3. Eraser Residue Cleanup

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Recall that the classic Pink Pearl eraser did an effective job at removing tarnish from tin plating. However, it left a lot of small eraser residue on the pads and board. What type of cleaning is necessary for the board to be reasonably rid of eraser residue?

Large and small eraser residue from Pink Pearl removing tarnish from a PCB pad (60x magnification).

Large and small eraser residue from a Pink Pearl eraser removing tarnish from a PCB pad (60x magnification).

After erasing the tarnish, no attempt was made to blow away, brush away, or otherwise clean the eraser residue from the tin-plated pad. As you can see, relatively-larger size eraser crumbs of approximately 1/100th of an inch in diameter appear on the left side of the eraser’s path. Smaller residue appears scattered all over.

Huffing and Puffing

After a few quick exhales, the larger eraser residue was blown away from the pad (60x magnification).

After a few quick exhales, the larger eraser residue was blown away from the pad (60x magnification).

Every student knows that you blow on the eraser bits to get them off of your paper. Indeed, a couple of puffs rid the circuit pad of the larger particles. But, many obvious pieces of smaller residue remain in the same location as the previous image.

Blowing on a board isn’t a very clean method, because:

Brushing Off Residue

After rubbing the board with fingers, the smaller eraser residue is deposited on the pad edges and cracks (60x magnification).

After rubbing the board with fingers, the smaller eraser residue is deposited on the pad edges and cracks (60x magnification).

After trying to blow away the eraser residue, all test-takers know that the next approach is to attempt to wipe, flick, brush, or rub away the particles with your hand or fingers.

On the PCB, such techniques moved the smallest visible particles from the top surface of the pad. However, the particles were crammed against the pad edges. It is also possible that the smallest eraser dust got smushed into cracks or scratches on the top surface.

I suppose wiping the pads with your fingers is somewhat effective in providing a surface to solder against. It isn’t exactly up to NASA specification, but it would work in a pinch. But, the tiny particles are likely to dislodge from the metal edges if exposed to air gusts (a fan) or vibration. Also, if you weren’t wearing clean gloves (or any gloves) then the plated surface now includes a light coating of dirt and finger oil.

This brings up an interesting question about how clean a PCB really needs to be. Many robot builders (myself included) often leave circuit boards completely exposed for easier experimentation. And, anyone who has every opened up an old PC comprehends the extreme levels of dust and dirt that accumulate on boards without obnoxious failure rates.

Perhaps it isn’t entirely unprofessional to declare “good enough” if the joint subsequently accepts solder. But, I’d be more careful if the robot or device controls a weapon, drives a powerful motor, is being sent into space, is used in critical (life saving) equipment, is being sold commercially, or includes fine-pitch circuitry.

Rinsing Off Eraser Remnants

The PCB pad appears fairly clean at 60x magnification after a tap water rinse and brief wash cloth dry.

The PCB pad appears fairly clean at 60x magnification after a tap water rinse and brief wash cloth dry.

Finally, the PCB was subject to the laziest possible washing. I held the board under a moderately running cold tap-water faucet for 3 or 4 seconds and then patted it dry with a cotton wash cloth. (That reminds me, you might not want to wash your face with the wash cloth in the bathroom nearest my office.)

No distilled water. No scrubbing. No soap or alcohol. The lack of soap or alcohol means oils and non-water soluble contaminants may still be present on the board.

Yet, the board looks fairly clean at this point. (FYI: The bright highlights at the top of the image are from the microscope’s lamp bulb. There are no particles embedded there.)

The Pink Pearl eraser doesn’t appear to have contributed any scratches, at least not beyond the ambient level of scratching already present on the board. Recall the eraser was rubbed vertically. So, the horizontal and diagonal scratches were not likely caused by the eraser.

There is still a decent distinction between the far left side of the pad with the original tarnish and the far right side that received the majority of the eraser strokes. The middle-left side should not be judged for tarnish as the eraser edge made only glancing strokes in that region based on the low precision of my hand.

We now know that the eraser method works fine with the Magic Rub or the Pink Pearl (after rinsing). Are there physical methods besides an eraser that may provide similar results?