We use only the finest, newest, fanciest top tier solder. The aerospace grade product that we are currently using employs what is known as a "no clean flux" which is safe (and designed) to be left on and not cleaned off the PCB.
We use a heavy cored wire solder for the connectors to ensure that everything is fully flowed with a complete flux covering. This actually ends up protecting the solder joints from moisture through the life of the board.
This coating left behind from the soldering process is NOT corrosive or hazardous like flux from ancient times. All of our boards have the through hole components soldered by a real human (machine selective soldering is a joke).
All surface mount components use the same solder alloy and type of flux but are reflow soldered in a large multizone proper conveyor oven.