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Black oxide treatment
Glossy copper surfaces on the inner layers of multilayer PCBs are clean and easily wettable, but have poor resin adherence and therefore must be roughened by a chemical process known as black oxide treatment. For this purpose, the printed board is immersed at 95 °C for 2 minutes into an oxidizing agent consisting of 31 g sodium chlorite, 15 g sodium hydroxide and 12 g sodium phosphate per liter of water (pH = 13.4). In this way, high peel strength of 1.8-2.1 kg/cm2 can be achieved. However, the duration of resistance to haloing is shortened by 2 minutes. The name of the process makes reference to the black color of the copper after treatment. If the color of copper is more brownish, we speak about brown oxide treatment. See also Brown oxide treatment and Haloing.