Masanori Usui

Effects of uni-axial mechanical stress on IGBT characteristics

This paper describes the impacts of mechanical stress on vertical power devices. The stress dependence of the DC characteristics of trench insulated gate bipolar transistors (IGBTs) was measured. The experimental results could be reproduced by the device simulation, which included stress dependence models of the carrier mobility and the band gap. We found that the stress dependence of the on-state voltage mainly arose from the MOSFET portion of the IGBT. Using the device simulation, we estimated the effects of mechanical stress on the surge voltage and the saturation current, which give us the important information for designing a power module with higher ruggedness.

Thermal simulation of joints with high thermal conductivities for power electronic devices

Abstract The thermal properties of new power modules joined by materials with high thermal conductivities, such as Ag or Cu nanoparticle joints, can differ from those of current modules joined by ordinary solders with low thermal conductivities. However, these properties have not been thoroughly investigated thus far. The overall thermal resistance of a simple simulation module was calculated by the 3-dimensional finite element method to study the correlation between the thermal conductivity of the joint layer and the thermal properties. The calculation results identified an optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink. This is presumed to occur because the thermal resistance decreases in the heatsink much more than it increases in the joint layer, owing to the increased uniformity of thermal spreading as the joint-layer thickness increases to the optimal value. This effect of thermal resistance reduction with thickening of the joint layer is seen when the thermal conductivity of the joint layer is sufficiently higher than that of the heatsink and the area of the joint layer is sufficiently smaller than that of the heatsink. The same effect is also expected in an actual module with a joint between a silicon carbide chip and a direct bonded copper substrate. This study reveals that the design concept for power modules should change to preliminarily estimate the optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink.

Degradation of a sintered Cu nanoparticle layer studied by synchrotron radiation computed laminography

Abstract The degradation process in a sintered Cu nanoparticle layer was studied by synchrotron radiation computed laminography (SRCL), which allows the high-resolution nondestructive observation of internal cracks in flat devices. A Cu layer was produced by sintering Cu nanoparticles between a Si chip and a direct bonded aluminum (DBA) substrate. This layer was then stressed by applying multiple thermal cycles, after which SRCL measurements were repeatedly performed. Images of the distributions of sintered Cu densities reconstructed from the SRCL observations show cracks with tortuous shapes generated around the dense Cu nanoparticles; these cracks propagated along the direction from the DBA substrate to the Si chip during thermal stress testing. Whereas crack propagation in typical soldering materials occurs along grain boundaries, the similar process in Cu nanoparticle layers is regulated by their sintered density distributions.

Behavior of stress generated in semiconductor chips with high-temperature joints: Influence of mechanical properties of joint materials

High-temperature joint materials are indispensable to realizing next-generation power modules with high-output performance. However, crack initiation resulting from stress concentration in semiconductor chips joined with high-temperature joint materials remains a critical problem in high-temperature operation. Therefore, clarifying the quantitative influence of joint materials on the stress generated in chips is essential. This study investigates the stress behavior of chips joined by Ni–Sn solid–liquid interdiffusion (SLID), which results in a high-temperature joint material likely to generate cracks after joining or when under thermal cycling. The results are compared with those fabricated using three types of solders, Pb–10%Sn, Sn–0.7%Cu, and Sn–10%Sb (mass %), which are conventional joint materials with different melting points and mechanical properties. Using Ni–Sn SLID results in the generation of high compressive stress (500 MPa) without stress relaxation after the joining process in contrast to th...

Warpage analysis of layered structures connected by direct brazing

Abstract In this study, we perform experiments and analyses to examine the warpage of Al/AlN layered structures connected by direct brazing. Al/AlN layered structures are considered under the high temperature fluctuations that occur in the brazing and baking processes. Three Al thicknesses are considered, using A3003 (JIS code) Al to clarify the variations in warpage. For the inelastic analysis, two types of constitutive model are used for Al: a non-linear kinematic hardening model considering cyclic hardening, viscoplasticity, and creep, and a conventional isotropic hardening model. Then, it is shown that the warpage in both brazing and baking processes can be simulated well using the kinematic hardening model.