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Understanding Effective Dielectric Constant vs. Bulk Dielectric Constant in FR4 Structures

This article by Ken Taylor from Polar Instruments explores the differences between effective dielectric constant (Er) and bulk Er in FR4 materials used in PCB structures. It highlights how both Core and Prepreg materials, although mixtures of resin and glass fibers, exhibit significant local variations in dielectric properties. Accurate modeling for differential structures is paramount to achieving precise impedance predictions. Recommendations for measuring and determining effective Er values for small differential structures are also provided, underscoring the importance of structure, dimensions, and material composition.

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Understanding Effective Dielectric Constant vs. Bulk Dielectric Constant in FR4 Structures

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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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