Copper Surface Roughness
Figure 2: Low-Roughness High-Periodicity Copper Foil
Figure 3: High-Roughness High-Periodicity Copper Foil
Figure 4: High-Roughness Low-Periodicity Foil
The most common transmission lines on PCBs are made from copper traces. Copper is highly conductive, flexible, and strong. In RF circuits, the most common copper weights are ½ oz and 1 oz copper. This specification refers to an amount of copper per square foot. ½ oz copper equates to about 0.0007” thickness and 1 oz copper is about 0.0014”
The roughness of the copper in a transmission line and its ground will impact the amount of insertion loss in a device or system. Rougher copper makes a trace electrically longer, meaning it increases the phase of the network. Since the phase is greater and the transmission line already has a set loss per unit length based on the parameters discussed above, plus its impedance and its trace width, a longer phase will induce greater insertion loss. Surface roughness is generally specified by a metric that describes the size of the peaks and valleys. It is generally reported as Ra (roughness average), RZ (tallest peak – lowest valley), or Rq (RMS value of the sample of peaks and valleys). While these metrics do a good job of describing the magnitude of the roughness, they do not cover the periodicity of this roughness, which will play a major role in the final phase length and overall insertion loss.
For this reason, the standard metrics are good for comparing runs of the same copper type, or differences in processing of foils of the same copper type, but may fall short when comparing copper foils that were created using two different methods, such as rolled copper vs. electrodeposited. The importance of copper roughness is related to the linewidth of the signal trace. The narrower the linewidth is, the more pronounced the impact of copper roughness on phase length. So, assuming a 50-ohm characteristic impedance, a thinner substrate will require a narrower line width. For example, on a 0.0073” thick substrate Rogers Corp’s RO4350B, a standard ED copper would produce 1.7 times the insertion loss per inch that their LoPro copper would for the same 50-ohm microstrip run. This would be of enormous impact to the microwave design engineer. Looking back to some of the tradeoffs discussed earlier, this could mean 42% fewer elements in a phased array, or the same reduction in power to the LNAs. However, if the same experiment is run on 0.0173” thick laminate of the same type, the higher profile copper is 1.35 times as lossy as the LoPro, so the advantage, while still substantial, is cut in half.
There are two main types of copper used in PCB formation – electrodeposited and roller. Electrodeposited copper is formed by dissolving bulk copper in an acid and plating it onto a drum. Its grain structure is quite random looking and has tight periodicity. In contrast, rolled copper is formed by passing a large slug of copper through progressively smaller mechanical rollers, stretching it out until it is the desired thickness. The grain structure of this copper will then be fairly tight periodicity in one axis and extremely long, stretched out peaks and valleys in the other. Because of this, rolled copper acts, electrically, almost identical to the theoretical models of perfectly smooth copper. While electrodeposited copper does sound inferior to rolled copper in this comparison, it does have its merits. Its grain structure allows for better adhesion to the laminates. It also can be used with different nickel oxide resistive coatings that can be used to create etched resistors directly on the PCB. There have also been some strides made in recent years on electrodeposited copper to drive down insertion loss by making the Ra surface roughness extremely small, sub-1 micron. By making the copper extremely smooth, its effect on insertion loss is getting very close to that of rolled copper. Rolled copper is also very expensive and only available on a restricted selection of laminates to which it can achieve acceptable adhesion.
Copper roughness is driven by more than just the way in which it is formed. After the copper supplier forms the copper foils, they are adhered in a heated press to a dielectric laminate system at the laminate supply house. The foil often needs an adhesion promotion processing step before this lamination occurs, to increase the roughness to a level needed for good adhesion and to pass the laminate supplier’s peel strength step. There is also a topology to the laminate itself, which, if rigid enough, may imprint on the copper being adhered. Beyond this, at the PCB shop there are a number of processes in the fabrication of the PCB which may increase the surface roughness of the copper. Many board processes require cleaning after, which may include a scrub that can add roughness to the copper. Some steps may require a sanding operation, which can do the same. The board shop may also require an adhesion promoter to traces before the different PCB cores are laminated together with a prepreg or other adhesive. Conversely, there could be plating steps at the PCB shop, which could smooth out the copper on the exposed side to some level.