How do machines in PCB fab plants solder components?

Wave soldering and re-flow soldering are both used. Try Google.

Re-flow might work at home, but would probably impractical.

Actually, I do hotplate reflow at home and it works really well. I have a syringe of paste, and I dab it on (like a pen, not like toothpaste) each pad, place the parts, and turn on the hotplate. I move the board around to heat evenly, and when I see the paste melt I give it a few seconds and remove the board.

I thought reflow was only for SMD parts. You cant do that on dip/lead parts can you?

Yup. I use mostly SMD at home anyway, less drilling. For through-hole parts I just hand solder them.

Usually it is. Samtec makes a series of connectors that are thru-hole but can be reflow soldered. They have a little disk on the leads that allow them to do this.

These are wave soldered. There is no simple home brew method of getting the same results. Probably the closest option would be to use a large solder pot and dip the bottom side of the board for approx 2-3 seconds. You would need to dip in flux first. Most solder wave machines use a bubbler (similar to a fish tank air pump) to make the flux foam up. The board is run over this prior to soldering to get an even layer of flux on the board.

You can usually find solder wave machines cheap on eBay. The shipping is often more than the cost of the machine, as most of them are quite huge. There are a few benchtop solder wave machines out there, but they never show up on eBay.

You can replicate the look of the connections by applying extra flux and "drag soldering" the connections after you've soldered them in place.

There are a few connectors out there which can handle reflow soldering with hand placed solder preforms, but they are very rare. Most plastic connectors cannot handle being reflowed, and the surface of the plastic will look terrible afterwards.

If you do come up with a comparable solution, please share!

Chris