Study of immersion tin plating for the production of printed circuit boards

• K.A. Orlova, N.S. Grigoryan, N.A. Asnis, K.N. Smirnov, A.A. Abrashov, T.A. Vagramyan and D.A. Zhirukhin

Mendeleev University of Chemical Technology, Russia, Moscow, Miusskaya sq., 9, 125047 Moscow, Russian Federation

Abstract: An immersion tin plating solution has been developed for deposition of finishing tin coatings onto the surface of the current-conducting pattern (hereinafter referred to as CCP) on printed circuit boards (hereinafter referred to as PCB). Finishing tin coatings are intended for the protection of CCP on printed circuit boards from corrosion to provide high surface wettability with solder and maintain the solderability and weldability for a long time (up to 6 months). Finishing immersion tin coatings are also designed to ensure coplanarity of the CCP surface. The solution contains (g (mL)/L): Sn²⁺ 12; CH₃SO₃H 40; C₃H₅O(СООН)3 300; PEG-400 170; CS(NH₂)₂ 100; Na(H₂PO₂) 25; TsKN-32М 1; and Ag⁺ 0.025. It enables depositing in 2 stages (at t=18–25°C and τ=2 min, then at t=70°C and τ=14 min) tin coatings the solderability of which meets the requirements and is not impaired after exposure to steam for 4 hours. It was revealed that with increasing temperature of the working solution, the thickness of the tin coating increased, while the crystal structure became coarser. Preliminary immersion tin plating in a cold solution was shown to results in a finer structure of the subsequent layer deposited in hot solution. It was found that sodium hypophosphite enhanced the stability of the solution and improved the reproducibility of the coating thickness and structure. An antioxidant was selected, TsKN-32M, which, by preventing the oxidation of tin(II) to tin (IV), enhanced the stability of the solution. It was found that no formation of whiskers in silver-doped tin coatings is observed after 3 months of aging.

Keywords: corrosion protection of contact pads on printed circuit boards, immersion deposition, immersion tin plating, finishing coating of the conductive pattern on printed circuit boards, metal coatings, inhibitor of whisker formation

Int. J. Corros. Scale Inhib., 2023, 12, no. 4, 1719-1732
doi: 10.17675/2305-6894-2023-12-4-17

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