he actually responded to this issue a few pages back. Shouldn't take too much time to find & he explains why he did this
OK find this:
"Exposing the copper on the bottom will increase the thermal coupling but i want to keep the heat sinks isolated.
(Important for water cooling, electrical corrosion)"
Well it makes difficult to solder heatsink on painted surface and using stripper will remove all paint. Not good. Electrical corrosion blah. If use same ground no broblem or use non conducting coolant.
You don't solder on heatsinks, you use thermal pads or paste.. If you could solder them on, then almost by definition they aren't good heatsinks.
He's probably asking this because he either has experience or has heard that good conductive solder will beat thermal paste or thermal pads in performance. Intel Ivy bridge Cpu line used thermal paste on their chip line which was different from the past when they normally solder the heat spreader to the die. You'd install the intel heatsink (or better aftermarket one) with thermal paste between the heatsink and chip spreader. So many enthusiasts were pissed about this on computer forums that they would pry the heat spreader off (voiding there warranties) and use a high quality thermal paste between the cpu die and heatsink. With this it allowed a relative improvement in temperature and overclocking headroom compared to the inferior or poorly applied intel thermal paste.
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