The Role of Nodules in the Growth of Zn Whiskers from Alkaline Cyanide-Free Zn Electrodeposits

Metal whiskers are spontaneous growths protruding from metal surfaces. It has been observed that tin, zinc and cadmium surfaces, especially their electroplated coatings are prone to develop whiskers. Many electrical and electronic failures have been attributed to the presence of these small metallic filaments. During the past 70 years, researchers have been striving to address the problems induced by metal whisker growth. However, most research effort has been focused on tin whiskers, whilst very little attention was paid to reliability issues associated with zinc whiskers. Only a few previous studies have been carried out to investigate the growth mechanisms of zinc whiskers, but the results of such studies are often conflicting and there is, at present, no widely accepted growth mechanism in place.

The current study focuses on whisker growth from zinc coatings on mild steel substrates electrodeposited from a commercial alkaline non-cyanide electroplating bath. Experimental work has been undertaken to investigate the effect of deposition parameters and storage conditions on whisker growth, and also to understand the formation and growth mechanism. Scanning electron microscope (SEM), focused ion beam (FIB), energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) have been used to characterise the as-deposited structure of the zinc coatings and to evaluate whisker growth.

Analysis showed that, for samples stored at room temperature, whiskers were present on samples deposited at current densities in the range of 20 – 50 mA cm^-2 within 4 weeks of electroplating. However, electroplating at low current densities significantly reduced whisker growth and no whiskers were observed on samples deposited at 5 mA cm^-2 after 12 months. Also, it was found that short period of post-electroplating heat treatment at high temperatures markedly reduces zinc whisker growth. In terms of the mechanisms for zinc whisker growth, our experimental data indicated that hydrogen evolution reaction that occurs on the steel, due to the low overpotential, was closely associated with zinc whisker growth. In addition, recrystallisation of the as-deposited columnar structure occurred at the whisker root.