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62 DESIGN007 MAGAZINE I APRIL 2019 crostrip design, and the only difference was the copper type. In these experiments, we con- sistently saw that circuits with a rougher cop- per 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 cir- cuit with the rougher copper will have a higher Dk value than the circuit with a smoother cop- per. Also, we found that wave propagation is more affected by the copper surface roughness in circuits made using a thinner substrate ver- sus circuits made with a thicker substrate. The effect that copper surface roughness has on wave propagation is also frequency depen- dent, and at low frequencies, the copper sur- face roughness may have little or no impact on the wave velocity. Frequency dependency is due to skin depth and at low frequencies where the skin depth is thick. The roughness is a small percentage of the cross-sectional area of the conductor being used by the RF current; the effect of the copper roughness has little in- fluence on the wave propagation. However, at higher frequencies—specifically, frequen- cies where the skin depth is equal to or less than the copper surface roughness—the rough- ness will have a significant impact on the wave propagation properties of the circuit. For an RF PCB using copper with a rougher surface, the copper surface will cause an in- crease in conductor loss, slower wave veloc- ity, and a higher effective Dk. For a high-speed digital PCB using copper with a rougher sur- face, the copper will cause an increase in in- sertion loss and propagation delay, and a de- crease of the eye opening in an eye-diagram, typically. Many years ago—when digital rates were slower, and the Nyquist frequency was lower—the influence copper surface rough- ness had on these circuits was minimal. Now, with faster digital rates and higher Nyquist fre- quencies, the potential effects that copper sur- face roughness has on these circuits can be substantial. With many new applications using milli- meter-wave technology, which is RF at very high frequencies, the copper surface rough- ness can have a very large impact on RF per- formance for these circuits. Insertion loss and phase response of millimeter-wave circuits can be greatly impacted by copper surface rough- ness. All copper types used in the PCB indus- try have normal surface variation. The natu- ral trend is that a copper with a rough surface will have much more roughness variation than copper with a smooth surface. The copper sur- face roughness variation can cause phase vari- ation from circuit to circuit. Even though my example uses a two-layer copper circuit, a common concern in build- ing multilayer PCBs is that the copper surface roughness may have more impact on the cir- cuit electrical performance than the designer would anticipate. The PCB fabricator will often have a choice of what type of copper to use on certain layers within the multilayer PCB. Typ- ically, their decision is based on their desire to build a robust circuit. To optimize the elec- trical performance of a circuit, copper surface roughness also needs to be considered. For more details on this subject, please con- tact your PCB materials provider. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. 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.