Microstructural study on the inhibition of immersion tin coating formation by the substrate surface contamination

The formation of an immersion tin coating from a hydrochloric acid based bath on the inner surface of clean and contaminated copper tubes was studied by scanning electron microscopy and transmission electron microscopy in order to explore the role of substrate surface contamination in the immersion tin coating build-up. The mechanism of inhibition by a drawing lubricant layer was the major attribute under exploration. The substrate surface contamination was found to influence both the coating nucleation and growth processes and the overall appearance of the immersion tin coating. A discontinuous, thin, nanocrystalline and irregular tin coating forms on a contaminated copper substrate instead of a continuous, crystalline, regular tin coating of a reasonable thickness, which builds up on a clean copper surface. However, the nucleation mechanism of immersion tin deposit is equivalent on clean and contaminated substrate surfaces. The oily contamination layer is, thus, only suggested to block the accessibility of aqueous plating solution onto the substrate surface, thereby hindering the coating nucleation at the areas of poor cleaning. Besides, no grain growth was encountered in tin coatings deposited on the poorly cleaned copper.