Tin Lead Plating

Tin-lead plating is a relatively simple process because the standard electrode potentials of tin and lead differ by only 10 mV. Tin-lead alloys have been deposited from electrolytes such as sulfonates, fluosilicates, pyrophosphates, chlorides, fluoborates, and, infrequently, phenosulfonates or benzenesulfonates. Of these, fluoborate and sulfonates (methane sulfonic acid, or MSA, also known as nonfluoborates, or NF) are available commercially. Tin-lead plating has traditionally been done with fluoborate solutions, but MSA solutions have become popular in the electronics industry because they are less corrosive to plating equipment, more uniform in deposition, easier to control, and more acceptable environmentally.

Fluoborate and methane sulfonate solutions plate tin from the stannous valance state. The term stannous valence state refers to the valence of tin in solution. In the case of fluoborate and MSA solutions, the tin is in the +2 valence state as Sn+2. Tin will plate only from the +2 state in acid solution. Alkaline stannate solutions plate tin from the +4 valence state. In fluoborate and MSA solutions, the stannous tin requires only two electrons to reduce it to metal:

Stannous fluoborate, along with lead fluoborate, fluoboric acid, and an addition agent, comprises the plating solution. The ingredients of the nonfluoborate MSA solution are stannous methane sulfonate with lead methane sulfonate, MSA, grain refiners (wetting agents), antioxidants, and fungicides. These components, as well as various addition agents, are available in commercial quantities. The solution operates at 100% cathode and anode efficiency.

Uses of Tin-Lead. Electrodeposition of tin-lead alloys was first patented in 1920, when these alloys were used to protect the interiors of torpedo air flasks against corrosion. When air was pumped into a flask under pressure, moisture in the air condensed and corroded the flask, weakening it. Lead coatings had been used to protect the interior against corrosion, but tin-lead alloy was found to be more corrosion resistant.

Today, tin-lead deposits are used as corrosion-resistant protective coatings for steel. The deposits usually contain 4 to 15% Sn, but the composition varies with the application. Automotive crankshaft bearings are plated with tin-lead or tin-lead-copper alloys containing 7 to 10% Sn, whereas an alloy containing 55 to 65% Sn is plated onto printed circuit boards. Tin-lead plating on circuit boards acts as an etch-resistant coating and facilitates soldering of board components after they have been inserted into the board. Copper alloys and alloy 42 (42Ni-58Fe) substrates are ordinarily plated with 80% Sn/20% Pb ± 10% MSA solutions in the manufacture of electronic components such as integrated circuits and surface mounts for postsoldering requirements. The shelf life, storage, and thickness of this composition have been proven by some Taguchi fractional multivariable experiments.

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