Regeneration of Electroless Nickel Solution by Ion Exchange

As plating proceeds, both nickel and hypophosphite ions are consumed. These deficiencies can be corrected by adding nickel and hypophosphite salts. However, as the byproduct phosphite accumulates, the quality of the deposit deteriorates. At some point the bath must be discarded and replaced-a practice that is expensive due to the cost of purchasing a new solution and disposing of the old one. The purpose of this study was to replace the phosphite with the active reductant, hypophosphite, and thereby reduce both costs. Unconsumed nickel ion would not have to be discarded because of high contamination with phosphite, and disposal costs could be substantially reduced or even eliminated. Spent plating solution was passed through a basic ion-exchange resin in the hypophosphite form. We anticipated that the anions (including phosphite) in such solutions would be exchanged with the bound hypophosphite; the exchange was found to occur. As our work progressed, we discovered the concept had already been proposed in the patent However, a recent paper3 reported the removal of only 50 percent of the phosphite. With the procedure described here, a larger proportion of the phosphite was regenerated to hypophosphite.