PCB Cleaning 101

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A Note Regarding the Author:

The information blow was gleaned from an old newsgroup entry, and was originally posted by a (presumably American) chap called Clay. In the interests of sharing this excellent information with a wider audience,I am putting it here in the Wiki for posterity.
Clay - if you are reading this, thanks for writing this excellent guide, hope you don't mind us reproducing it here. If you do, just drop us a note and we'll remove it for you. :)

Cleaning Circuit Boards

There are a variety of methods that can be used to clean circuit boards depending on the type of board, the type of grime, and experience level and available tools of the person doing the cleaning.
You must ask yourself one important question before cleaning a circuit board: "Is having the board clean worth the risk of breaking it in the process?" This is particularly true if the board is working, but just dirty. Cleaning a circuit board always involves a risk of damaging the board or the components on it. (If the board is already broken and you need to clean it to attempt repairs, the decision is obviously much easier.)

Dust (or other dry, free flowing solids)

To remove dust or other light debris from a printed circuit board it is generally advisable to simply brush the substance with either an old (dry) toothbrush, a dry (clean) paint brush, or the "apolstery" brush from a vacuum cleaner. Once the dirt is loose, the brush can be used to directly whisk the dust away, or a compressed air source can be used to blow the board clean. It is generally not advisable to use an operating vacuum cleaner to try to remove dirt from a circuit board unless it is a specially designed vacuum cleaner for electronics work. (A household vacuum cleaner can generate thousands of volts of static electricity at the head of the hose which can cause damage to some electronics.)

Grime (dust/dirt mixed with something tacky - oil, Pepsi, wax, etc.)

Grime is particularly common on monitor chassis boards where high temperatures cause outgassing of components coated with wax. The wax forms a sticky film on the board which traps dirt easily-- and since the high-voltage potential of the monitor anode will attract airborne particles, monitor boards can become particularly filthy. Other boards will sometimes become encrusted with dirt and gunk that simply won't brush away, so more aggressive cleaning can be become necessary.
Grime that appears to be mixed with some form of oil or wax (or spilled Coke, or Pepsi, or God-knows-what else) will need some form of detergent applied followed by mild scrubbing to expunge the offending contaminants.
Your first step should be to remove any socketed components (chips, cables,etc.). These parts can be cleaned separately using the technique described below-- and removing them now will let the cleaning process get under the sockets and clean much more completely than if you left the parts in place.(Of course, you will need to make note of where the socketed components go to make sure you get them replaced in the correct locations and with the proper orientation.)
Certain components may not react well to liquid-based cleaning. Old open-frame/carbon film potentiometers, any components contained in cardboard, and older non-watertight crystals may be damaged by contact with liquids. You may wish to desolder these components to be safe if you have them on the board to be cleaned. Also note that many types of stickers will be destroyed by this process, so EPROMs may need to be recovered, etc.
The actual cleaning of the board can usually be safely accomplished by spraying the surface with a generous amount of a household, non-phosphate based cleaner. "Simple Green" and "409" seem to work quite well. Let the cleaner sit for 10-15 seconds to loosen the dirt and then scrub lightly with an old toothbrush or clean paint brush. (A 2.5" wide nylon paintbrush will make short work of grunge on a monitor chassis with a liberal spray of '409' applied!). Once the board is scrubbed down, you will need to rinse it *very* well with water. Technically speaking, a demineralized/purified water should be used, but tap water is probably fine for 99% of applications. A spray attachment from a kitchen sink works well for rinsing and for monitor chassis' a garden hose with a spray-head is quite effective.(Random semi-related warning-- do NOT clean the silver/grey coating off of the back of a monitor CRT! It is a required coating for proper operation of the monitor!)
After rinsing, carefully blot the circuit board dry with a clean terry-cloth towel (and shake it to get hidden water out). Compressed air (either from a can or a compressor) can also be used to blow off excess water.
Thorough drying of the circuit board is critical. While hand-drying the board, preheat a household oven to about 170 degrees F. Once the oven is up to temperature, turn the oven OFF, and place the cleaned PCB inside the oven and close the door. Allow then board to dry for 2-3 hours. (Overnight is fine.) It's a good idea to warn roommates/spouse when you do this. (Having someone preheat the oven the 500 degrees to broil a roast while you board is still in the oven is *bad*.)
After the drying cycle is complete you can remove your board from the oven and perform a visual inspection for any damage. If unsealed potentiometers are on the board (like a monitor chassis) they may need to be turned back and forth several times to remove any residue from the wiper area to regain smooth operation.
An alternate method for cleaning this type of grime is to use the clean/rinse cycles of a household dish-washer. Make sure the dishwasher is free of any food particles and place the board inside like you would a large plate. A small amount of dish-washing detergent can be added to help the cleaning process, and if possible the water temperature should be set to the lowest setting. You will want to remove the board from the dish washer at the end of the rinse cycle (do not use the dish-washer's dry cycle as it is not well temperature-regulated-- use the oven technique listed above).

Solder Flux (brownish crusted material on the solder-side of the PCB)

Many solder fluxes used in PCB repair and some types of manufacturing are water resistant. They will leave a yellow-to-brownish "crust" on the pins of chips on the circuit board's solder-side. This is usually the result of a previous repair that was not properly cleaned.
Solder flux can usually be removed with household rubbing alcohol and a toothbrush. Wet the toothbrush with the alcohol, and gently scrub.Repeated applications may be necessary. If available, anhydrous alcohol (used for electronics cleaning) works much faster. Blot the cleaned area with a non-linting tissue (like Kimwipes EX-L) or a clean towel.
There are also commercial cleaners used for flux and grease removal. Some types of cleaners are available from Radio Shack, but are generally overpriced and of marginal quality. A professional grade cleaner like Tech Spray's "Envi-Ro-Tech 1676" will perform best but can be more difficult for a hobbyist to obtain.

Paint (spray point, latex, etc.)

Paint in and of itself is usually not harmful to the operation of a game PCB. Since paint can be difficult to remove, unless you need to clean the board for repair work it may be best to leave it alone.
If you have to clean the paint off, try to determine what type it is. Spray paints will generally come off with standard mineral spirits or paint thinners. Follow up use of any thinners or paint removers with the cleaning procedure for "Grime". Latex paints can be removed with latex cleaners, or sometimes soap and water. Your mileage may vary.
Before selecting a cleaner to try, remember that many old chips have their numbers painted or silk-screened on (modern chips are usually laser etched).Paint removal techniques can often strip transistors or chips of their markings making identification very difficult.
Also note that most PCB silk-screening and printed circuit boards themselves are epoxy based-- an aggressive paint removal product that works against epoxy (MEK, JASCO, etc.) may damage the PCB or silkscreen.

Corrosion (acid damage, salt water spray, etc.)

Corrosion should be neutralized and cleaned as soon as possible. The most common forms of corrosion on game PCB's will usually be salt-air exposure and battery acid (from Lithium or NiCad batteries).
In the case of battery acid leakage, the board should be neutralized with a mixture of baking soda and water forming a thin paste. Gently scrub the paste into the PCB with a toothbrush and let it sit for 20-30 minutes. Note that acids can often get under the protective solder-mask coating of a PCB and travel along the traces to areas quite far from where the visible battery damage has occurred. A neutralizing "soak" with the base mixture will help mitigate any further ingress of the acid.
After soaking/scrubbing, rinse the board thoroughly and follow-up with the "Grime" cleaning procedure listed above.
For boards suffering from salt-air or water-based corrosion, the cleaning procedure for "Grime" is usually all that is required.
Salt and acid damaged boards will often form hard oxides on metal surfaces that won't yield to gentle scrubbing. Media blasting (glass or plastic beads or crush walnut shells or corn cobs) will often help remove more stubborn oxides and tarnish.

Specialty Techniques ('other')

Depending on the equipment you have available, other cleaning methods can be beneficial.
Ultrasonic cleaning such as with a Bransonic cleaner can get rid of 'hidden' contaminants and reach areas direct scrubbing can't. It will also remove fluxes and oils if used in conjunction with appropriate detergents.
Media blasting ("bead blasting") is a dry cleaning method which is particularly good for removing oxidation build-up and some types of paint/coatings.
Media tumbling is most often used for cleaning small parts or spent ammunition casings ('brass'). It is also useful for removing oxidation or polishing of small parts for arcade games.
Metal cleaners/polishes ("Brasso", etc.) can be used to remove some types of metal oxidation/tarnished from chips and connectors.