Electroless Deposition of Nickel

Electroless (autocatalytic) plating involves the presence of a chemical reducing agent in solution to reducemetallic ions to the metal state. The name electroless is somewhat misleading, however. There are no external electrodes present, but there is electric current (charge transfer) involved. Instead of an anode, the metal is supplied by the metal salt; replenishment is achieved by adding either salt or an external loop with an anode of the corresponding metal that has higher efficiency than the cathode. There is therefore, instead of a cathode to reduce the metal, a substrate serving as the cathode, while the electrons are provided by a reducing agent. The process takes place only on catalytic surfaces rather than throughout the solution (if the process is not properly controlled, the reduction can take place throughout the solution, possibly on particles of dust or of catalytic metals, with undesirable results). Brenner and Riddell [1] invented electroless Ni plating in 1946 rather accidentally when they observed that the additive NaH2PO2 caused apparent cathode efficiencies of more than 100% in a nickel electroplating bath. This led them to the correct conclusion that some chemical reduction was involved. Further research resulted in the development of the original process that the inventors named electrodeless plating. The name soon lost the de, and later the name autocatalytic was formally adopted, although electroless is still widely used. The words are synonymous. Autocatalytic plating is defined as the deposition of a metallic coating by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited. Such plating has been used to yield deposits of Ni, Co, Pd, Cu, Au, and Ag as well as some alloys containing these metals plus P or B. Electroless Cr deposition has also been claimed. Chemical reducing agents have included NaH2PO2 (the one originally used by the inventors for Ni and Cu deposition and still the most important and widely investigated), formaldehyde— especially for Cu—hydrazine, borohydrides, amine boranes, and some of their derivatives. Electroless plating possesses several characteristics not shared by other techniques, and that accounts for its ever-growing popularity. Its throwing power is essentially perfect, at least on any surface to which the solution has access, with no excessive buildup on edges and projections. Deposits may be less porous than electroplates and hence have better corrosion resistance. Power supplies, electrical contacts, and the other apparatus necessary for electroplating are not required. The process is usually an integral and necessary step in plating on nonconductors such as plastics (see Chapter 15 of this volume). The process is particularly important in the printed circuit industry. Some electroless deposits have unusual or even unique magnetic properties. Experience shows that each substrate requires its own specific techniques; depositing active metal onto the surface of a non(semi)conductor is still somewhat of an art. The surface preparation (i.e., cleaning process) requires very careful selection and application. It must be stressed that cleaning may affect the porosity of the metal deposit. Residues from cleaners and deoxidizers may create inactive spots that will not initiate electroless deposition. This may result in the necessity to have a thicker deposit before continuity is achieved. In extreme cases continuity is never reached. In general, deposition requires one or more of the following steps (see Fig. 18.1): (1) cleaning, (2) surface modification, (3) sensitization, (4) catalyzing or (30) catalyzing, and (4) activation (acceleration). Rinsing is required between the steps. We refer to the steps 3 and 4 (shown in Fig. 18.1) as sensitization and catalyzing. By these terms we mean