the ferric state the circuit patterns were treated to produce a black copper oxide coating on the exposed surfaces this was done using proprietary solutions supplied by the enthone co and following their recommended procedure a similar treatment using a basic solution of sodium chlorite is described in u s pat no 2 364 993 to meyer the use of black oxide treatments to improve adhesion of copper to resins is well known two such pieces each 6 34 u00d7 8 34 152 u00d7 203 mm were prepared with simulated circuit patterns covering a 4 u00d7 4 inch 102 u00d7 102 mm region on one side and with a 4 u00d7 4 inch square of copper removed from the other side centered in the piece of laminate and opposite the circuit patterns pieces of the unsintered poly tetrafluoroethylene tape described above were laid on the patterned side of one circuit bearing layer the full 6 34 u00d7 8 34 area was covered with the tape the other circuit bearing layer was then laid patterned side down on top of the tape layer the assembly was placed in a press between two pieces of light weight aluminum foil and with a sheet of rubber above the top piece of aluminum foil to distribute the pressure with the press at room temperature a force of 40 000.