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Archive for September 25th, 2010

Sharpie as PCB Etch Resist

Because my hombrew circuit boards don’t have plated-through holes, I solder Z-wires from top to bottom. This entails little more than a solder blob around the wire on each side, but this time I wondered if having a slightly larger solid-copper area on each surface would be an improvement. Regrettably, I wondered this after masking the board.

Because I use an Ultra-fine-point Sharpie to touch up pinholes & suchlike, I decided to try it on larger areas by simply coloring in a few of the openings in the ground-plane grid.

Sharpie etch mask - Results 1

Sharpie etch mask - Results 1

Short answer: doesn’t work so well.

However, I’m using direct etching: rubbing ferric chloride on the masked PCB with a sponge. The abrasion probably wears the Sharpie ink off the surface and then the copper begins etching as usual. If I were doing this with normal agitation / aeration, perhaps a Sharpie mask would work better.

This is also 1-ounce copper, so there’s twice as much etching going on. Perhaps half-ounce copper would vanish fast enough that the Sharpie mask would remain effective.

A bit more detail, with another Z-wire pointed right at you.

Sharpie etch mask - Results 2

Sharpie etch mask - Results 2

The grid is 20-mil wide on 50-mil centers, with 25-mil isolation to other signals. The “via” holes use a 24-mil drill.

The row of dents just below the wire came from tiny openings in the mask that happen when Eagle poured the ground plane against the isolation surrounding the trace at the bottom. The toner-transfer resolution isn’t quite good enough to leave a clean opening and the etchant can’t quite reach the bottom to dig out the copper.

Memo to Self: Next time, try a 100-mil square pad around the via, centered on a grid intersection to fill in four adjacent openings.

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