Characterisation of crystallisation behaviour of electroless Ni-P plating during heating

Electroless-plated nickel film has many potential advantages, such as selective autocatalytic metal deposition, good solder wettability, and in application as solder diffusion barrier. It has high potential for lower cost under bump metallurgy and flip chip bumps themselves for flip chip packaging. Crystallisation and phase transformation behaviour of electroless-plated nickel-phosphorus (Ni-P) deposits during thermal processing have been the subject of various investigations, since these processes have vital roles in determining the materials properties of the plating. In the present work, differential scanning calorimetry (DSC) and x-ray diffraction (XRD) analysis were used to study the crystallisation and phase transformation behaviour in the electroless Ni-P platings with 3similar to5 wt% (NiP4) or 6similar to9 wt% (NiP75) phosphorus contents. X-ray line broadening technique was used to estimate the grain size and microstrain, with the aid of the software PROFIT to separate the reflections of crystalline nickel from the amorphous phase. Results showed that in both Ni-P platings the grain size increases when temperature increases, with sharp increase at temperatures above 400degreesC. The microstrain in the platings decreases with increasing temperature. The crystalline nickel formed is largely randomly orientated, with possible preferential orientation (220). It was also shown that the NiP4 specimen heated up to the termination point of the DSC peak at 40degreesC/min has larger grain size and microstrain than that at 5degreesC/min. The amount of Ni3P phase formed at this point is the same regardless of the heating rate. The increase of grain size may contribute to the exothermal peak in the DSC curve.