Signal Scattering from Impurities in PCBs Paul G. Huray University of South Carolina

1. Signal Scattering from Impurities in PCBsPaul G. Huray University of South Carolina First ITESO-Intel International Workshop on Signal Integrity12:00 – 12:30 AM, April 7, 2005 Guadalajara, México

2. Talk Outline • Who is participating in the project? • Importance of the project. • TDR tool development. • Preliminary Results. • Analytic Scattering Theory. • Numerical Scattering Outcomes. • Future Directions.

3. ParticipantsIndustryAcademia Intel Richard Mellitz, Columbia, SC Paul Hamilton, Hillsboro, OR Jim McCall, Hillsboro, OR Janjie Zhu, DuPont, WA University of South Carolina Paul G. Huray, Professor Yinchao Chen, Assoc. Prof. Peng Ye, PhD candidate Femi Oluwafemi, DuPont, WA

4. Importance of the project • Silicon density approximately doubles every 18 months (Moore’s law). • PWB electrical technology improvement is much slower. • PWB’s have afforded excess electrical capability since the 1970’s. • Now, GHz signal presents new signaling challenges for PWB. • PWB properties could throttle system speed improvements.

6. PCB Manufacture • PCBs are made from dielectrics that have been clad with copper foil. • They are available in different materials and thicknesses • FR4 (Flame Retardant ε=4) is a glass fiber epoxy laminate

7. Glass Cloth Samples 1080 glass 2116 glass 7628 glass

8. Copper Surface Roughness

9. Can we develop a sensitive, simple TDR Tool for Manufacturers? • Should be a simpler method than a VNA. • It is sensitive enough to show differences in board and copper types? • Can PWB manufacturers use the tool for performance analysis?

10. Pulse Application and Measurement of Transmitted Energy 50-ohm resistive Splitter / combiner TDR heads on extension cables 1250 micron Cascade probes

11. Analysis Options Pulse Height Pulse Width * Pulse Height Pulse Width @ 50% Area

12. PRELIMINARY RESULTS: Peak Analysis

13. PRELIMINARY RESULTS: Shape Analysis Input pulse Output pulses

14. PRELIMINARY RESULTS Sensitivity Analysis Differentiates di-electric material and rough vs smooth copper.

15. PRELIMINARY RESULTS: pulse amplitude response tracks S21 Conclusion: Normal TDR with superposition can measure PWB line loss

16. PRELIMINARY Derivative Peak Analysis

17. Analytic Theory Steps • An external pulse at z=0 on a microstrip waveguide leads to a Magnetic Vector Potential, , in a volume of homogeneous FR4 that can be calculated by Green’s Theorem. • The Magnetic Vector Potential yields ~ TEMz electric field intensity, Einc, and magnetic field intensity, Hinc, in homogeneous FR4. • An inclusion (bubble or fiberglass cylinder) in FR4 provides a scattering center for incident Einc and Hincfields. • A conducting hemisphere on the surface of a microstrip trace provides another type of scattering center for incident fields. • Scattered fields lead to a redistribution of the current density in the microstrip trace and in the ground plane. • Use multiple scattering centers of various radii (absence of FR4, conducting hemispheres) to model manufactured PWB traces with statistical distribution of bubbles, fiberglass cylinders and rough surfaces.

18. Dimensions

19. Variables of FR4 inclusion model

20. Orthogonal View of inclusion Model

21. Variables of surface hemisphere

22. Orthogonal View of surface hemisphere

23. Step 1: Calculate Az(x,t)

24. Step 2: Calculate Einc(x,t) and Hinc (x,t)

25. Step 3: Calculate Esc(x,t) from a spherical inclusion Center is a sphere of radius a that produces absence of FR4. Center may absorb and scatter external fields. Fields are outgoing waves at infinity that may be expanded as:

26. Scattering Parameters • Coefficients α±(l) and β±(l) are determined by the boundary conditions at r=a. • If the spherical surface impedance isZs, Etan=Zs aRxH and

27. Cross Sections

28. Step 4: Calculate Hsc(x,t) from a spherical scattering center Equations are the same as Step 3 with Boundary Conditions at r=a:

29. Step 5: Calculate Jz(x,t) due to scattering from centers Scattered fields lead to a redistribution of the current density in the microstrip trace and in the ground plane. For the microstrip:

30. Step 6: Calculate for Multiple scattering centers • Evaluate Jz(x,t) for a variety of scattering radii (volume bubbles and surface hemispheres) • Evaluate the effect of off-center spheres • Evaluate the effect of a random distribution of volume bubbles and surface hemispheres

31. Initial Numerical CFDTD with PCB impurities

32. Time domain field distribution

33. Time domain current distribution

34. Initial CFDTD model with PCB impurity

35. Time domain field distribution

36. Time Domain current distribution

37. Comparison of field distributions

38. Comparison of current distributions

39. Comparison of field distributions Without impurity With air bubble

40. Comparison of field distributions With dielectric bubble εr=10 With PEC bubble

41. Compare model predictions, numerical outcomes with measured output • Validate model by comparing measured outputs and numerical outcomes with “manufactured” spherical inclusion samples. • Validate the model by comparing measured outputs and numerical outcomes with “manufactured” rough surface samples. • Refine the model to compensate for irregular shaped inclusions or trace surface features.

42. Customize TDR input pulses for differential “measurements” • Determine if a choice of input pulses can differentiate scattering from volume sphere inclusions and surface conducting hemispheres. • Plan regimen of input pulses for unknown samples to “measure” distribution of FR4 inclusions and microstrip trace roughness.