Effect of thermal exposure on microstructure, orientation relationship, residual stress and failure mechanism of ultrasonic-assisted Kovar/SnSb10/Kovar joints

Abstract

Thermal exposure is often accompanied by the accumulation of inhomogeneous element diffusion, continued solid-state phase transformation, and occurrence of residual stress, causing the failure of the joint directly. In this work, we focused on the orientation relationship (OR) alternation, residual stress distribution, and failure mechanism of the reflow/ultrasonic-assisted Kovar/SnSb10/Kovar joint after thermal exposure. Especially, the role of Ni element during thermal exposure was analyzed in detail. It was noticed that the thickness of interfacial intermetallic was merely lower than 4 μm, even after thermal exposure for 45 d. The interfacial ORs of IMC/Kovar, whether ultrasound was applied or not, changed to an incoherent state. However, it did not deteriorate the ultimate shear strength, which reached over 50 MPa even after thermal exposure at 160 °C for 45 d. Compared to the reflow process after thermal exposure, the joints with ultrasonic improves the strength by about 10%. However, it dramatically decreased to only 20 MPa after thermal exposure at 200 °C for 45 d. The rationale was that ultrasonic-induced supersaturated Ni elements in solder and the Ni elements from the Kovar alloy diffused into the IMC region simultaneously, forming the (Fe, Ni)Sn2 IMC layer with relatively high residual stress and acting as the fracture origin region. The current findings demonstrated that the Kovar/SnSb10 interface proposed outstanding microstructure stability and mechanical reliability and was a promising alternative for a more extensive range of high-temperature applications.