Reliability study of mounting structure for high temperature power semiconductor device chip using high purity aluminum

Abstract

We propose a new concept of mounting structure for high- temperature operable power semiconductor devices such as Silicon Carbide (SiC) or Gallium Nitride (GaN) with high reliability. The proposed structure is composed by high purity aluminum (Al) as a circuit metal on substrate and high- temperature resistant joint material as a chip joint layer. In this structure, the circuit metal can deform easily instead of the joint layer, which is usually hard, by the stress caused by Coefficient of Thermal Expansion (CTE) mismatches between the chip and substrate. So, the structure leads Al as the circuit metal to fulfill the function of reducing stress on the chip in the advanced SiC Power Module instead of a solder joint layer used in the conventional Power Module. We also expect that Al has a possibility to endure the intense thermal cycling tests such as -50/+300degC range. In order to demonstrate this new concept based on above mentioned hypothesis, mechanical material tests and Finite Element Analysis (FEA) were executed. From the test results, it became clear that the plastic behavior (stress-strain properties) of Al is similar to that of a tin-based solder alloy comparatively. By using these data, the FEA simulating the intense thermal cycles at several structures was carried out to evaluate the stress on the chip and the inelastic strain on Al. The calculated stress suggested that such a high stress causing cracks on the chip is not generated on a structure which is composed by Al with a ceramic substrate, even though a displacement caused by CTE mismatch between the chip and substrate is increased by expanded temperature range. Continuously, in order to estimate fatigue life of the each structure against the thermal cycles, mechanical fatigue tests were carried out by using specimens with a notch for strain concentration. From the test results, the low cycle fatigue property of Al was obtained. By that property and the value of inelastic strain calculated by the FEA, it was expected the proposed structure has a high potential to secure sufficient reliability at the high-temperature operable chip joint. Furthermore, additional tensile tests using three kinds of Al were carried out to investigate influence by purity of Al. In the results, it was suggested that an application of 99.99% Al on the proposed structure brings the best solution to reduce the stress and to secure reliability at the chip joint compared with 99.5% Al or 99.7% Al.