'Stackable bus connectors' are common nowadays on Arduino and other boards where a single connector has both the male pins (for the board below) and female sockets (for the board above)
In DIY sites there are instructions on how to solder these to the board. This isn't tooo hard.
But what do you do when you have a PC104+ 4x30 (120 pin) connector?
Soldering these by hand isn't easy and very time consuming but possible.
BUT, what do you do in mass production? you can't move these connectors through wave soldering as solder will get attached to the male pins and you can't have paste on the holes.
Any ideas on how this is done? I have a hard time believing this is done by hand. In the past there were press-fit connectors (still available today) but these are rare and quite expensive.