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70 The PCB Magazine • March 2014 impedance by using wider conductors and larg- er gaps. In digital designs, limitations of line widths restrict this freedom of design [4] . When the depth of penetration, or "skin depth," of the electromagnetic field exceeds the root-mean-square roughness height, the in- fluence of roughness on loss is minimal. With increasing frequency, the skin depth becomes smaller and currents flow near the surface, fol- lowing the contour of that surface and thus re- sulting in an increased resistance because of the additional distance the currents must flow. At the highest frequencies, the depth of the pen- etration is much smaller than the roughness height and the additional loss caused by the roughness becomes almost independent of the frequency. At 10 GHz, the skin effect of copper is below 1 µm. The reduction of the foil's treatment profile has essential advantages since conductor loss is widely reduced and therefore allowing new op- portunities in the (very) high-speed sector. Methodology and Results IPC-4562 defines three major classes for roughness profiles, irrespective of the foil's thickness. Below 5.1 µm (200 µin) is defined as "very low profile." However, this definition is behind the industrial progress. Another gap is found among definitions of Rz since Rz ISO is not identical to Rz JIS. For very low roughness, this difference is signifi- cant and close to 20–25%. The smoother the roughness, the higher the risk for reaching erroneous conclusions by mea- suring with a contact profilometer, as required by IPC-4562. A standardized contactless mea- surement is needed. Figure 1 illustrates the existing IPC classifi- cation together with our internal guideline in- cluding such smoother roughness profile types: Copper foil surface roughness from micro- sections along with a digital translation into a pixel mapping for calculating the foil's profile is currently investigated [5] . In order to significantly reduce the foil's profile, a new ED copper foil type called BF foil was developed. Prior treatment, such foils have a roughness Rz (ISO) of ~1 µm and less than 2.5 µm after treatment, as shown by Figure 2. ED-COPPER FOILS FOR HIGH-SPEED DIGITAL PCBS continues Figure 1: roughness definitions and revised classification.