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26 The PCB Design Magazine • February 2015 materials exhibit surface irregularities. If the av- erage incline of the "teeth" of the conducting surface is 60 o , forming an equilateral triangle, then the increase in surface resistance would be approximately twice that of a dead flat surface. Less severe geometries induce smaller effects. However, a better measure of the surface rough- ness is the root-mean-square (RMS) height (h RMS ) of the surface bumps, since the exact pro- file cannot be established for a working design. Taking the worst case equilateral ridge geometry as a guide, one would expect the RMS rough- ness to be in the order of 0.29 times the average peak to valley roughness. The onset frequency (ω) of the RMS surface roughness effect is given by: where is the magnetic permeability (4πx10 - 7 H/m) and is the copper conductivity (5.98x10 7 S/m) and h RMS for ED copper. Therefore, inserting the numbers into the above equation, the onset frequency (ω) of the RMS surface roughness effect is 1.28GHz for ED copper. The smoother the copper surface, the higher the frequency at which surface rough- ness takes effect. Also, the effective dielectric constant and hence capacitance is increased due to the in- creased surface area of the teeth of the rough conductor. This, in turn, lowers the characteris- tic impedance slightly. The capacitance increas- es by about 5% for microstrip and 10% for strip- line traces that exhibit more roughness. So the effect on impedance is -2% to -5%, respectively. Although the surface roughness is not gen- erally stated in material manufacturer's data- sheets, typical values are 3 µm for the outer surface and 6 µm for the inner surface. For ex- ample, Panasonic's high-performance laminate, Megtron 6 core material, has a dielectric con- stant (Dk) of 3.4 and loss tangent (Df) of 0.004 at 18GHz. This material is available in standard ED, VLP, SVLP and HVLP (very/super/hyper low profile) foil with average surface roughness of 7–8, 3–4 and 1.5–2 µm respectively. Isola uses mostly VLP type coppers for their HSD product offerings. The RTF coppers that Isola offers have roughness values of 4–8 mi- crons. This is standard for FR408HR and IS415 products. VLP2 (Isola's designation) copper is incorporated in high-end products that require the best insertion loss. The roughness for this copper is 2 microns. So, the smoothest surface HVLP material with 1.5 µm average surface roughness, will therefore not exhibit any noticeable surface roughness effect up to 20GHz. Consequently, one would be well advised to take note of the surface roughness properties if your design is running over 1GHz to ensure you choose an ap- propriate material for the application. Points to Remember • At speeds below 1GHz, the effect of cop- per surface roughness on dielectric loss in negligible. • As frequency increases, the skin effect re- gion drives the current into the surface of the copper dramatically increasing loss. • When the copper surface is rough, the ef- fective conductor length extends as cur- rent follows along the contours of the sur- face up and down with the topography of the copper surface. • Smooth surfaces considerably increase the possibility of delamination during the thermal stress of the PCB fabrication and assembly processes. • Skin effect is the tendency of an alternat- ing current to become distributed within a conductor such that the current density is largest near the surface. • Skin depth decreases with increased fre- quency. • The nickel content of ENIG surface finish has a ferromagnetic property that can ad- versely affect electromagnetic fields in the high frequency domain. • At approximately 2.7GHz, the resonant be- havior of the nickel component in ENIG increases insertion loss. It is for this reason, that SMOBC processing should be consid- ered for all high-speed designs. • Most substrates are copper clad with either RA, ED, or RTF copper. • To the naked eye, copper clad laminate ap- EFFECTS OF SURFACE ROUgHnESS On HIgH-SPEED PCBS continues beyond design