Electronic – De-soldering Through-Hole Solder

If you don't want the leads to protrude on the bottom side you essentially want to mount your TH components as SMDs. So I would suggest to use SMD pads, push the leads onto the pads, and solder them there.

For large/heavy components this might give a mechanical reliability problem: a round wire soldered at both sides (or soldered at the bottom only but with a component immediately at the top) can't easily rip the copper from the PCB, but a lead soldered to an SMD pad can!

PCB material is not optimized for conducting heat, so you might want to put lots of via's in the appropriate places to conduct the heat.

I read in your edit that you want to do this for trough-hole screw connector blocks. My gut feeling is that this will seriously impact the mechanical reliability. You could consider using those two-level connectors for screw terminals, which would free room so you could put your heatsink under only a part of your PCB, leaving the area of the screw terminal terminals to be soldered at the bottom.

To do dip soldering you first flux the board (for example by dipping it into liquid flux). There is no rush to solder after this, you can even let the flux dry. It helps if you can preheat the board to approximately 100°C.

The solder should be at the proper temperature before starting. Skim the dross to one side, leaving a shiny surface, then 'roll' the PCB into the solder starting at one end until it is flat in the solder, then lift the part up that was lowered first. Total time should be a few seconds. There will be much sizzling as the flux boils off. If the solder is too cool it won't clear the pins well. If it is too hot you can damage parts or the board itself.