Surface Topography of Copper in High Frequency RF applications
The irregularity of a machined, rolled, or electrodeposited metal surface is the result of the manufacturing process, including the choice of tools or deposition method, machine geometry, and environmental conditions. When examined under high magnification, what seems to be a smooth surface can actually be quite irregular, with peaks and valleys. These peaks and valleys can be measured and used to define the condition and sometimes the performance of the surface. There are more than 100 ways to measure a surface and analyze the results, but the most common measurement of the surface texture is the roughness measurement.
In North America, the most common parameter for surface texture is Average Roughness (Ra). Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.
In Europe, and in the electronic copper foils market, the more common parameter for roughness is Mean Roughness depth (Rz). Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore extremes have a much greater influence on the final value. Over the years the method of calculating Rz has changed, but the symbol Rz has not. As a result, there are three different Rz calculations still in use.
Depending on the manufacturing method of a given copper laminate, the foil can be graded depending on this Rz Mean Roughness depth value. Standard HTE foils can look quite rough under an SEM (Scanning electron microscope), as evidenced in the picture above. One can easily see how a signal can degrade as it travels over this uneven surface, or how a signal detection at 10Ghz or above can be missed.
The reason is the Skin Effect. The Skin Effect is the tendency of an alternating
current to become distributed within the conductor its traveling along, such that the current density is largest near the surface, and decreases with greater depths
in the conductor. The higher the frequency, the greater the tendency for current to take the path of low inductance on the outer surface of the conductor. The skin depth is given by the following formula below where the skin depth in microns is calculated.
δ = SQRT(2/(2(π) f µ σ)
f is the frequency in MHz, and as it increases, the skin depth (δ) decreases. By the time 15GHz is reached, the outermost portion of the conductor is utilized by the current, and as a result, the path becomes uneven and can result in increased resistance, as well as signal degradation and speed loss.
In order to accommodate this need, the circuit board and RF laminate industry has developed foils with finer grain sizes, allowing for unparalleled planarity at this level. Foils such as VLP, VLP-2, and VSP are all the buzzword in the RF materials arena. They also come with a hefty additional price tag, and must be specified. I know of quite a few design engineers trying to work in the 10-15GHz range who will order expensive Teflon based materials without assuring VLP foil is used.
Additionally, new foils have been developed with even smaller Rz for both higher RF frequencies, as well as higher speed signals for PCBs. HVLP and ULP coppers are available at a premium.
The easiest way to assure you will get the best quality foil without having to spell it out is to order material with thinner copper. Standard 2 oz., 1 oz., and 1/2 oz. copper clad materials and foils all have the same basic HTE topography, but 1/4 oz. and 3/8oz. copper foils and substrates do not. As a result of manufacturing a thinner foil, the bonus is automatically a smaller grain size and increased planarity at the micron level. If you are currently experimenting with different designs, you may want to consider these thinner foils. (This is not as good of an idea is you are expecting to require heat dissipation…)