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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

Polar Instruments

Why Effective Er is not the same as Bulk Er

Ken Taylor

September 2002

© Polar Instruments 2002

www.polarinstruments.com


Fr4 dielectric
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


Typical e field distribution
Typical E-field distribution

Embedded Microstrip

er


Microphotograph of fr4 structure
Microphotograph of FR4 structure

Glass fibers Er = 6

Core

Resin Er = 3.2

Prepreg

P

P

5 mil

C


Fr4 structure
FR4 structure

Bulk Er value in this direction is 4.2 approx

Er = 6.0

Er = 3.2



Field distribution in differential pair
Field distribution in Differential Pair

Impedance value

Increases as

Er and C decrease


Fr4 structure1
FR4 structure

3.1

Impedance Value Increases

4.2

3.1


Resin layer in differential microstrip
Resin Layer in Differential Microstrip

Resin-only region

Glass/Epoxy mixture



Conclusions
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%


Conclusions1
Conclusions

  • Your predictions must consider

    • Structure

    • Track dimensions

      • Thickness

      • Widths

      • Spacing

      • Etch taper

      • Symmetry

    • Dielectric layer composition and Effective er values


Conclusions2
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


Recommendation
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.)


Polar instruments 1336217

Thank You

  • Questions?….

    ken.taylor@polarinstruments.com