Characterization of PCB Material & Manufacturing Technology for High-Frequency

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The third transmission mode is SIW. The two investigated substrates have different values of Dk, however, for a comparison of the fabrication processes the same dimensions have been used for both materials. Therefore, the cutoff frequency varies slightly according to the difference of Dk. The measurement results for material type A are presented in Figure 14. For frequencies close to f_cutoff there is a slight difference noticeable in between the two manufacturing processes, nonetheless, this difference tends to be negligible as frequency increases. The losses for different SIW lengths of 2.4 mm, 4.8 mm, 7.2 mm and 9.6 mm are 0.34 dB, 0.73 dB, 1.06 dB, and 1.43 dB at 85 GHz, respectively. It can be concluded that there is a linear increase of the losses over L with an average of 0.358 dB for 2.4 mm SIW length. This value is valid from 65 GHz up to 100 GHz as losses are approximately constant for this frequency range. As the Dk of material type B is slightly larger than for material type A a small shift of f_cutoff can be noticed in Figure 15 compared with Figure 14. Material type B has almost overlapping results for both fabrication processes even for frequencies close to f_cutoff. Again, a close to linear increase of loss can be observed for line lengths varying in between 2.4 mm and 9.6 mm with an average of 0.53 dB for 2.4 mm SIW length at 85 GHz. According to measurements, this value is valid from 60 GHz up to 90 GHz.

Figure 14: Material type A SIW.

Figure 15: Material type B SIW.

Conclusion and Outlook

Structuring processes show different deviations of line width and trapezoidal degree of the cross-section, which also impairs the applied immersion nickel/gold overplating. In a comparison of RF-losses, the fabrication processes have demonstrated certain characteristics for the investigated transmission modes, MS, CBCPW, and SIW. MS has shown that for line lengths up to 5 mm there is no observable difference in terms of manufacturing process. Nonetheless, for longer lines a degradation of panel plating over pattern plating occurs, which increases quite linearly for the investigated materials. CBCPW exhibits slightly larger overall losses as MS. Due to the measurement results, the frequency dependence has shown a quite divergent behavior, as for low frequencies there is already a noticeable loss term present, however, with less overall increase over frequency than for MS. This behavior has been observed for both investigated materials. The SIW shows independent losses regarding the manufacturing process. For SIW, the losses can be assumed to be constant for frequencies above 65 GHz up to 100 GHz, for example, for material type A.

In a future work, these described observations will be extended to investigate the impact of different interconnection methods and top-end surface platings. Additionally, for the CBCPW further designs will be made and measurements will be carried out for different gap widths. This is done to try to evaluate when the discussed change of loss difference starts to emerge compared to the MS line structure.

References

1. B. Rosas, “Optimizing Test Boards for 50 GHz End-Launch Connectors: Grounded Coplanar Launches and Through Lines on 30 mil Rogers 4350 with Comparison to Microstrip,” Southwest Microwave Inc., Tempe, Arizona, 2007.

2. D. M. Pozar, “Microwave Engineering,” 3rd edition, Wiley, 2005.

3. J. Coonrod, B. Rautio, “Comparing Microstrip and CPW Performance,” Microwave Journal, July 2012.

 

Editor's Note: This article was originally published in the 2015 IPC Apex/Expo proceedings.

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