I need to join a 316 stainless steel tube to a copper plated fibreglass circuit
board which can only withstand lead tin soldering temperatures (180 C).
..
If I 'tin' the stainless using silver solder (Weldcote Metals 50 N) will I be
then be able to use 60/40 tin/lead solder to join it to the copper?

If that isn't going to work then I'll have to silver solder a copper ring around
the tube then solder to that.

Mike