Abstract: Micro-wave GaAs power chip is brazed to MoCu substrate by Au80Sn20 eutectic filler metal. The effect of process parameters such as brazing temperature, scrub numbers on eutectic brazing is studied. The rule of process parameters control is introduced. Microstructure, elemental distribution and void rate are analyzed by scanning electron microscope(SEM), energy dispersive spectroscopy(EDS) and X-ray equipment. Interfacial interactions between AuSn filler metal and the plating layers of GaAs and MoCu are studied. Solidification behavior of the brazed joint is revealed. Au plating layer at the back of GaAs dissolves into AuSn filler metal partly. However, the Au plating layer on the surface of MoCu dissolves into AuSn filler metal completely and the brazed zone is bonded to the Ni plating layer. The brazed zone is composed ofζ-Au₅Sn at the side of base metals and Au-Sn eutectic at the center.