I'd braze the joint, or possibly use silver solder.

Ordinary soft solder is simply not suitable for what you are trying to
accomplish.

I have an Oxy/Acet torch available, but I normally just use an
inexpensive MAP gas torch on jewelry and small items. Silver soldering
is arguably a bit easier than brazing, but either technique should work
for your application. Both require specialized fluxes to do a good job.
The flux should be available from the same place that you purchase your
silver solder or brazing rods.

Hope this helps.

Harry C.