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.
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