Design of PCB and MCM for high speed digital systems

1. Design of PCB and MCM for high speed digital systems The art of compromise Torstein Gleditsch SINTEF Electronics and Cybernetics

2. What this course is and what it is not • This course is an introductory course to designing Printed Circuit Boards and Multi Chip Modules for high frequency digital applications. • It will introduce you to the basic concepts of transmission lines and how utilize this theory into practical designs. • It will introduce you to modeling of lines, drivers and receivers to help you get a better understanding of the effects of the different measures. • It will not make you a proficient SPICE user. • At last it will make you understand why high speed digital design is difficult and how to improve a design.

3. What is so special with high-speed digital • Broad band • Must cover every frequency from DC to GHz • No “Dirty tricks” possible • Higher order harmonics is necessary for edge integrity. • High number of critical signals • Digital signals is more tolerant to distortion • Switching create high currents in short periods

4. A digital signal • Frequency 50MHz • Risetime 1ns ( 10% - 90% ) • Falltime 1ns ( 10% - 90% ) • Bandwidth 350MHz

5. Spectrum of a single pulse 10 ns risetime 1 ns risetime

6. Spectrum of a pulse train

7. The effect of missing harmonics 1st harmonic only 1st to 5th harmonic 1st and 3rd harmonic only 1st to 21st harmonic

8. Why transmission line calculations • Transmission line theory describe the behavior of the signals on a PCB or MCM • Simple models like RC-calculation do not apply at higher frequencies • It is possible to predict the quality of the signal. • It is possible to experiment with different types of termination.

9. When do I have to take into account transmission lines effects • Thumb rule: When the rise time is less 2.5 times the time delay of the signal on the trace. (Transit time) • Another: If the trace length is larger than 1/7 of the largest wavelength of the signal. (At upper frequency edge) • Example with two different effective dielectric constants Trace length (mm) Trace length (mm) Risetime (ns) Risetime (ns)

10. Reflections from a 1ns risetime signal 40 mm non terminated line 300 mm non terminated line

11. Crosstalk 40 mm non terminated line 300 mm non terminated line

12. Transmission Lines A brief introduction

13. Basic Transmission Line Types Microstrip W t H Stripline er tand W H t

14. Dielectric constant (Relative permittivity) (er ) • Describe a materials ability to hold charge compared to vacuum when used in a capacitor. • The permittivity of a vacuum is: e0 = 8.85419 ·10-10 • The permittivity of a material is: e= e0 er • If we put a dielectric material between two capacitor plates of area A and a distance D the capacitance in Farad is:  A [F] C = D

15. Loss tangent ( tand ) • Describe the materials “resistance” to change of polarization • s is the conductivity of the dielectric at frequency . (S/m) • e is the permittivity of the material. (F/m)  tan  = 2 f  

16. Magnetic permeability (m) The magnetic permeability describe a magnetic property of a material. It is the ratio between the magnetic flux density (B) and the external field strength (H). B [H/m]  = H The relative permeability of a material is the permeability relative to vacuum. µ = µ µ 0 r  4 µ = 0 7 10

17. Properties of glass reinforced materials

18. Basic transmission line diagram Transmission line Driver Termination resistor

19. Some Transmission Line Properties • Signal velocity (m/s) • Influenced by dielectric constant of materials • Impedance • Mostly influenced by dielectric constant of material, width of line and distance to ground plane(s) • Loss • Influenced by conductivity of conducting material, frequency of signal and loss tangent of the material. • Dispersion • Influenced by frequency dependant dielectric constant

20. The four describing parameters • R Resistance per unit length [ohm / m] • C Capacitance [ F / m ] • L Inductance [ H / m] • G Conductance [ S / m ]

21. Signal velocity (  ) • The signal velocity  is measured in m/s • For a practical calculations the signal velocity is only dependant on the relative dielectric constant er • With c0 the velocity of light in vacuum, we get: c 0 v [m/s] =  e r

22. Impedance ( Z0 ) • Impedance is measured in Ohms • Impedance describes the AC resistance a driver will see when driving a signal into a indefinitely long transmission line.  R L j +  [ohm] Z = 0 G C j +  For a loss less line this simplifies to:  L [ohm] Z = 0 C

23. Loss ( a ) • Loss a is measured in dB / m • Loss is the reduction of signal voltage along a line due to resistance and leakage • The resistive losses is due to resistance in conductor and ground plane. • The dielectric losses is due to the energy needed to change polarization of the dielectric material. • The radiation losses is the energy sent from the conductor acting as an antenna.

24. Resistive losses ( aC ) The resistance is based on the cross-section of the conductor and the bulk resistivity of the conductor material: s = bulk conductivity [S/m] w = line width [m] t = line thickness [m] 1 [ohm] R = w t  The loss factor is then calculated by: 8,68589 R [dB/m]  = 2 Z 0

25. Skin depth ( ds ) At higher frequencies only a thin layer of the conductor transport the current. The thickness where the current density is reduced to 1/e is called the skin depth ds.  1 [m] d = s f   µ

26. Resistance at higher frequencies When the conductor is much thicker than the skin depth one have to substitute the skin depth for the thickness in the resistance formula: 1 = R [ohm]  d w s 1 = R  1  w   µ f The resistance has now become frequency dependent !

27. Dielectric loss ( aD ) The dielectric loss aD is due to the energy needed to change the polarization of the dielectric material. The conductance is then: [S/m] =   G 2 f C tan The dielectric loss is then: 1  = 8,68589 G Z [dB/m] 0 D 2

28. Radiation loss • All lines are radiating more or less • Radiation loss is often negligible from a signal integrity standpoint but important from a EMC standpoint. • Radiation loss is difficult to calculate

29. Dispersion • Dispersion is an effect caused by different velocities for the different frequency components. The result is a different phase change for each of the frequency components. • The reason for this is that the dielectric constant of the material vary with frequency. • Dispersion is one of many sources of signal distortion. • It is not easy to calculate dispersion because the lack of frequency dependant material data. • Dispersion may cause trouble when exact edge position is important.

30. Crosstalk • Crosstalk is coupling of a signal form one line to another • There are two key parameters used to describe crosstalk • Kf the forward crosstalk coefficient • Kb the backward crosstalk coefficient (normally the largest) • Td is the transit time delay ( ) 1 L m = - - K C Z 0 m f 2 Z 0 L m - C Z 0 m Z 0 = - K b 4 T d

31. Reflections In this case Z1 = R2 = 25 ohm, and Z0 = 50 ohm. Then r = -0.333 => -1.66 overshoot

32. Via modeling • Inductance and capacitance of via’s is difficult to calculate without 3D field analysis tools. • A 1nH inductance and a 1pF capacitance can be used as a start • Careful use of coaxial line equations can also give an indication of the values.

33. Building a good board Some hints for the high performance designer

34. What is important for a good board • Symmetrical around center • Tolerant for etch variations (+/- 1 mil is not unusual) • Lines spaced for acceptable crosstalk • Standard dielectric thickness’ • Tolerant for variation in dielectric thickness • Acceptable high loss. (The loss may be your friend) • Acceptable total thickness • Power and ground layers close together (typically 100um)

35. Laminate tolerances Dictionary: Toleranse = Tolerance Tykkelse = Thickness Klasse = Class Glassvevtype = Glass fabric type

36. Prepreg Tolerances

37. Impedance tolerance to line width As line width increases, dependence on absolute tolerance decreases Border lines on +-1mils absolute line width tolerance

38. Impedance tolerance to laminate tolerance

39. Minimum line widths vs. er and dielectric thickness Curves on equivalent dielectric thickness

40. 8 layer high-speed lay-up example 1 Dictionary: Kobber = Copper rent = pure

41. 8 layer high-speed lay-up example 2

42. Pack32 A desktop calculator for transmission lines

43. Idea behind the program • Easy to use • PC based ( Windows 95 or NT ) • Good enough results (better than textbook formulas) • Easy to organize material, component and lay-ups • Easy for the developers to add new functions. • Data is stored one place, no tedious typing • Fast, no calculation take more than one second

44. Workflow for transmission line analysis 1. Enter material data if not in database 2. Define a Lay-up 3. Calculate the line properties for a single line 4. Adjust Lay-up and re-calculate until satisfied 5. Calculate for double lines to check for crosstalk and dual line impedance. 6. Generate a SPICE model for a critical net in the design, single or double lines as appropriate 7. Simulate and adjust line parameters (width, spacing, lay-up) 8. Use the obtained parameters as design rules for your critical nets.

45. Material database Sorry! You have not got the English version! • For transmission line calculation, • one need: • Electrical conductivity • Dielectric constant • Dielectric loss factor • Magnetic permeability

46. Lay-up definition Sorry! Norwegian text!

47. Single line analysis Stripline Microstrip Buried microstrip

48. Dual Line Analysis

49. Exercise Design a lay-up with these properties: • 50 Ohms impedance • FR-4 Dielectric • No Solder resist • 4 Signal layers • Two Power layers • Two Ground layers • No more than 3dB loss for a 10cm line at 1GHz • Tolerant for +/- 1 mil etch error.

50. Analog simulation of digital signals Time for sacrifice