Why is it so hard to desolder iMac capacitors?
Trying to replace capacitors on an iMac motherboard and having a very difficult time desoldering them. Any ideas why this is more difficult than other manufacturers boards?
The main problem(s) with desoldering electrolytic capacitors from an iMac motherboard (or any logic board) is due to lack of soldering skills and/or knowledge of soldering - possibly too low of wattage soldering iron. But, I do it on a regular basis with a 40 watt iron - though "the authorities" claim you need a 60 watt stain glass iron for the job.
Kyle correctly pointed out that lead-free solder to comply with RoHS. Lead-free solder melts at a much higher temperature. He over looked that if you melt some lower temp solder into the existing joint that this factor goes away.
BAC correctly points out that this is a thick, multilayer board and it acts like a heatsink. Again, this factor becomes negligible if you melt some low temp solder into the joint.
I know it sounds crazy saying to desolder something that you need to add fresh solder to it- preferably with a lower melting point - to make the process of desoldering go faster and smooth, but the method is tried and true.
I don't know the specific details of the logic board you're working with, but in general multi-layer circuit boards are often difficult to desolder components because the many copper layers within the board can too easily conduct the heat away from the area you're trying to heat, especially if you're using a low wattage soldering iron. in any case you need to be careful about overheating the board and possibly delaminating nearby traces.
Are you heating the wick, then the lead?
You can try just pulling the Capacitor off, leaving the attaching legs/wires, then solder the new capacitors to the remaining wire. This requires no soldering directly on the motherboard.
Radio-Shack have a very nice iron with a hole in the tip; I found this makes working on the G5 iMac board much easier, as it heats all round the lead.
I recently re-capped a G5 iMac board, and it went very smoothly thanks to this $12 iron.
Replacing the capacitors can be done without having to clear the holes, by cutting the leads short so they can be pushed in with a little pressure as one heats from the other side.
...preheating the area with a heat gun, and radiating out from the work-site makes the process go much better, if you don't preheat the work area, when you put the solder tip on the board the cold board is acting like a heat-sink and drawing the heat from the work area.
be mindful of the melting temperature of any components associated with the use of the heatgun...a good hair-dryer will work in the absence of an industrial heatgun...also try adding fresh solder to the connection after you preheated the board before wicking the solder, degradation of the properties in the old solder prevents it from wicking properly as well
There is a large copper plane sandwiched in the middle of the Logic board near the large bad caps on a 20-inch iMac G5 and possibly similar models. It sucks the heat away.
You can get a 45-watt or more nice soldering iron to deal with it. You can get a very small drill-bit (1/32' or less) and drill away the leeds for the old caps.
Don't go all out and use a propane torch on the board.
I find many mac models have conformal coating on the logic boards. This can also make unsoldering more difficult. And if you are soldering/desoldering anything attached to the ground plane, you need a soldering iron that can source a lot of heat.
ground plane connected part,
RoHS (I think most intel iMacs are RoHS compliant),
means you need something more than the standard pencil type soldering iron. I use an old Weller WTCPN, which is a beast of an iron. The power supply weighs about 8 pounds.
step up wattage on sodering gun.