Bright Acid Tin Plating

Overview

Electroplating bright tin is always considered a part of the imaging process where positive artwork is used, since the resulting tin plate is probably the best etch-resist available when using acidic or ammoniacal etchants. Acid tin plating tanks are identical to the tanks used in tin/lead plating with the exception that the anode bars are metallic tin instead of tin/lead.. The items that are not easily fabricated in a home shop are available from a variety of sources. A long lasting, very reliable plating solution can be mixed using readily available materials.

The Downside of Tin Plating

As good as it is, tin plating suffers from a tendency to form an intermetallic alloy with copper which changes the conductive properties of the bulk metal and can cause an increase in brittleness. Tin plating can also cause the growth of metallic whiskers between circuit elements leading to very hard to find shorts. As a result, the tin layer must be stripped after your circuit has been etched. Luckily, the thin layer of tin that is necessary for effective resist action is easily removed using a replenishable, environmentally friendly stripping solution.

Basic principles

An acid tin electroplating solution is a mixture of water, organic acid, and stannous tin. To this is added a number of organic constituents that serve to regulate and distribute the delivery of tin ions to the surface being plated. The two basic organic brighteners are commonly referred to as the "brightener" and the "starter".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of tin bars (or baskets of nuggets) arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical tin/lead electroplating bath has an effective full load operating "impedance" that ranges between 0.015 Ohms and 0.035 Ohms). Situated halfway between these anode "banks" is the copperclad substrate that is to be plated. It is variously referred to as the cathode or the workpiece.

In the simplest terms, metal deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of tin ions to the cathode where the ionic charge is neutralized as they plate out of solution. At the anode (in a properly maintained bath), sufficient tin erodes into the electrolyte, to exactly make up for the deposited material, maintaining a constant concentration of dissolved tin metal. As in all electrolytic solutions, there is a tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more metal ions than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of tin instead of the smooth, bright surface needed for reliable resist action inside the etching tank. Inhibiting and controlling this nonlinear behavior is where the organic brighteners come in to play.

Organic brighteners

In a well controlled plating bath, the starter supports the formation of a skin on the anode material which serves to regulate the diffusion of tin ions into the electrolyte. The material is also attracted to, but not co-deposited on the cathode (work piece) forming a layer (film layer) in close proximity to the surface that controls the rate of tin metal deposition.

The brightener works within the film layer to control alloy deposition on a microscopic level. It tends to be attracted to points of high electro-potential, temporarily packing the area and forcing metal ions to deposit elsewhere. As soon as the deposit levels, the local point of high potential disappears and the brightener drifts away. (i.e. brighteners inhibit the normal tendency of the plating bath to preferentially plate areas of high potential which would inevitably result in rough, dull plating) By continuously moving with the highest potential, the brightener prevent the formation of large clumps of tin whiskers, giving the smooth, bright deposition that results from a properly maintained and operated acid tin plating bath..

Mark Brelsford of QMS in Toronto, ON likens the action of the starter to the function of a doorman at a theater who regulates the flow of people into a theater, but doesn't really care where they go once inside. The brightener would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.