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Considering water as a given, the third most important ingredient in a copper electroplating solution, as a short survey of related recipe webpages will show you, is hydrochloric acid (HCl). Here's a sample link: http://www.thinktink.com/faqs/cupltfaq/cupltf01.htm

I can guess that the role of sulfuric acid (the second most important ingredient; the first being copper sulfate) is to provide you with sufficient anions to allow copper cations to easily enter the solution at the anode. But in almost all recipes that I have seen, a small amount of hydrochloric acid is added. I wanted to know how useful it is and what is the opportunity cost associated with it.

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    $\begingroup$ This is an hypothesis: i think HCl form a complex with $Cu^{2+}$ such as $CuCl_4^{--}$, this remove copper ions from solution and so allows further oxidation of copper to take place. $\endgroup$ – G M Oct 12 '13 at 8:03
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As already mentioned by @GM it indeed is used to take out $\ce{Cu^{2+}}$, but it is also used to take out $\ce{Cu+}$.

I know this because I once used a mixture of $\ce{HCl}$ and $\ce{H2O2}$ to etch copper boards, in that process both $\ce{Cu^{2+}}$ and $\ce{Cu+}$ were dissolved by the formation of $\ce{CuCl2}$ and $\ce{CuCl}$ with $\ce{Cl-}$ from $\ce{HCl}$.

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The role of the HCl in copper plating and nickel plating solutions is to increase solution conductivity and increase anode corrosion and thus inhibit anode passivation which can deplete the bath of copper sulfate.

Be careful with with adding too much HCl since it is a very difficult component of a bath to selectively remove, dummy plate or filter and high levels tend to cause deposit stress and low anode efficiency. HCl is good at removing the oxides from copper but does not form a ionic species as sulfuric acid does.

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