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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.)
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