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

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
Core
Glass fibers Er = 6
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 typical embedded 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?….