Indirect bonding of Ni-electroless plated AlN and Cu by hot pressing method

Abstract

The electroless Ni (EN) plating method was employed to metallize the AIN ceramic substrates. The EN-plated AlN substrate was bonded with the Cu foil to form a sandwichlike AIN-EN/Cu/EN-AlN assembly by hot pressing in vacuum with a pressure of 6.5 MPa for 30 min. For the bonding temperature below the Ni-P eutectic temperature of EN at 88O"C, the samples were bonded through solid state diffusion. On the other hand, the samples were bonded via a liquid phase media through both wetting and diffusion if the bonding temperature was above 880°C. An optimum adhesion strength around 10 MPa occurred within bonding temperature range 600-700°C. The fracture took place in the ENKu interface for samples bonded below 6OO"C, while fracture took place in the AIN/EN interface above 700°C. The increasing temperature enhanced interdiffusion of Cu and EN to form a strong bond, yet resulted in a large residual thermal stress in the AIN/EN interface. The bonded samples with as-received AlN exhibited higher adhesion strength than those with polished AlN because there existed a residual compression perpendicular to the .41N/EN interface at surface irregularities in the as-received AlN, which resulted in an additional shear strength offset in the adhesion test. The adhesion strength of samples with etched AIN was the highest as compared to those of as-received and polished AlN, although the surface roughness of the etched AlN was the same as that of the as-received one. It is argued that the etched surface of AlN with micro-etched holes provides the anchor sites for interlocking with the EN film, which results in a good mechanical bonding in the joint. However, a mechanical trimming on the edges of bonded samples would damage the joint, and a low adhesion strength is instead observed.