Differential Impedance

1. Differential Impedance Effect of Etch Taper, Prepreg, and Resin Flow on the Value of the Differential Impedance Dr.Alan Staniforth, Martyn Gaudion Ken Taylor Polar Instruments

2. Discrepancies • Software accuracy • Closed form equations • Empirical data tables • Field Solver • Structure Cross Section • Track profiles • Dimensions • Predicted • Microsection measurements • Dielectric Value • Non-homogenous • Bulk specification

3. Closed Form Equations • Used very successfully until 3 ~ 5 years ago • Valuable Approximations • Various degrees of complexity • See IPC-2141, Wadell, Cohn, Hilberg, others • Good for larger track dimensions • > 20 mils • Relatively few, simpler structures • Unsuited for differential structures • Can be performed on a typical scientific calculator

4. Field Solver • Highly complex mathematical modeling • Accurately predicts E-field vectors • Accounts for track cross sectional profile • Charge distribution • Method of Moments • Green’s Function •  < 0.5%  < 2% • No practical lower dimension limit . . . yet • Requires PC

5. Field Solver • Assumes regular trapezoidal track cross section • Equal etch tapers • Assumes track identical pair • Requires good fab process • Reduces calculation resource

6. Track Assumptions

7. Charge Distribution and Track Profile

8. Field varies with Track Profile

9. Impedance Varies with Etch Taper

10. 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

11. Typical E-field distribution Embedded Microstrip er

12. FR4 structure C 5 mil P P P C

13. FR4 structure

14. FR4 structure 3.1 4.2 3.1

15. Field distribution Impedance value increases

16. FR4 structure 3.1 Impedance Value Increases 4.2 3.1

17. Resin Layer in Differential Microstrip

18. Resin Layer in Differential Stripline

19. Conclusions • Simplified modeling of differential structures leads to impedance discrepancies of several ohms • A field solver is required which takes into account • Structure • Track dimensions • Thickness • Widths • Spacing • Etch taper • Symmetry • Dielectric layer composition and er values

20. Conclusions • Resin flow into region coplanar with tracks increases the impedance of typical • embedded microstrip by 3.5 ~ 4 ohms • embedded stripline by 3 ~ 3.5 ohms

21. Conclusions • Accurate implementation of predicted impedance requires • Accurate production 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 • Accurate knowledge of dielectric composition • Dielectric layers • Resin region and component er value

22. Thank You • Questions now?…. • Questions later? • Americas: 800 328-0817 ken.taylor@polarinstruments.com richard.smith@polarinstruments.com • Europe: martyn.gaudion@polarinstruments.com • Asia: amit.bhardwaj@polarinstruments.com