Laser bonding of copper ribbons and clips on SiC power MOSFETs with sintered copper bond buffers

Abstract

The source (emitter) contact of MOSFETs (IGBTs) is one of the most sensitive joints of power semiconductor modules. Due to limitations with regards to current density and power cycling reliability, state-of-the-art aluminum wire bonding is replaced, for example, by copper wire or ribbon bonding. In this study, a novel laser micro welding technology has been implemented to connect the source contact of 1.2 kV SiC MOSFETs reinforced by silver-sintered copper bond buffer top plates. The feasibility of laser bonding of 0.3 mm thick copper ribbons or 0.15 mm thick copper clips has been investigated on bond buffer top plates of 0.1 mm or 0.15 mm thickness.The laser bonding process was optimized by optical inspection of the weld seam, shear strength measurements of the bonds, analysis of polished micrograph sections, and electrical characterization of the power MOSFETs. Shear strength values exceeding 200 MPa have been repeatedly achieved for all material combinations. Results of electrical characterization are promising, and optically impeccable welding seams strongly correlate with electrically functional semiconductor devices. Power cycling of encapsulated power modules is currently ongoing, and results will be presented.An outlook is given on future measures to further improve the laser bonding process, and we discuss how laser bonding compares to other advanced topside interconnection methods.