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At frequencies below 1GHz, the effect of copper surface roughness on dielectric loss is negligible. However, as frequency increases, the skin effect drives the current into the surface of the copper, dramatically increasing loss. When the copper surface is rough, the effective conductor length extends as current follows along the contours of the surface up and down with the topography of the copper surface. At high frequencies, the effective resistance of the copper increases relative to the additional distance over which the current must transverse the contours of the surface. The total loss comprises of the sum of the conductor loss and dielectric loss.
Whilst it may be possible to manufacture copper foil with a perfect mirror-smooth finish, the foil-to-resin adhesion would be compromised. This would considerably increase the possibility of delamination during the thermal stress of the PCB fabrication and assembly processes. For this reason, a reduced oxide coating is applied to the inner core layers to promote adhesion of the prepreg resin as it flows under the applied heat and pressure to cure.
Skin effect is the tendency of an alternating current to become distributed within a conductor 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 lower inductance on the outer surface of the conductor.
In a previous column,
Beyond Design: Surface Finishes for High-Speed PCBs, I pointed out that the nickel content of ENIG surface finish has a ferromagnetic property that can adversely affect electromagnetic fields in the high frequency domain. One could argue that since the nickel is plated on top of the microstrip surface, that it would have little effect on properties of the trace. And that due to the skin effect, the current will travel the path of least inductance, which is on the lower surface of the copper closest to the reference plane. However, it has been found that at approximately 2.7GHz, the resonant behavior of the nickel component in ENIG, increases insertion loss. It is for this reason that solder mask over bare copper (SMOBC) processing should be considered for all high-speed designs.
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