Sunday, March 15, 2009

Panel vs. pattern plating

There are two main approaches commonly used when electroplating PCB substrates, panel plating and pattern plating.

During panel plating, the entire copper surfaces on both sides of the substrate, as well as the hole walls are plated up to a desired final thickness. While this requires a fairly hefty current source for even a modest size PCB, the end result is a smooth, bright copper surface that is easy to clean and prepare for later processing. A major problem for folks without access to a photoplotter, is the need to use negative artwork to expose the circuit pattern into the more common contrasting reversing dry-film photoresists (contrast preserving films have been introduced from time to time but never seem to stay around for very long). When you etch a panel plated board, you end up removing most of the material that you plated, so the burden of extra erosion of the anode banks is exacerbated by an increased copper loading in your etchant.

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 copper from the anode bank and requires that less copper be removed during etching reducing bath analysis and maintenance. The downside of the technique is that it requires that the circuit pattern be plated with either tin/lead or an electrophoretic resist material prior to etching and then stripped prior to soldermask application. This increases the complexity and adds another set of wet chemical baths to the process.

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