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The secret life of FR4 boards (Part 2)

Posted: 02 May 2011     Print Version  Bookmark and Share

Keywords:FR4  printed circuit boards  PCB 

Fabrication of the traces that will carry signals throughout a multi-layer printed circuit board (PCB) begins with a single inner layer to which the copper layer has already been applied. At this stage, the copper covers the entire surface of the inner layer. In the next few process steps, most of the copper will be removed in order to leave just the traces.

There are varieties of finished products: single-layer boards, multi-layer boards, and both single- and double-sided boards. The sequence of processes described here likewise has many variations for specific products. Typically processes are carried out on a multi-board panel that is later broken into separate boards.

A layer of photo resist is applied to the copper and subjected to pressure and heat. Once the resist is in place, the artwork template for the traces for this layer of the panel is laid on top of the resist. The artwork typically arrives as what is known as a Gerber file, which guides a photo plotting machine to create the artwork template.

In the artwork, the areas that will become traces are left open, while all other areas are solid. The resist is highly sensitive to ultraviolet (UV) light. In the next step, the board layer is exposed to UV light, which hardens the exposed photo resist overlying the copper that will become traces. The unexposed photo resist is unchanged.

The next several steps involve etching by liquid chemicals to remove various parts of the assembly. The etchant may be sprayed onto the board layer, or the panel may be immersed in a tank containing the etchant. Chemical reactions between the board layer and the etchant solution usually give off toxic, corrosive fumes that must be safely removed.

The panel is treated with a liquid developer that removes the unhardened photo resist but that leaves the hardened photo resist in place. Where the unhardened photo resist has been removed, the copper is now exposed.

In the next step, a liquid is used to remove all of the copper that is not covered by the hardened photo resist. In these areas, the original epoxy/woven fibre layer is now exposed.

Finally, a chemical step removes the hardened photo resist on top of the traces, leaving only the traces themselves.

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