4. Test Results for Scotch Brite and Buffing Wheels for Removing PCB Tarnish and Oxides

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When exposed to air, the plating on printed circuit boards tarnishes. Depending on the particular plating, the resulting surface compounds may reduce electrical conductivity enough to lead to a soldering or operational failure.

On the previous pages, we’ve seen that a couple of types of pencil erasers work well at removing tarnish. Before closing this topic, I want to compare the performance of an eraser to a Scotch Brite No-Scratch pad and to a Dremel Rotary Tool with a cotton buffing wheel attachment.

A method that I didn’t try is using extra-fine (000) or super-fine (0000) steel wool. My biggest concern with steel wool is all the little conductive metal filaments that it sheds.

Another method that I didn’t try was sandblasting. That would be even messier than rubber pencil eraser residue and most people don’t have access to that type of equipment and media.

But What About Chemicals?

There are chemical treatments for reversing or etching-away tarnish, rather than using physical abrasion. Unfortunately, many of the chemical processes require full-emersion, electrical connectivity (such as to migrate anions to less noble metals), or commercial facilities.

You may find some retail tarnish remover chemicals at your local hardware store. They may or may not result in long term problems due to chemical incompatibility. And, they may leave a “protective” film that prevents successful soldering or electrical contact.

I did try topical (local) applications of soap, isopropyl alcohol, acetone, and electrical cleaner without any effect on the tarnish. That was as I expected, since those products are more applicable to removing dirt, oils, and residues.

Various Abrasion Technique Tests

The alternate abrasive processes for removing tarnish were tested on a commercial PCB. However, unlike the prior tests, this board includes a solder mask, silkscreen, and the surface is plated with silver rather than tin/lead.

Each tarnish removal method was applied on a comparatively tarnished board-hole ring pad for whatever length of time and pressure was necessary until there was a noticeable difference in surface tarnish.

Eraser Tarnish Removal Baseline

A microscopic view of a circuit board hole pad with solder mask, silkscreen, tarnished plating, and cleaned plating.

A microscopic view of a circuit board hole pad with solder mask, silkscreen, tarnished plating, and cleaned plating.

The Magic Rub pencil eraser is the standard bearer of the tarnish removal test. The far-right side of the image shows the area cleaned by the eraser. There is no scratching and no eraser residue.

There is a little bit of a faint brown silver-oxide barely perceptible in the deeper grooves of the cleaned portion of the pad. But, to be fair, anytime you try to clean half a pad (less than 1/20th of an inch wide) by hand, you’re not going to be as successful as you normally would be when you can be more aggressive.

The above image is interesting in that it shows a close-up view of the green solder mask overlaying the silver-plated copper trace. A solder mask on a circuit board covers the entire board except for the chip pads and holes. This accomplishes several things:

The above image also shows a microscopic view of the very top layer on most circuit boards -- the silkscreen. A PCB silkscreen usually indicates part outlines, part numbers, or other text. When etching homemade circuit boards, most hobbyists try to include that information on the top or bottom copper layer. But a printed silkscreen layer is better because it has higher contrast ink, is non-conductive, and doesn’t use up precious board space that could be devoted to electrical traces.

Scotch Brite No Scratch Pads

The first challenger to the pencil eraser is the Scotch Brite pad.

Scotch Brite No-Scratch multi-purpose scour pads.

Scotch Brite No Scratch multi-purpose scour pads.

If you make your own circuit boards, you may use standard Scotch Brite pads to remove oxidation before etching and to remove resist after etching. I recently ran across these “No Scratch” versions that I figured would be worth a try.

Zoomed in view of a tarnished circuit board hole before and after using Scotch Brite No Scratch scour pads.

Zoomed in view of a tarnished circuit board hole before and after using Scotch Brite No Scratch scour pads.

The no-scratch pads scratched.

The product box cover does have the disclaimer “No Scratch... For Most Household Surfaces”. Admittedly, they didn’t make this product for cleaning circuit board tarnish, and PCBs are not “most household surfaces”. Furthermore, if the company made the product too wimpy, then it probably wouldn’t be useful for most household chores.

In the end, it took a lot more effort to get a lot less tarnish off, and it produced many more scratches than the Magic Rub eraser. Scour pads aren’t a good choice for PCB pad cleaning unless you baked dinner on the board.

Dremel Rotary Tool with Buffing Wheel

Cotton buffing discs are often used as one of the final steps in polishing a delicate surface. They should be much more gentle than scouring pads.

Left: Dremel High-Speed Rotary Tool with buff. Right: Right side of PCB pad moderately buffed.

Left: Dremel High-Speed Rotary Tool with buff. Right: Right side of PCB pad moderately buffed.

The buffing wheel was applied only to the right side of the PCB hole to contrast it against the original level of tarnish. It seems like it did a good job -- but it’s hard to tell in this image. Perhaps if I had targeted the entire hole there would have been a bit more cleaning action.

On the upside, there aren’t any scratches and there isn’t any residue (although a loose cotton strand might appear on occasion).

A buffer is an acceptable choice for a connector, gold-plating, or a situation where scratching is absolutely unacceptable. But, the angle and size of the tool, and the high rotational speed, make it awkward and slightly risky to use. I have melted plastic by being too aggressive with a buffing wheel on a Dremel.

The Winner and Still Champion - The Pencil Eraser

After testing, I feel more comfortable using an eraser to remove tarnish from printed circuit board pads and connectors. Pencil erasers are cheap, readily-available, effective, portable, non-conductive, and vapor free.

Admittedly, I don’t know about long term effects and I don’t have an objective method of measuring the solderability of the cleaned pad. The results are based on subjective visual inspection of microscope images.

If you have some tips or suggestions related to this topic, please let me know.