In article ,
Doug White wrote:

Mike B someone@noplace wrote in
:

On Sun, 14 Oct 2012 08:54:28 -0400, "Steve W."
wrote:

Mike B wrote:
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

Why? Take the stainless tube. Clean it VERY well on the end. Use a
stainless wire brush and while you brush the stainless use regular
solder to tin the stainless. Then solder as normal.

I've had zero luck soft soldering stainless with 60/40. Years ago I
tried a silver based soft solder and that didn't work either.

It works fine if you have the right flux. I used to solder to stainless
RF connectors all the time, using a small bottle of clear liquid flux.
Unfortunately, I don't remember the brand or name of the stuff. The
catch is that it is an acid, and it boils & spatters when you solder with
it. It eats soldering iron tips, and the spatter will corrode everything
in sight. It's best to have a separate iron & work in a well protected
or separate environment from whehre you do most of your work. And
definitely wear safety glasses!

It's called Tinners Flux, and is made and sold under many names, and
some kinds are specialized for use on stainless steel (usually because
the flux has some Phosphoric Acid in it), but I've used ordinary
hardware-store tinners flux on stainless without difficulty.

Careful mechanical cleaning of the steel to be soldered is always
helpful.

Joe Gwinn