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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. Discrepancies. Software accuracy Closed form equations Empirical data tables Field Solver

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Presentation Transcript
differential impedance
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

discrepancies
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
closed form equations
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
field solver
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
field solver5
Field Solver
  • Assumes regular trapezoidal track cross section
      • Equal etch tapers
  • Assumes track identical pair
      • Requires good fab process
      • Reduces calculation resource
fr4 dielectric
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
typical e field distribution
Typical E-field distribution

Embedded Microstrip

er

fr4 structure
FR4 structure

C

5 mil

P

P

P

C

fr4 structure14
FR4 structure

3.1

4.2

3.1

field distribution
Field distribution

Impedance value

increases

fr4 structure16
FR4 structure

3.1

Impedance Value Increases

4.2

3.1

conclusions
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
conclusions20
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
conclusions21
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
slide22
Thank You
  • Questions now?….
  • Questions later?
    • Americas:

800 328-0817

[email protected]

[email protected]

    • Europe:

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    • Asia:

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