Substrate thermal expansion coefficient effect on cracks induced by the high-heat treatment of electroplated Ni-P films for power devices

Abstract

Automotive power semiconductors are required not to fail at high-temperature mounting (∼400 °C), and a structural design that prevents defects as well as cracks is essential. In this study, we focused on the thermal expansion coefficient of medium-phosphorus Ni films (P content: 8–9 wt%), which are commonly used for soldering or wire-bonding, with the substrates on which they are electroplated to investigate the effects of high-temperature heating on cracks induced in the Ni films. The Ni-P films were deposited on Al, Cu, Ni, and Fe substrates via electroplating technique using a Watts bath containing phosphorous acid. By heating to approximately 360 °C, the Ni-P film shrank because of the phase transition to a phosphorus compound mainly composed of Ni3P. As a result of the Ni film indentation test after heating to 400 °C, cracks were induced in the Ni film on the Cu substrate but not on the Ni substrate. The thermal expansion coefficients of the Cu and Ni substrates at approximately 360 °C were 17.4 and 14.8 µm m−1 °C−1, respectively; suggesting that the residual stress at the interface between the Ni film and the substrate affects cracking.