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Figure 6: Trace cross-section.
The top surface of the traces is further evaluated by Scanning Electron Microscopy (SEM). It can be seen in Figures 6 and 7 that there is some overlap caused by the immersion of nickel/gold onto the dielectric surface on trace edges of the copper conductor. The panel plating evaluation board showed an overlap from the trace edge to dielectric surface of 9 µm ± 2µm while the measurement of the pattern plating test vehicle showed just 4 µm ± 2µm. The primary reason for this phenomenon can be found in the treatment process of the copper foil. Both structuring processes pictured in Figures 6 and 7 use the same base material and copper foil. Nevertheless, the slightly protruding copper residues are removed more thoroughly for the pattern plating process. Consequently, the end surface overlap into the dielectric material is reduced. A main result is the consequence that this observation has to be considered for the design preparation by the PCB manufacturer, to preserve the line characteristics as well as to ensure low losses for the higher frequency domains.
Figure 7: SEM analyses.
Analysis and Measurement Results: RF-Losses
Firstly, a process comparison is made for the MS line mode for three different line lengths L. As can be seen in Figure 8, for material type A for an MS line with L = 5 mm there is basically no measureable difference between the applied fabrication processes. However, for increasing line lengths there occur proportionally larger losses as shown for L = 10 mm and L = 25 mm. At the maximum measured frequency of 110 GHz, pattern plating shows about 0.65 dB more loss for L = 25 mm. For material type B (Figure 9) the same observations can be made for L = 5 mm where there is no difference at all for the two manufacturing processes, and for L = 10 mm and L = 25 mm losses increase with L. It is interesting to note that, at the maximum frequency, the longest line shows approximately the same difference between the losses of the compared processes as has been reported for material type A. The insertion loss can be appropriately fitted in a least-square manner with with and being the factors to be determined over frequency f[2]. By fitting the insertion loss, measurement deviations are reduced and a smooth curve for the additional loss of panel plating over pattern plating can be obtained.
Figure 8: Material type A MS.
Figure 9: Material type B MS.
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