IMMERSION TIN: ITS CHEMISTRY, METALLURGY, AND APPLICATION IN ELECTRONIC PACKAGING TECHNOLOGY.

The surfaces in copper-plated through-holes in printed circuit boards for complex electronic packaging can be made solderable by immersion deposition of tin. The properties of the prepared surfaces vary from those of 'white immersion tin,' which is easily wetted by molten tin solder, to those of 'gray immersion tin,' which is nearly nonwettable. In this paper, the rate law for tin deposition in the tin immersion plating bath is studied. Certain effects of chemical composition of the plating bath upon the character of the tin layer are investigated and the effects of thermal annealing of the plated surface upon the composition of the tin layer are determined. The differences in composition of white and gray immersion tin surface coatings are revealed by X-ray diffraction Rutherford backscattering spectroscopy and Auger electron spectroscopy. Solderability tests on Sn, Cu, Cu//3Sn, and immersion tin surfaces are included.