Study of Immersion Tin Plating for the Production of Printed Circuit Boards

An immersion tin plating solution was developed for depositing the finishing tin coatings onto the surface of the conductive pattern (hereinafter referred to as CPR) of a printed circuit board (hereinafter referred to as PCB). The finishing tin coatings are intended to protect the CPR of a printed circuit board from corrosion in order to ensure good wettability of the surface with solder and to retain the ability to solder and weld for a long time (up to 6 months), and the finishing immersion tin coatings are also intended to ensure coplanarity of the CPR surface. The solution contains (g (ml)/l): Sn²⁺ – 12; CH₃SO₃H – 40; C₃H₅O(СООН)₃ – 300; PEG-400 – 170; CS(NH₂)₂ – 100; Na(H₂PO₂)∙H₂O – 25; Ag⁺ – 0,025. It allows to deposit tin coatings with the required solderability in two stages (at t = 18…25 °С and τ = 1…2 min; and then at t = 60…70 °С; τ = 12…16 min) that do not deteriorate after being held in a steam atmosphere for 4 hours. As the temperature of the working solution increases, so does the thickness of the tin coating, while the crystal structure becomes larger. Preliminary immersion tinning in a cold solution leads to a refinement of the structure of the subsequent layer deposited in a hot solution. The addition of sodium hypophosphite increases the stability and service life of the solution, and improves the reproducibility of the thickness and structure of the coatings. It was found that in silver-doped tin coatings, whisker formation is not observed after 3 months of aging. The maximum operation concentration of copper ions that accumulate during the immersion exchange is 8 g/L, which is comparable to a foreign analogue.

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