Abstract: The heat dissipation structure of aluminum alloy with multilayer microchannels is commonly used in power electronics owing to its efficient heat dissipation capabilities. Achieving high-quality bonding of the alloy is essential for engineering applications. The effects of bonding pressure, holding time, and temperature on the interface microstructure and mechanical properties of the joint created by liquid phase-assisted diffusion bonding of 6063 Al using AlSiMgCu brazing alloy were explored. The results show that the method eliminates the structural inhomogeneity of brazed joints by extruding liquid brazing alloy with a smaller pressure of 0.5 MPa. It also reduces the stringent requirements of diffusion bonding on the surface quality of base material and process parameters. Additionally, it offers high dimensional accuracy and strong adaptability of process parameters. The interface microstructure of the joint is similar within the parameter ranges: pressure of 0.5 ~ 2 MPa, temperature of 530–550 °C, and time of 1 ~ 4 hours. It typically consists of an Al alloy matrix and a small amount of solidified liquid brazing alloy, which fills micro gaps of the same scale as the surface roughness. The tensile strength of the joint remains almost unchanged, which is equal to base material under the parameter of 550 °C/1 h/2 MPa, with a tensile strength of 107 MPa and a deformation rate of only 0.10%.