Sunday, March 15, 2009

Tin/Lead (Solder) Plating


Electroplating tin/lead alloy is often considered a part of the imaging process, since the resulting solder plate is a very effective etch resist when used in conjunction with both acidic and ammoniacal ethcants. Most of the equipment, like tin/lead plating tanks, is easy to make and will last for many years if properly maintained. The items that are not easily fabricated in a home shop are available from a variety of sources. A high-performance plating solution can be mixed using readily available materials.

The Downside of Tin/Lead Plating

Laminating soldermask over solder plate is not recommended for boards that are going to be wave soldered or passed through an infra-red reflow oven. As the solder melts it can cause the mask to wrinkle and crack, creating pockets where solder flux and moisture can be trapped against circuit elements. Generally speaking, solderplating is only used on boards that are going to be hand assembled and soldered, or boards that are not going to be coated with soldermask. It is possible the strip away the tin/lead coating after etching your circuit, but the resulting bath will contain dissolved lead and will be difficult to dispose of responsibly. If you intend to make boards at production levels, and want to use a metallic etch resist, you might consider bright acid tin instead of tin/lead. Acid tin is plated with exactly the same equipment as tin/lead and has the advantage that it can be stripped off using a replenishable bath that is very environmentally friendly.

Basic principles

A tin/lead (solder) electroplating solution is a mixture of water, organic acid, stannous tin, and lead sulfate. To this is added a number of organic constituents that serve to regulate and distribute the delivery of tin and lead ions to the surface being plated. The two basic organic additives are commonly referred to as the "additive" and the "carrier".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of tin/lead 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 both tin and lead 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 and lead erodes into the electrolyte (at the proper ratio), to exactly make up for the deposited material, maintaining a constant concentration of dissolved tin and lead. This all sounds quite nice, except for the annoying 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 and lead spears instead of the smooth, matte surface needed for reliable resist action inside the etching tank. Inhibiting and controlling this nonlinear behavior is where the organic additives come in to play.

Organic additives

In a well controlled plating bath, the carrier supports the formation of a skin on the anode material which serves to regulate the diffusion of tin and lead ions into the electrolyte in the proper ratio. 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/lead alloy deposition.

The additive 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 additive drifts away. (i.e. additives 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 additive prevents the formation of large clumps of poorly mixed alloy, giving the smooth, matte deposition that is the hallmark of a properly functioning solder plating bath..

Mark Brelsford of QMS in Toronto, ON likens the action of the carrier 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 additive would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.

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