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Polar Instruments

Polar Instruments. Why Effective Er is not the same as Bulk Er Ken Taylor. September 2002. © Polar Instruments 2002 www.polarinstruments.com. FR4 Dielectric . Core and Prepreg have different e r Both are Mixtures of Resin and Glass Fibers Materials are non-homogenous

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Polar Instruments

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  1. Polar Instruments Why Effective Er is not the same as Bulk Er Ken Taylor September 2002 © Polar Instruments 2002 www.polarinstruments.com

  2. FR4 Dielectric • Core and Prepreg have different er • Both are Mixtures of Resin and Glass Fibers • Materials are non-homogenous • er specified for laminate is the bulk value • er for glass ~ 6.1 er for epoxy ~ 3.2 • So significant local variations occur for er

  3. Typical E-field distribution Embedded Microstrip er

  4. Microphotograph of FR4 structure Glass fibers Er = 6 Core Resin Er = 3.2 Prepreg P P 5 mil C

  5. FR4 structure Bulk Er value in this direction is 4.2 approx Er = 6.0 Er = 3.2

  6. Field in FR4 structure 3.1 4.2 3.1

  7. Field distribution in Differential Pair Impedance value Increases as Er and C decrease

  8. FR4 structure 3.1 Impedance Value Increases 4.2 3.1

  9. Resin Layer in Differential Microstrip Resin-only region Glass/Epoxy mixture

  10. Resin Layer in Differential Stripline

  11. Conclusions • Over simplified modeling of differential structures leads to impedance discrepancies of several ohms • Resin flow into region coplanar with tracks increases the impedance of typicalembedded microstrip or stripline by 4%

  12. Conclusions • Your predictions must consider • Structure • Track dimensions • Thickness • Widths • Spacing • Etch taper • Symmetry • Dielectric layer composition and Effective er values

  13. Conclusions • Accurate implementation of predicted impedance requires • Accurate knowledge of dielectric composition • Dielectric layers • Resin region and local er value • Accurate build of predicted dimensions • Track width • Track spacing • Track etch taper • Increased etch taper leads to enlarged resin region • Track symmetry • Loss of symmetry also affects differential performance

  14. Recommendation • Build samples of small differential structures • Measure impedance using CITS500s • Microsection and micro-dimension the sample • “Back” calculate to obtain the Effective Er using Si6000 • In future, use that value of Er for • similar structures • same dimension range • same materials • For small differential structures in FR4, expect Er(effective) ~ 3.7 (About 10% less than spec.)

  15. Thank You • Questions?…. ken.taylor@polarinstruments.com

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