If your out of warranty try this: (The Heating Reflowing part is optional, I would try to just clean it out and repaste the cpus first.)
1. Remove warranty sticker and rubber foot that’s positioned above hard drive bay. This will invalidate any warranty.
2. Using a small flat blade screw driver or slotted torx bit, unscrew cover plate screw.
3. Slide cover plate towards the left until it stops and lift off.
4. Remove all Phillips screws marked noting size and position.
5. Hinge case towards you until disengaged and then tilt towards the left. See below.
6. Remove ribbon cable by carefully lifting black part of connector (locking tab).
7. Lift/tilt blue ray drive towards the right whilst disconnecting power cable and the large ribbon (lift tab) underneath.
8. Remove 5 screws and the earth wire screw on psu, and unplug connector plug. Before removal of psu unplug small bnc connector (black wire) and remove tape on side of psu, now lift off psu.
9. Now remove the wide ribbon from connector (lift tab) unscrew the four screws on the Bluetooth board and remove.
10. Remove hard drive cover.
11. Unscrew blue hard drive screw and disengage hard drive by sliding towards gap in the bay.
12. Remove hard drive, see below.
13. Unscrew 4 screws holding the bracket, on/off and eject control panel. Remove small ribbon (lift tab) and remove control panel.
14. Unscrew 8 screws.
15. Lift enclosure out of plastic base, see below.
16. Now unclip black rear panel and remove.
17. Unscrew 4 screws and remove sprung brackets, see below.
18. Lift of metal cover plate whilst allowing ribbons to pass through gaps in plate without damage, see below. (Taking note where heat sink rubbers are positioned).
19. Supporting fan assembly turn board over, remove fan power connector and fan.
20. Remove 2 screws holding plate to hard drive connector and unplug battery connector.
Now remove plate.
21. Clean off old heat sink compound from the 2 mainprocessors using Isopropyl alcohol making sure all traces of the old heat sink compound has gone.
22. Clean both touch plates on bottom of fan/heat sink assembly with Isopropyl alcohol until all traces of the old heat sink compound has gone.
23. Now using a vacuum cleaner or compressed air blower get rid of all dust build up in the case and on all component parts of the ps3 (pay special attention to the fan assembly and heat sink fins (MUST BE CLEAR OF DUST).
24. Once cleaned you should now have a nice pile of bits that looks something like this, see below.
Now for the tricky bit!
25. Check the top metal cover plate (underside) for burn-in marks, as you can see below this ps3 got very hot before it yloded!
26. Place the ps3 mother board on a flat heat resistive surface (kitchen work top or melamine shelf will be fine).
27. Turn heat gun on and set temperature to 350 deg and the fan speed to low (to high and it will move the surface mount bits).Allow gun to get up to temperature before continuing!
28. Using a circular motion hold heat gun over areas outlined below (keep heat gun an inch above
board/components at all times).
29. CPU side first.
30. After 15 to 20 seconds move onto the next area.
31. Once all marked areas have been heated/reflowed on the mother board LEAVE for at least 10 to 15 minutes to cool. (DO NOT MOVE MOTHER BOARD WHILE COOLING).
32. A popping noise may be heard during cooling, don’t worry this is normal.
33. NON CPU side.
34. After 10 to 15 seconds move onto the next area.
35. Once all marked areas have been heated/reflowed on the mother board LEAVE for at least 10 to 15 minutes to cool. (DO NOT MOVE MOTHER BOARD WHILE COOLING).
36. A popping noise may be heard during cooling don’t worry this is normal. Heat sink compound still on in this picture, remove before reflowing. Heat sink rubber pads still on in this picture remove
before reflowing also hold ribbons out of way while reflowing
You have just repaired you’re yloded ps3 mother board.
37. Now its time to apply new heat sink compound to the 2 processors, when doing this its always best to follow the manufacturers instructions but I believe the process is the same with all types (please check instructions that come with paste), before continuing
38. The paste I am using is called AKASA 450 it’s a silver based compound with a thermal conductivity of 9.24w/m deg c and an operating range of 0 to 200 deg c. You can buy it from most pc shops but I get mine from my local Maplin ( I have been told that there is a better compound on the market called arctic silver but I haven’t tried it as yet so the choice is yours).
39. Use syringe supplied and dispense a small worm of compound from top to bottom on the left hand side of processor chip, now using credit card supplied spread compound evenly and thinly over processor face (to much is bad and not enough is also bad) the ideal amount is when you can almost see through the paste.
40. Now repeat on the other processor.
41. Reassemble the PS3.