Bonding mechanism of electroless Ni-P film with AlN substrate and Cu foil

Electroless Ni (EN) plating method is employed to metallize Y2O3-doped AlN ceramic substrates with different surface morphologies, including the as-received, polished, and etched AlN. The EN-plated AlN substrate is bonded with the Cu foil to form a sandwich-like AlN-EN/Cu/EN-AlN assembly by hot pressing in vacuum with a pressure of 6.5 MPa for 30 min. The bonding strength of the joint is determined by the adhesive abilities of EN/AlN and EN/Cu interfaces. Bonding between the Cu foil and the EN film is achieved by the interdiffusion of Cu and Ni atoms to form a solid solution interlayer, while the EN film is adhered on the AlN substrate mainly by the mechanical interlocking through the rough interface, etched holes, and open pores. At the bonding temperatures lower than 400°C, no reaction occurs between the Cu foil and the EN film. Increasing bonding temperature enhances the interdiffusion of Cu and Ni to form a strong bonded solid solution interlayer. However, the depletion of the element Ni in EN film results in the formation and enlargement of the pores at EN/AlN interface, which makes a poor adhesion between EN film and AlN substrate. An optimum bonding temperature is observed around 600 and 700°C for various surface conditions of AM substrates. Theoretical calculations of the residual thermal stresses in AlN, EN, and Cu layers with respect to the bonding temperatures are related to the variation of the bonding strengths. The relations of the adhesion strengths, interfacial morphologies, elemental distribution, and calculated residual stresses is proposed