Role of Cu²⁺ as an additive in an electroless nickel-phosphorus plating system: A stabilizer or a codeposit?

The effect of cupric (Cu²⁺) ion as an additive in the acidic electroless nickel plating (ENP) bath on the characteristics of the resulting nickel-phosphorus (Ni-P) alloys was investigated mainly with X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray (EDX) spectrometry. Cupric ions in the acidic ENP bath using hypophosphite anions as reductants have been reported ambiguously as stabilizers or codeposit constituents. In this work, the critical concentration of added copper salts, CuSO ₄·5H₂O, that starts to inhibit the ENP process was determined to be ca. 536 mg/L. In general, the deposition rate, surface morphology, and pit formation on the surface of as-deposits are significantly improved with Cu²⁺ addition at concentrations less than the critical value. The electroless nickel alloys were shown as a mixture of an amorphous deposit and a crystalline copper metal rather than as amorphous alloys alone.^1,2 During the initial stage of the electroless plating process, the copper contents in as-deposits were found to decrease rapidly with plating time from the EDX analyses. The X-ray photoelectron spectroscopy result also confirms that copper is the preferred deposited species during the initial stage of the ENP process. The theoretical model³ is revised by taking into account the effect of adsorbed cupric ions on the shift in the depth of the net nuclear potential of the electroless nickel frontier, and successfully predicts the deposition rates. Moreover, as predicted by the revised model, the adsorbed cupric ions on the just-deposited Ni-Cu-P frontier could enhance the adsorption of hypophosphite anions and, accordingly, the deposition rates.