Via and Return Path Discontinuity Impact on High Speed Digital Signal

1. Via and Return Path Discontinuity Impact on High Speed Digital Signal Qinglun Chen, Intel WPD Jin Zhao, Sigrity Inc. Contributor: Weimin Shi, Intel DPG, Raymond Chen (Sigrity) Chi-te Chen Intel WPD

2. Agenda • The motivation of this study • How via impact on high speed digital signal • How Signal return path discontinuity impact on high speed digital signal • Correlation and measurements • Summary

3. Motivations of This Study • Rambus needs less than 5 ps skew accuracy and +/- 125 ps skew budget. What are the major skew contributors for Rambus channel interconnection? • Via discontinuity and coupling • Return path discontinuity • Manufacturing variation • Impedance discontinuity • Buffer Strength difference • Cross talking ---even and odd modes • Load variation

4. Via Discontinuity How via length impact the signal wave form and delay? How via coupling impact the signal wave form and delay? How termination via impact the signal wave form? Test Board for Rambus Study

5. How via length impact on high Speed digital signal quality Signal trace Via GND 1 GND 2 3 4 Via Inductance depending on its length -> impedance either

6. Waveform and Delay Comparison

7. How via coupling impact on high Speed digital signal quality Signal trace GND GND Via even coupling  increase via inductance  increase impedance Via odd coupling  decrease via inductance  decrease impedance

8. Via Coupling impact on waveform and Delay

9. 35 Ohm 28 Ohm Vcc ~ Gnd Pwr 35 Ohm 28 Ohm Vcc ~ Gnd Pwr Figure 6. Configuration of Rambus signal net How termination via impact the signal wave form?

11. PCB Signal Return Path Discontinuity How does a power reference plane impact the signal wave form and delay? How does via return path impact the signal wave form and delay? What is the difference between single line vs multiple line switching for RPD? How do different layer traces impact the signal wave form and delay?

12. How a power reference plane impact the signal wave form and delay? Signal trace Via GND VCC • AC path through inductance and capacitance • Local capacitance is frequency dependent • Current have to flow to the ground plane through local capacitance and induce imaging current • Referring a power plane will create return path discontinuity

13. Return Path Discontinuity & Via Coupling impact on waveform and Delay

14. What is the difference between single line vs multiple line switching for RPD? At Receivers: 1 line switching on S1 1 line switching on s6 8 line switching on s6

15. How via return path impact on high Speed digital signal quality Signal trace GND VCC Via GND GND Delay order: near ground via, far ground via, no ground via

16. How Ground Via impact on Skew and Waveform

17. Measurement Results for Four layers with 3 Line coupling

18. OR840 WS Board Benefits From Fundamental Studies With Speed97 More than 4000 outrigger system boards are shipped per week Fundamental studies: 1. Return path; 2. Via coupling; 3. Die-to-Termination channel analysis Outrigger Board Stack-up: Rambus signals S1 GND S3 VCC This is a very competitive product: Rambus IS on 4 Layer stack-up That is why IBM has ordered a big volume

19. Correlation

20. Test Board For Correlation(1998) Propagation delay test pattern for manufacturing variation Via coupling test pattern Via characterization Pattern Power plane as ground return test pattern Via characterization pattern

21. Via Coupling Pattern S1 GND S3 S4 VCC S6 62 mils 400 mils Via 50 ohm Trace 100 mils

22. Test Board Measurement of Via Even Mode Coupling Red Line --- no coupling, Blue Line -----Odd coupling

23. Test Board Measurement of Via Odd Mode Coupling Red Line --- no coupling, Blue Line -----Odd coupling

24. Via Coupling Skew Measurement and Simulation (Even/odd Modes) Via coupling skew depends on input signal skew

25. Two Series Via Patterns for Correlation 50 ohm Trace S1 GND S3 S4 VCC S6 62 mils Via 400 mils S1 GND S3 S4 VCC S6 62 mils

26. Speed97 Simulation Vs TDR MeasurementPattern 1 Red Line --- Measurement, Blue Line -----Speed97 ( done in 12/1998 at New York University at Binghamton by Weimin Shi )

27. Speed97 Simulation Vs TDR MeasurementPattern 2 Red Line --- Measurement, Blue Line -----Speed97 ( done in 12/1998 at New York University at Binghamton by Weimin Shi )

28. Summary • Via discontinuity and coupling become a major signal skew contributor for high speed digital signals • Power reference plane is another major signal skew contributor for high speed digital signals • Synchronized signals should reference a same ground plane to avoid additional signal skew • Return path discontinuity is due to non-uniform inductance and capacitance in signal paths