It has been well known in the RF industry for many years that copper surface roughness impacts RF performance of PCBs. This analogous statement also applies to high-speed digital applications. To explain the copper surface roughness issue, take a simple two-layer copper circuit, for example.
The circuit is a microstrip with a signal conductor on the top copper plane and a ground plane on the bottom of the circuit. Additionally, the concept of skin depth and wave propagation should be considered. Skin depth is the depth within the cross-sectional area of the copper where the majority of the RF current resides and is dependent on frequency. At lower frequencies, the RF current will have a thicker skin depth and use more of the conductor. 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) material. Using a lower-Dk material, the wave propagation 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 comparison 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. However, 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 microstrip design, and the only difference was the copper type. In these experiments, we consistently saw that circuits with a rougher copper surface had a slower wave propagation compared to circuits with a smoother surface. A rougher copper will slow the wave, and a slower wave is perceived by the circuit as a higher Dk, even though the Dk of the material is the same for both circuits. When extracting the Dk value from circuit performance, the circuit with the rougher copper will have a higher Dk value than the circuit with a smoother copper. Also, we found that wave propagation is more affected by the copper surface roughness in circuits made using a thinner substrate versus circuits made with a thicker substrate.
To read this entire column, which appeared in the April 2019 issue of Design007 Magazine, click here.