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.
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?