Surface properties and solderability behaviour of nickel-phosphorus and nickel-boron deposited by electroless plating

X-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), atomic force microscopy (AFM) and wetting force measurements have been applied to various nickel deposits prepared by standard electroless plating for probing the origins of solderability behaviour of 'electroless nickel'. By controlling the plating chemicals and plating conditions, three kinds of electroless nickel deposits have been prepared, namely electroless nickel-high phosphorus (EN-HP), electroless nickel-low phosphorus (EN-LP) and electroless nickel-boron (EN-B). Wetting balance measurements confirmed the solderability strength decreased in the order of EN - B > EN - LP > EN - HP. Unexpectedly, among the three EN deposits, EN-B was found by XPS to have the thickest surface oxide, which opposes the traditional correlation of good solderability with thin surface oxide. On the other hand, SAM and AFM studies showed that the average nodule size increased in the order of EN - B < EN - LP < EN - HP. Accordingly, there appeared to be a correlation of small nodule size with good solderability. Solderability of 'electroless nickel' thus depended more on surface morphology than on surface oxidation. Nevertheless, further studies of the wetting kinetics of the EN deposits revealed that among the three EN deposits EN-HP had the oxide structure that was most difficult to penetrate and thus the slowest to 'wet'. The structure of the surface oxide therefore was found to affect the wetting kinetics of 'electroless nickel'. Copyright (C) 2001 John Wiley & Sons, Ltd.