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March 2014 • The PCB Magazine 73 be significantly increased on a certain number of low- and very low-loss resins. Studies and Results In a first study carried out by Isola, the 18 µm (1/2 oz./ft²) thick ultra flat profile foil BF- TZA (Rz ISO ~2 µm) was compared to an 18 µm (1/2 oz./ft²) thick typical reverse treated TZA-B foil having a normal very low profile (Rz ISO ~ 4 µm). Insertion loss (SET2DIL method) at 4 GHz and 8 GHz was measured and shown in Figure 8. For very low-loss resin systems, the flat pro- file foil BF-TZA generates significantly lower conductor losses compared to a reverse treated foil. The roughness profile on the bonding side of the copper foil is not the only contributing factor for conductor losses. By applying a regu- lar innerlayer bonding treatment on the foil's shiny side, it becomes evident that gains in con- ductor losses from a flat profile foil are strongly attenuated by the innerlayer oxide treatment's roughness (Figure 9). ED-COPPER FOILS FOR HIGH-SPEED DIGITAL PCBS continues Figure 6: almost no profile foil as next step flat profile BF foil (l); almost no-profile BF-anp foil (center); combined with 4µ proprietary primer coating (r). Figure 7: peel strength of BF-anp (almost no profile) foils combined with a primer coating.