foil having a thickness of 35 u03bcm polyimide from step 1 and the polyimide bmi from step 3 described above were stored in storage tanks a and b respectively a nitrogen blank was applied in both storage tanks a and b a gear pump 2 was used to deliver the coating composition from each storage tank each of the two coating compositions first passed through a filter 3 then a debubbler 4 a pressure sensor 5 was attached to each of the debubblers 4 to detect any pressure anomaly the pressure readings were displayed in pressure displays 6 the two coating compositions converged at the double layered extrusion die 7 before they were applied onto a moving copper foil 8 which was rolled pass the extrusion die 7 by the pulling of a chill roller 9 the upper layer had a thickness of 250 u03bcm the lower layer had a thickness of 250 u03bcm after drying the upper layer and the lower layer had thicknesses of 25 u03bcm and 25 u03bcm respectively thus providing a final total thickness of 50 u03bcm the coated copper foil prepared above was pre heated at 150 u00b0 c for 20 minutes prior to the start of the.