Interfacial reactions and shear strength on Cu and electrolytic Au/Ni metallization with Sn-Zn solder

The interfacial reaction between eutectic Sn-9 wt% Zn solder and two different kinds of ball-grid-array substrates (Cu and Au/Ni electroplated Cu) during aging at 150 °C and the shear strength of the resulting joints were investigated. In the Sn-9Zn/Cu joints, only Cu₅Zn₈ intermetallic compound (IMC) was observed between the solder and Cu layer during the first 100 h of aging. After aging for 1000 h, the layer-type Cu₅Zn₈ IMC layer was disrupted at the interface, causing it to act as a channel for the diffusion of Sn. As a result, Cu₆Sn₅ and Cu₃Sn IMCs were formed underneath the Cu₅Zn₈ IMC layer. This interfacial reaction significantly degraded the joint strength. In the case of the Au/Ni/Cu substrate, an AuZn₃ IMC layer formed at the interface because of the fast reaction between Au and Zn. In addition, the AuZn₃ IMC layer became detached from the interface during reflow. The detachment of the AuZn₃ IMC layer is presumably from the mismatch in the coefficients of thermal expansion and weak adhesion between the AuZn₃ IMC layer and Ni layer caused by the depletion of the Au layer. When the aging time was extended to 100 h, Ni₅Zn₂₁ IMC was observed on the Ni substrate. The shear strength of the aged Sn-9Zn/Au/Ni/Cu joint was significantly related to the detachment of the AuZn₃ IMC layer. After aging at 150 °C, fracturing occurred on the detached AuZn₃ IMC layer. To ensure the reliability of the Sn-Zn/Au/Ni/Cu joint, the detachment of the AuZn₃ IMC needs to be prevented.