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Background:

I use Sodium persulfate to etch a copper board to manufacture home made Printed Circuit Board (PCB).

On the etching powder's plastic, it is written that:

1.5-2dl of etch powder in 1 L of hot water or corresponding volumes

So I add 2 cup powder for 10 cup hot water. In the first time usage it works perfect and it etches the board (double layer 8x16 cm FP4 board) in 20-30 min. However, when I use it for the second time (in another day), the solution works tooo slow. The last time I waited 13 hours for board to be ready. As the board stayed too long on the enchant, the acid ate away many traces on the PCB.

I add extra sodium persulfate (around 2 cup again) when I use the solution second time.

I use microwave (900 W) to warm the enchant again. I have not measured the temperature of the acid but I guess it might be around 40-60 C and I don't have any temperature stabilization so that the solution get colds in 5-10 min again and I repeat the same procedure maybe 6-8 times per process.

Question

Why it takes too long to etch the copper when I use the solution second time although I add the same amount (2 cups) of sodium persulfate to the solution?

What can I do to speed up the etching process apart from mixing the solution?

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    $\begingroup$ Only what you don't want: prepare a fresh solution each time as the disolved copper is making things slower. You can't use another reagent? I know of ferric chloride and the mix of hidrochloric (or muriatic) acid/peroxide. $\endgroup$ – f p Jul 12 '13 at 13:50

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