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

Abstract

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.