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SEPTEMBER 2019 I DESIGN007 MAGAZINE 89 circuits. With this in mind, it seemed prudent to show the same information in Figure 7 but using a test vehicle that is a GCPW trans- mission-line circuit. This compari- son is shown in Figure 8. Several things should be consid- ered when comparing Figures 7 and 8. Different Y-axis scales are used for insertion loss in the two graphs, and the curves for GCPW depict much greater losses than the curves for microstrip. Another item of in- terest is the differences between the insertion loss for circuits with thin nickel compared to circuits with thicker nickel. Plated thickness variations for immersion tin will have less im- pact on variations in insertion loss because the immersion tin is very thin. It is applied by means of a self-limiting process and the typical immersion tin thickness is about 47 μin. (1.2 μm). With such a thin layer, the tin does not have a sig- nificant influence on the composite copper-tin conductivity at the side- walls of the microstrip signal con- ductor until about 20 GHz. Figure 9 used the same test vehi- cle as used for Figures 7 and 8 but with different thicknesses of im- mersion tin. As is evident, the im- mersion tin thickness variation has some impact on microstrip inser- tion loss at higher frequencies, but not as much as the impact of ENIG finishes with varying thicknesses of nickel as can be seen in figure 7. The tin thickness for these circuits was 24 μin. (0.6 μm) for the thin tin plating and 79 μin. (2.0 μm) for the circuits with thick tin plating. Another study has looked at an electro- less palladium immersion gold (EPIG) plating which does not have nickel. The same test ve- hicle was used for the other finishes, and EPIG plated finish shows an improvement in inser- tion loss compared to the other finishes. How- ever, a different substrate material was used in the EPIG study, a material with a Dk of 3.2, Df of 0.0033, and copper surface roughness of 0.9 μm RMS. With this material, the bare cop- per circuit will have more losses than in previ- ous studies in which bare copper circuits have been mentioned. However, the comparison be- Figure 8: These insertion loss curves for GCPW transmission-line circuits compare circuits with bare copper conductors to circuits with ENIG finishes with different nickel thicknesses. Figure 9: The microstrip insertion loss curves compare circuits with bare copper conductors to circuits with immersion tin (ImSn) finishes with different tin thicknesses.