A Study of the Growth Rate of Cu-Sn Intermetallic Compounds for Transient Liquid Phase Bonding during Isothermal Aging

Recently, research and development of power electronics, such as inverter systems and power modules, in electric vehicles (EV) and hybrid electric vehicles (HEV) are increasing. In the power electronic systems, the chip junction temperature is extremely high during operation. Therefore, the new bonding materials and methods are needed for operation at high temperatures for power electronic packaging. Some good candidates for high temperature die-attach applications include high-temperature solders such as Au-Sn, Ag sinter pastes, and transient liquid phase (TLP) bonding materials. Among them, a TLP bonding technology utilizes cheap metals such as Cu, Ni, and Sn. This process is also similar to the conventional soldering process and is reliable when subjected to high temperatures over long durations. TLP bonding is a die-attach technology wherein an intermetallic compound (IMC) is formed via a diffusion reaction by incorporating a low melting point metal between metals with high melting points. This paper presents the growth rate of forming Cu-Sn IMCs in the composite preform that is used as a die-attach material for a TLP bonding. To overcome the drawbacks of the TLP bonding such as a long bonding time, the composite preform was fabricated, and the feasibility for high-temperature power electronics applications was evaluated in this study. The composite preform is composed of a Cu core layer with a high-melting temperature and a low-melting temperature Sn coating at both sides of the Cu core layer. During aging treatment, the Cu-Sn IMC layer was rapidly formed by consuming the both low-melting point Sn layers. The IMC formation by the reaction between Cu and Sn is very important because it affects the thermal, electrical and mechanical reliability for electronic packaging technologies. After isothermal aging treatments at $150^{\circ}\mathrm{C}$ for various times, the thicknesses of Sn, Cu, and Cu-Sn IMC layers in the composite preform were investigated using field-emission scanning electron microscope (FE-SEM, INSPECT F, FEI, USA) equipped with an energy dispersive X-ray spectroscope (EDX) and focused ion beam (FIB, NOVA-600, FEI, USA).