Laminated transient liquid phase preform and bond characterization for high-temperature power electronics application

Transient liquid phase (TLP) bonding is a diffusion bonding process for joining metals using low temperature. Although, TLP is applicable to power electronics systems intended for high-temperature application, the brittle inter-metallic compounds formed at its inter-phase have low resistance to thermomechanical stresses experienced during the operational life of the device. Laminated TLP (L-TLP) bonding, which uses a preform with ductile core layer, has the potential to mitigate thermally induced stress. L-TLP performance depends on the quality of the preform, as well as processing parameters. However, little emphasis has been dedicated to the characterization of the preform and bond performance. In this work, variations in fabrication process and surface roughness were assessed in terms of final bond quality for varying core layer thicknesses. Scanning electron microscopy (SEM), confocal scanning acoustics microscopy (CSAM), energy dispersive x-ray (EDX) and shear tests were used to evaluate the bond quality. Two preform fabrication processes comprising of electro-cleaning-dilute nitric acid-zincate (EH_dZ) and alcohol-concentrated nitric acid-zincate (AH_cZ) were considered. The results show that AH_cZ produces superior performance. In addition, it is found that a rougher core layer resulted in better adhesion of plated layers. The bond shear force exceeded the relevant MIL-STD 883 acceptable threshold of 2.5 kg for shear area greater than 4.12 mm² for core layers of 38 and 76 μm. Numerical simulation using ANSYS shows, that L-TLP bond, with thinner core layer thickness resulted in in higher thermally induced stress at the intermetallic compound (IMC) layer.