Abstract Scope |
In recent years, there is an increasing demand for high-density power modules, because of their wide application fields, such as turbines, hybrid electric vehicles, medical devices, and locomotive traction. Thick aluminum wire (>100 μm) bonding is the most commonly used interconnected method to connect chip and lead frame for the power module, due to its excellent high-temperature stability and low cost. Hence, a reliable and robust bonding joint of aluminum wire is of great importance for the power module packaging.
Sufficient interfacial elemental diffusion and reaction are the prerequisites for forming a strong metallurgical bond. Over the past decades, ultrasonic ball and wedge bonding methods have been extensively studied to realize thin aluminum wires diffusion bonding for integrated circuit packaging. However, the weak bonding joints and insufficient interfacial reaction caused by low output power limited its application in thick aluminum wire bonding. Ji et al. found that even if the 25-μm thin Al wire is bonded with the Au/Ni/Cu pad by the ultrasonic wedge method, no obvious element diffusion, and new phase formation were observed. Oldervoll et al.reported that only a small amount of purple IMCs were observed after storing under 225 °C for three months of the ultrasonic wedge-wedge bonded 25-μm aluminum wire with plated gold pad. Compared with ultrasonic bonding, parallel gap micro resistance welding seemed to be a good alternative with controlled output power and working stability. During the ultrafast welding process, adequate elemental diffusion and reaction can be formed at the bonding interface due to enough input power, and a higher bonding strength was obtained. Nerveless, one crucial issue associated with this bonding method is the lack of research on the interfacial microstructure, resulting in instability of the bond quality.
In this work, the interfacial phases of the Al/Au/Cu joint after parallel gap micro-resistance welding were observed by high-resolution transmission electron microscopy (HRTEM). Obvious Al, Au, and Cu elemental interdiffusion can be observed by energy dispersive spectrometer (EDS) mapping, indicating the occurring of sufficient interfacial reaction. The interfacial phases mainly contain three phases, Cu-based solid solution, (Au, Cu)Al2, and (Cu, Au)9Al4. Thanks to the effect of electric current and high temperature, liquid Al at the center region rapidly reacted with Au and form (Au, Cu) Al2. Excessive Al atoms would also diffuse to the interface between Cu-based solid solution and Cu, forming (Cu, Au)9Al4. |