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Design of PCB and MCM for high speed digital systems

Design of PCB and MCM for high speed digital systems. The art of compromise. Torstein Gleditsch SINTEF Electronics and Cybernetics . What this course is and what it is not.

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Design of PCB and MCM for high speed digital systems

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  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

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