Electroless deposition of Ni–P/Au coating on Cu substrate with improved corrosion resistance from Au(iii)–DMH based cyanide-free plating bath using hypophosphite as a reducing agent

Abstract

The green cyanide-free gold deposition is an important development direction in electroless gold plating. However, the commonly Au(I) based cyanide-free gold plating bath always suffers from severe corrosion to the Ni–P layer and unsatisfactory service life of the bath. In this work, a green and environmentally friendly cyanide-free gold plating bath was developed with hypophosphite added as a reducing agent into the Au(III)–DMH (5,5-dimethylhydantoin) based plating bath to retard the corrosion of the Ni–P layer. SEM micrographs combined with XRD and XPS analysis indicated that the electroless deposited gold was pure and compact. And XRD also revealed that the oriented deposition of gold was growing preferentially on Au (111). Corrosion tests, including salt spray tests, potentiodynamic polarization tests, and electrochemical impedance spectroscopy tests, indicated that the obtained Cu/Ni–P/Au coatings had significantly improved corrosion resistance performance with the loading of hypophosphite as the reducing agent. The baths remained transparent and no turbidity or precipitates were detected for 210 days, reflecting good stability. The detection for the Ni²⁺ concentration in the bath showed that adding hypophosphite could retard the replacement reaction between Au³⁺ and the Ni–P layer in part and this is important for decreasing the severe corrosion of the Ni–P layer. Moreover, Au was inactive for catalyzing the oxidation of hypophosphite during the deposition process which was confirmed furthermore via quantum chemical calculations. Therefore, our developed green cyanide-free electroless gold deposition process can beneficially provide research value and application prospects in microelectronic industry.