Intermetallic reactions in Sn-8Zn-20In solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflow process of Sn-8Zn-20In solder joints in the ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the Au and Ag thin films react with liquid solder to form γ₃-AuZn₄/γ-Au₇Zn₁₈ and ε-AgZn₆ intermetallics, respectively. The γ₃/γ intermetallic layer is prone to floating away from the solder/Ni interface, and the appearance of any interfacial intermetallics cannot be observed in the Au/Ni surface finished Sn-8Zn-20In packages during further aging treatments at 75°C and 115°C. In contrast, ε-CuZn₅/γ-Cu₅Zn₈ intermetallics are formed at the aged Sn-8Zn-20In/Cu interface of the immersion Ag BGA packages. Bonding strengths of 3.8N and 4.0N are found in the reflowed Sn-8Zn-20In solder joints with Au/Ni/Cu and Ag/Cu pads, respectively. Aging at 75°C and 115°C gives slight increases of ball shear strength for both cases.