Reliability evaluation of thick Ag wire bonding on Ni pad for power devices

Abstract

In recent years, silicon carbide (SiC) semiconductors have gained traction in high-performance power devices due to their ability to operate at high temperatures (above 200 °C). However, traditional interconnection materials are not sufficiently reliable at such elevated temperatures. Typically, in power devices, thick Al wires with diameters of 200 μm or larger are commonly used to bond Al electrodes of semiconductor chips to their corresponding Cu leadframes or substrates. Nevertheless, the low melting point of Al can lead to recrystallization at high temperatures, thereby raising concerns about reliability and a decrease in wire strength and fatigue properties. Although thick Cu wires with higher melting points have been explored as potential solutions, their high hardness and susceptibility to work hardening during bonding can damage the chip beneath the bonding area, making them impractical. In response to these challenges, we investigated the use of thick Ag wires. However, direct bonding of thick Ag wires to Al pads can form intermetallic compounds (IMC) in the bonding area, leading to concerns about decreased reliability. To address this issue, we experimented with a bonding method that utilized thick Ag wires on Ni pads. In this study, we evaluated the reliability of thick Ag wire bonding under various stress conditions, including high-temperature storage tests (HTST) and temperature cycling tests (TCT). Our results showed that thick Ag wire bonding on Ni pads exhibited high bond strength and prevented IMC formation after HTST. Nonetheless, bond failure of the Ag wires could occur after TCT due to the mismatch in the coefficient of thermal expansion (CTE) between the Ag wire and SiC chip, particularly under extremely severe TCT conditions. We found this issue could be mitigated by introducing a soft metal layer, such as Al, between the Ni pad and the SiC chip. This reduced internal stress and enabled the device to pass TCT under harsher conditions with reliable performance.