cu layer.