A combination of analytical methods was used on printed circuit board coupons to conclude the following: (1) while steam ageing deteriorates solderability of tinned coupons, it does not increase the oxide thickness or tin‐lead ratio of the surface (top 50 Angstroms) of tinned coupons; (2) therefore, some other factor, probably oxidation of the copper or tin/copper intermetallic substrate, determines solderability of the coupons.
Marshall, J.L., Miiller, D.E., Sees, J., Matteson, S.E., Weathers, D. and Lichtenberg, L. (1991), "Correlation of Chemical Microstructure with Solderability of PCBs", Circuit World, Vol. 18 No. 1, pp. 17-21. https://doi.org/10.1108/eb046150
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