A role of Ti-Sn diffusion layer formed at the interface between Pb free solder and TiNiAu multilayer

The intermetallic compounds such as a nickel-tin (Ni-Sn) layer are formed at the interface between a solder and a backside electrode composed of a titanium/nickel/gold (Ti/Ni/Au) multilayer for power devices. In this study, a Pb-free solder was compared with a Pb-based solder and investigated the adhesion properties and the interfacial microstructures between the solders and the Ti/Ni/Au multilayers for the power device reliability. In the case of the Pb-based solder, the Ni-Sn layer existed at the Ti layer interface after a reflow soldering process. However, the Ni-Sn layer was separated from the Ti layer after a thermal shock test, and the device was peeled off between the solder and the Ti layer by a pull method. There was no diffusion layer at the interface between the Ti layer and the solder. In the case of the Pb-free solder, the Ni-Sn layer separated from the Ti layer after a Pb-free reflow soldering process. Contrary to our expectation, the device was not peeled off at any interfaces but cracked in the silicon (Si) die even after the thermal shock test. By transmission electron microscope (TEM) observation, the existence of a Ti-Sn layer was confirmed at the interface between the Ti layer and the Pb-free solder. As a result, the Ti-Sn layer was closely related to adhesion properties.