This is probably the most common cause of PCs crashing - overheating. We've covered this in other threads...
A decent quality 1" paint brush with the hoover nozzle nearby is the best way (with the PC unplugged) - the dust is basically human and animal debris & a whole bunch of dust mites - if you are allergic to dust mites (I am!) then keep the nozzle of the hoover close to the brush as you clean.
An air compressor is excellent for getting to difficult places, but DO THIS OUTSIDE else your office/workshop will just have all that nasty stuff just re-distributed... Compressed air is ideal for cleaning up inside fans - rotate the fans by fingertip - if you feel that there is even the slightest "stickyness" after cleaning, replace the fan ASAP with a good quality ball bearing (not sleeved) one.
One point that is often missed is to clean through the PSU - its a classic dust-trap, and should be thoroughly cleaned out too... as has been pointed out by others, the PSU has capacitors charged up to about 400V, so if opening up the PSU case, leave it well alone for 10 minutes after unplugging before cleaning. Note that even when a PC is turned off and no fans are running, the PSU is still providing 5V standby power to the motherboard and is thus still a danger.
If you must remove the CPU heatsink to clean it (not always necessary), then please note that you shouldn't reuse the old thermal compound as it dries out over time and is never so good second time round when trying to re-seat a heatsink.
Important point here too - the stuff that's generally used is silver-loaded and is conductive, i.e. bad news if not where it's supposed to be, so clean it off very carefully with IPA (not the beer, Isopropyl alcohol aka 2-propanol) and a cotton bud, being VERY careful not to get any on the PCB or on other components - only use IPA in well-ventilated areas - you will get a blinding headache otherwise! Been there, done that, got the headache
Whilst IPA is best, methylated spirits is OK too...
EDIT: Should have mentioned that sometimes a thin thermally conductive rubber pad is used. If there was one there, don't re-use it (they don't reform well). Remove it carefully with a craft knife, clean with IPA, and proceed with using Ag-loaded compound as below...
Important: If you remove the CPU chip itself from its socket, either deliberately or accidentally, do not handle it without an anti-static strap round your wrist that is properly earthed (not to the PC case as that is unplugged, hopefully!)
Its important to use the same sort of Ag-loaded compound (sometimes called "Arctic Silver") to re-seat the heatsink - use extremely sparingly - the thinnest possible coat that completely covers the metal part of top of the CPU chip (actually, stopping maybe 0.5mm - 1mm short of the edge is good) and absolutely no extra that might "splodge"/squeeze over the edge when clamped - a thin bit of flexible plastic used as a spatula is excellent for this.
Put a sticker on the back of the case saying when it was last cleaned out (I do this for batteries in smoke alarms & test equipment too).
Lastly, think about where you site your PC - if its on a carpeted floor it will collect a whole bunch of rubbish - just raising it off the floor by 4 or 5 inches can really help.
A decent quality 1" paint brush with the hoover nozzle nearby is the best way (with the PC unplugged) - the dust is basically human and animal debris & a whole bunch of dust mites - if you are allergic to dust mites (I am!) then keep the nozzle of the hoover close to the brush as you clean.
An air compressor is excellent for getting to difficult places, but DO THIS OUTSIDE else your office/workshop will just have all that nasty stuff just re-distributed... Compressed air is ideal for cleaning up inside fans - rotate the fans by fingertip - if you feel that there is even the slightest "stickyness" after cleaning, replace the fan ASAP with a good quality ball bearing (not sleeved) one.
One point that is often missed is to clean through the PSU - its a classic dust-trap, and should be thoroughly cleaned out too... as has been pointed out by others, the PSU has capacitors charged up to about 400V, so if opening up the PSU case, leave it well alone for 10 minutes after unplugging before cleaning. Note that even when a PC is turned off and no fans are running, the PSU is still providing 5V standby power to the motherboard and is thus still a danger.
If you must remove the CPU heatsink to clean it (not always necessary), then please note that you shouldn't reuse the old thermal compound as it dries out over time and is never so good second time round when trying to re-seat a heatsink.
Important point here too - the stuff that's generally used is silver-loaded and is conductive, i.e. bad news if not where it's supposed to be, so clean it off very carefully with IPA (not the beer, Isopropyl alcohol aka 2-propanol) and a cotton bud, being VERY careful not to get any on the PCB or on other components - only use IPA in well-ventilated areas - you will get a blinding headache otherwise! Been there, done that, got the headache
Whilst IPA is best, methylated spirits is OK too... EDIT: Should have mentioned that sometimes a thin thermally conductive rubber pad is used. If there was one there, don't re-use it (they don't reform well). Remove it carefully with a craft knife, clean with IPA, and proceed with using Ag-loaded compound as below...
Important: If you remove the CPU chip itself from its socket, either deliberately or accidentally, do not handle it without an anti-static strap round your wrist that is properly earthed (not to the PC case as that is unplugged, hopefully!)
Its important to use the same sort of Ag-loaded compound (sometimes called "Arctic Silver") to re-seat the heatsink - use extremely sparingly - the thinnest possible coat that completely covers the metal part of top of the CPU chip (actually, stopping maybe 0.5mm - 1mm short of the edge is good) and absolutely no extra that might "splodge"/squeeze over the edge when clamped - a thin bit of flexible plastic used as a spatula is excellent for this.
Put a sticker on the back of the case saying when it was last cleaned out (I do this for batteries in smoke alarms & test equipment too).
Lastly, think about where you site your PC - if its on a carpeted floor it will collect a whole bunch of rubbish - just raising it off the floor by 4 or 5 inches can really help.
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