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60 DESIGN007 MAGAZINE I APRIL 2019 It has been well known in the RF industry for many years that copper surface roughness impacts RF performance of PCBs. This analo- gous statement also applies to high-speed digi- tal applications. To explain the copper surface roughness issue, take a simple two-layer cop- per circuit, for example. The circuit is a microstrip with a signal con- ductor on the top copper plane and a ground plane on the bottom of the circuit. Addition- ally, the concept of skin depth and wave prop- agation should be considered. Skin depth is the depth within the cross-sectional area of the copper where the majority of the RF cur- rent resides and is dependent on frequency. At lower frequencies, the RF current will have a thicker skin depth and use more of the con- ductor. At higher frequencies, the skin depth is thinner, and less of the conductor is used by the RF current. For wave propagation, the electromagnetic wave that propagates on the microstrip circuit will be slower for a circuit using a higher dielectric constant (Dk) materi - al. Using a lower-Dk material, the wave prop- agation is faster, and the propagation delay is reduced. Rogers has studied copper surface roughness extensively and has a lot of information to share for the interested reader. We know if a compar- ison is done on two identical circuits with one using a high-Dk material and the other a low- Dk material, the circuits will have slower and faster wave propagation, respectively. Howev- er, we found other circuit properties can alter the wave propagation speed, and one of them is the copper surface roughness. To be specific, the copper surface roughness is the roughness at the substrate-copper interface. Further, we have done experiments where the same material was used with the same mi- How Copper Properties Impact PCB RF and High-speed Digital Performance Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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