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Tin/Lead Electroplating Module
-Operation/Additives-Panel vs. Pattern I

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.

Panel vs. pattern plating
Electrolytic tin/lead, as used in the printed circuit industry, is virtually always deposited via pattern plating. The only exception is when dummy plating is used to sweep the the electrolyte of particulate contamination.

Pattern plating, as the name implies, involves masking off most of the copper surface and plating only the traces and pads of the circuit pattern. Due to the reduced surface area, a much smaller capacity current source is generally needed. Further, when using contrast reversing photopolymer dry-film plating masks (the most common type), a positive image of the circuit is all that is needed. For many prototype PCBs, this artwork can be reliably produced on a relatively inexpensive laser printer or pen plotter. Pattern plating consumes less material from the anode bank and usually requires that less copper be removed during the etching process. This decrease in load on the etchant results in a reduction in etchant bath analysis and maintenance requirements.

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