Over time, dust and grime inevitably builds up inside your computer directly on the circuitry. The fans won't clear it out, and you can't ignore it because it can lead to overheating and, ultimately, premature component failure. If you have a tower-style desktop computer with a large enclosed hardware environment, you can usually clean it from the outside. Sometimes, however, you will need to open it up for direct cleaning, which is hazardous for the electronics. With the right tools and a careful method, you can mitigate these risks and clean circuit boards and peripheral cards safely.
Using Compressed Air From the Outside
Handling a circuit board directly isn't something you should do unless you must. The electronics on a circuit board are very sensitive, and being handled by a human is just about the most dangerous part of a circuit board's life. So the first rule for cleaning one is to be as unobtrusive as you can. Usually, you can clean everything that needs cleaning simply by shutting down the computer and spraying compressed air into the fan exhaust ports. This will dislodge most free-flowing dust inside the machine and blow it right back out the ports. Keep the air can as close to upright as possible, and use a series of short bursts until no more dust blows out.
Opening the Computer Tower
If you notice a lot of noise or heat coming from the tower or showing up on your system monitor, and spraying compressed air from the outside doesn't help, then it's time to have a look inside. Shut down the computer and unplug all the cables, both power and data. Make sure your hands are clean, then sit down at your work station and ground yourself. Use a screwdriver if necessary to open the tower and slide out the chassis. Holding the canister upright, spray compressed air all over the circuitry, keeping the nozzle a couple of inches away from the surfaces.
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Explaining Grime, Soldering Flux and Corrosion
Before you touch anything, make a visual inspection of the electronics and check for grime and soldering flux. Grime is any kind of dust mixed together with oily or waxy substances so that it sticks in place, and will be the biggest remaining issue after you spray away the loose dust. Soldering flux is the leftover detritus from a sloppy soldering job, and is more hazardous to remove than grime. If you live somewhere with salty air, or if a battery burst on the circuit boards, then you may also see corrosion. Corrosion from salty air looks similar to grime and can be cleaned the same way. Corrosion from a battery should look pretty noticeable.
Deciding What Grime and Flux to Clean
Cleaning off grime and soldering flux can be even more hazardous than leaving it to accumulate. This is especially true of soldering flux, which isn't likely to spread farther than it already extends. Generally, you shouldn't remove it unless it's particularly bad or you're confident you can do so safely. If grime is building up near a heat generator like the CPU or looks like it's sitting on top of circuit pathways -- or if it appears to be an especially large deposit -- then you should cleaning it. The rest you can probably leave -- except for the grime in the sockets where your cables plug into the computer, which you should clean freely since there are no sensitive components there.
Performing the Spot-Cleaning
For spot-cleaning grime and soldering flux, use cotton swabs and isopropyl rubbing alcohol, preferably with an alcohol content in the 90 to 100 percent range -- the higher the better. Lightly dampen a swab with the alcohol. You don't want it to drip or leave puddles when you touch the components, so if you soak too much onto the swab, get a paper towel and squeeze the excess liquid out. Then take the swab and gently brush it across the grime or flux to dislodge it. Use patience rather than force. Many gentle strokes will eventually get the job done, and with far less risk. After the alcohol dries, remove any remaining dislodged bits with compressed air.
Treating Significant Corrosion
You should always treat significant corrosion when you see it. Try the grime-cleaning method first. If that doesn't work, try lightly rubbing the corrosion off with a pencil eraser, which can be especially helpful on copper components. If you need more, mix together baking soda in a little bit of water to make a runny paste. Soak a cotton swab in it -- this time so that it is almost dripping -- and apply it gently and thoroughly to the corroded areas. Let it dry completely, at least one full day, then repeat the grime-cleaning method. Replace the offending battery as well. You may need to specially order one if none are available in your area.