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March 2014 • The PCB Magazine 71 With an extremely dense and very uniform treatment, the new BF foil types offer a large contact area to the dielectrics. This is illustrated in Figure 3 by treatment imprints after copper etching. From a mechanical point of view, this foil type behaves like any regular copper foil. Such foils are produced down to 9 µm thick- ness (1/4 oz./ft²). Although this level of thick- ness is not yet needed for high-speed digital ap- plications, it will likely be necessary for sequen- tial build-up HDI boards for mobile devices and for IC packages. A large number of new resin systems with low- to very low-loss properties (with Df data ranging from 0.003–0.010 @ 10 GHz) have been introduced in the last few months. Since their requirements and limitations are very different in view of their final application, Figure 4 illus- trates an internal classification and suggestions for the market, along with the treated foil's side roughness and its name. With several highly filled resin systems, me- chanical anchorage of the treatment dendrites is the leading factor for bonding securely. With the new BF foil type, the reduced roughness profile compensates the mechanical anchor- age by a much higher contact area. This is also improved because of a much higher uniformity of the fine dendritic treatment. Figure 5 docu- ments typical peel strength ranges on a certain number of copper clad laminates. A further reduction of the foil's roughness profile is currently investigated with so-called almost no profile foils (BF-ANP foil). In order to maintain secure bond strength, such ANP (or profile-free Cu-foils) are combined with a very thin high TG primer coating layer. This potentially next generation for very high- speed applications is illustrated by Figure 6. The cross-section of Figure 6 illustrates the absence of the typical nodular treated side. By combining with a proprietary halogen-free ep- oxy-based primer coating, peel strength could ED-COPPER FOILS FOR HIGH-SPEED DIGITAL PCBS continues Figure 2: surface comparison prior treatment. left: matte side: rz ~ 7 µ. middle: reverse (shiny) side: rz ~ 2.5 µ. right: new flat profile type (BF foil) with rz ~ 1 µ. Figure 3: treatment imprints after etching. left: matte side: rz ~ 7 µ. middle: reverse (shiny) side: rz ~ 4 µ. right: new flat profile type (BF) foil with rz ~ 2 µ.