Thiourea should be very easily rinseable, it is present in the etchant at only
ppm levels, and is very water soluble, so therefore should rinse real easily.

If you are leaving residues, it could be that you have consumed all the
thiourea (it's purpose is that it is a Cuprous ion chelate), and are dropping
out Cuprous salts, which are the least soluble of anything in the etchant.

If this is the case, the solution is to pull more air through the etcher.

You may want to look at our home page, http://www.pcBfab.com which has a
discussion of etchant chemistry.

Best regards,

Rudy Sedlak
RD Chemical Company

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