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I don't use any flux with 2% 5% or even 10% silver solder on copper to copper.
The only time I've used flux was when I was going over soft solder with 5% silver.
The two will form a new alloy but. . . you need to flux it.
See here middle of the page for what I did on SP2.
SP2If you are not sure. . . practice on scrap
Also to keep your joins neat,
Where you don't want the solder to run (as silver solder has a habit of doing)
Put liquid paper / white out or whatever they call it.... on the place you don't want the solder to go.
:D :D :handgestures-thumbupleft: