In this paper, we discuss the bad pair condition causing a singular stress at a vertex of the interface in three dimensional (3D) prismatic butt joints in comparison with the bad pair condition along the side. 3D finite element (FE) eigenanalysis is applied to obtain the singular index. We found that the bad pair condition at a vertex is slightly different from the well-known 2D bad pair condition α(α-2β) > 0, which can be expressed using Dundurs' parameter (α, β). It should be noted that in most cases the bad pair can be judged by (α, β) except for some cases where the bad pair depends on the material combination even under the same (α, β). To clarify the difference, the bad pair conditions are compared on the (α, β) diagram. For convenience, a simple expression for a bad pair is proposed on the basis of the theoretical consideration, and the validity is confirmed numerically. We also confirm that the bad pair condition along the side of the interface can be expressed using Dundurs' parameter as α(α-2β) > 0.

We bonded a Cu terminal and Cu substrate together using ultrasonic vibration and evaluated its mechanical properties and interfacial microstructure in order to investigate the factors which influence the initial reliability, and the formation mechanism of a high reliability interfacial microstructure. We controlled the bonding load and amplitude, and evaluated the initial strength of the joints as well as their thermal reliability. It was suggested that the formation of the initial bonding-nucleus at the early period of ultrasonic bonding and a bonding condition of high load and amplitude were the principal factors that increased the initial strength of the joints. It was also suggested that long-term reliability was improved because the interfacial microstructure between the Cu terminal and Cu substrate was comprised of fine grains which resulted from using high strength Cu alloy as the Cu substrate.

This paper reviews the previous research on the methodologies for evaluating structural integrity of wire bonds and die-attachments in power modules. Under power module operation, these parts are subjected to repeated temperature variations which induce repeated thermal stress due to the mismatch in the coefficients of thermal expansion of the constituent materials. Thus, thermal fatigue phenomena are critical issues for the structural integrity of power modules. In the present paper, we also deal with the evaluation methodologies for thermal fatigue in the temperatures over 200°C, which are expected operational temperatures for wide bandgap semiconductor power modules. We also propose a research methodology for confirming the structural integrity of power modules in the design phase.