Abstract The fundamentals of whiskering in pure Sn coatings electrodeposited on pure Cu and brass (65 wt. % Cu: 35 wt. % Zn) substrates with and without electrodeposited Ni underlayer was studied. These coatings were subjected to isothermal aging at 50 °C to accelerate the whisker growth, and whisker growth was studied using scanning electron microscope (SEM) equipped with focused ion imaging. Texture characterization was conducted using X-rays and electron backscatter diffraction (EBSD). It was observed that whisker growth was more accelerated in case of brass substrate compared to pure Cu substrate. No whisker growth was observed in coatings with Ni underlayer. It was observed that diffusion of substrate atoms along Sn grain boundaries plays major role in whiskering. Ni underlayer inhibited diffusion of Cu and Zn atoms into Sn coating and hence was very effective in mitigating whisker growth.