I was waiting for some Acrylic for the outside of my enclosure, I had taken my heated bed and moved it to the side while adding the upgraded hot end. I thought I should put it back now I have the new Marlin Firmware & I could adjust the hot end height etc. In the past I had levelled the bed quite easily but now for some reason it was just was not levelling at all. The bed was totally twisted.

I was trawling Amazon and saw the MK3 aluminium clad heated bed. I read about the MK3 and how it should have an even heated surface.
Ordering it was the easy part.
I have always found that the LED on the bed was a good sign of if the bed was on or not. I examined the bed when it arrived it seemed OK. Quite thick no visible warping. I also noticed that the LED would now be facing down and not visible. I thought I would just wire the LED to a resistor two wires and add it to the loom no big deal.

I tried to solder the heater wires to the pads with my 40 Watt soldering iron. It kept sticking to the pad it was like it was a magnet.
OK, I thought, this needs the 100 watt soldering iron gun. I tried heating up the gun until it had been on for several minutes. This gave me about 4 seconds of usable time then it started sticking too. After about 90 minutes I had managed to get the solder to flow on one of the pads. But the pads 2&3 had to be joined together there was no way I was going to solder this properly.

I thought about the reflow soldering process so I got the oven that my wife uses for her polymer clay. The oven was too small to put the board inside so I put the part of the board with the pads on the outside top of the oven and turned the oven on high.
You would think this would work!
The board was now hot and it gave a small improvement but not enough to solder both pads.
After spending 90 minutes fussing with the oven and the 100w gun. I added my 40 watt iron success the solder flowed. I needed an extra set of hands and got help to hold the wire in place and feed the solder while I held the 2 soldering irons together on the pad.
So if you are trying to upgrade to an aluminium heated bed, then now you know the key is to preheat the board so that the aluminium does not suck away all the heat from the soldering iron.
They are not the best solder joints in the world the one on the right is better but the 2 on the left wow they were difficult to solder neatly.