physics chemistry

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    What's the best flux liquid for soldering copper onto stainless steel, HCl or H3PO4?

    What's the best flux liquid for soldering copper onto stainless steel, HCl or H3PO4, with how many percent concentration of each ? How's logical explanation on solder sticking process will actually work some suggest Phosphoric acid better, actually I tried both works, ...many thanks in advance !