Tom: Although one ounce of copper per square foot equates to .0014", most fab houses purchase cores with copper cladding with an associated tolerance (ex: .0014" +/-.0002"). It seems that the copper folks very shrewdly send us copper which tends to be on the low end to save money. Our experience has been an average of .0013", but the material is still within spec at .0012". Further board processing can remove as much as .0001" during cleaning, so a bare minimum (copper at the low end minus processing) would be .0011" (1.1 mils). That is how the terms "one ounce copper" and "one mil copper" have become somewhat erroneously interchangeable in our industry. Regards, Tom Coyle Field Services Engineer HADCO Corporation