Solder joint fracture on copper cuff

I’m soldering round pewter medallions onto the center face of copper
cuff bracelets with limited success. Copper does not have the
"spring" qualities of brass or silver, and when you adjust the
copper bracelet for size, the copper bends instead of flexing. The
soldered joint holding the pewter medallion tends to fracture. I’m
thinking perhaps I should switch to brass instead of copper, or maybe
soldering the medallion into an inlaid pocket instead of on the