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Suspended Substrates and Their Applications in High Speed Communications. By Ron Miller,   Consultant, Signal Integrity Design, GHz Data, Newark California. Suspended Substrate(SS) embedded in PCB’s. Sketch – End-on-trace Signal Problems at Ghz Data Rates. Performance improvements using SS

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Suspended Substrates and Their Applications in High Speed Communications

By Ron Miller,   Consultant, Signal Integrity Design, GHz Data, Newark California


Suspended Substrate(SS) embedded in PCB’s

  • Sketch – End-on-trace
  • Signal Problems at Ghz Data Rates.
  • Performance improvements using SS
  • Historical SS
  • Manufacturing SS PCB’s
  • Improved Tolerances with SS
  • Impedance Control
  • Cost Savings
  • Industry Applications

Description of SS

  • See Sketch of End-on trace
  • A thin laminate with a single trace on the top side.
  • Channels are milled in a bottom plate
  • The dielectric under the trace is mostly air
  • The laminate is like a bridge supporting the trace.
  • The channel is like a valley that the bridge crosses
  • A top plate with a Mirror-Image channel covers the bridge.
  • The trace is like a coax with a length-wise dielectric supporting the center conductor

Frequent Asked Questions

  • Does the Layers of dielectric insulate ground layers from each other?
  • No. At higher frequencies the thin dielectric creates a capacitance that ties them together.
  • How are vias made if the dielectric around the trace is air?
  • Dielectric material is pressed in at the vias to support the trace, pad and via.
  • What is the effective dielectric constant?
  • Er ~1.2

Numbers and Formulas

  • Dielectric FR4 SS
  • Dielectric Constant (Er) 4 1.1
  • Velocity =C/Sqrt(Er) C/2 C
  • Impedance K/Sqrt(Er) ~50 70
  • Constant W/H
  • Tand .04 .000
  • Tand vs freq varies .000
  • Dielectric Constant vs Freq Varies .000

Signal problems at Ghz Data Rates

  • High Frequency attenuation of PCB traces.
  • Degradation of state transitions (dispersion of signal edge)
      • From attenuation of high frequency components
      • The dielectric loss tangent
      • Frequency dependency of loss tangent
      • Skin effect
      • From group delay (phase shift) versus frequency
      • Frequency dependency of dielectric constant
      • Frequency dependency of impedance variations (stubs etc.)

Signal problems at Ghz data rates

  • Delay of path is about double the speed of light delay
        • From the dielectric constant.
  • Reflections from changes in impedance in the path.
        • Impedance variations.
        • Frequency dependence of stubs.
  • Multiple reflections from several reflective points in path.
  • Susceptibility to external and on-board error sources.
        • Crosstalk
        • EMI
  • Eye pattern amplitude distortion and closure.
  • Eye pattern jitter moves sampling point.
  • Data errors.

Performance improvements using SS

  • The dielectric constant is 1.
  • The dielectric loss tangent is 0.
  • Cross-talk and EMI is eliminated.
  • No frequency dependency of dielectric.
  • No manufacturing Variations in dielectric.
  • Delay is only 10 percent greater than speed of light.
  • Dielectric thickness tolerances are greatly reduced because
  • very accurate rolled metal sets the thickness of the air.

Historical Suspended Substrate(SS)

  • Suspended Substrate has been used in RF and Microwave
  • applications.
  • A thin substrate layer was sandwiched between the top and
  • bottom conductive plates, with channels milled into these
  • planes above and below the trace
  • The trace was supported by the substrate layer enclosed in
  • the top and bottom channels.
  • Mechanically the substrate layer acts like a bridge carrying
  • the trace through the channel
  • The channel depth sets the air thickness and the impedance
  • The structure is assembled using nuts and bolts.
  • An end-on view of a trace resembles a coaxial structure.

Manufacturing Suspended Substrate PCB’s

  • Fab of Spacer Layer
  • Milling
  • Etching
  • Stamping
  • Drawing
  • Suspended Substrate Vias
  • Drill oversized holes in the metal planes
  • Fill the oversized holes with dielectric – squeegee
  • Laminate and process as usual.
  • PCB’s may combine Standard and Suspended Substrate
  • layers

Improved Manufacturing Tolerances 1

  • Better impedance control than standard PCB
  • SS depth tolerance is set by metal thickness and is
  • accurate.
  • FR4 dielectric thickness varies because of soft material.
  • The dielectric constant of air is constant and does not vary,
  • compared with standard materials which vary as much a
  • 10%.
  • The use of air dielectric provides a very low dielectric loss
  • factor for high frequency, microwave and high speed
  • digital signals up into the gigahertz and gigabit
  • frequencies, compared to standard dielectric materials.

Improved Manufacturing Tolerances 2

  • The use of air dielectric provides the shortest time delay or
  • the fastest transition time for a given trace length,
  • compared to standard dielectric materials. Epoxy
  • fiberglass material has a delay of approximately 2 X the
  • free space velocity of light while this application of SS is
  • approximately the free space velocity of light.
  • Air dielectric also minimizes the dispersion of the
  • transition of the signal from one voltage to another caused
  • by frequency dependent dielectric losses and phase shift
  • which are not present in air.

Improved Manufacturing Tolerances 3

  • The use of air dielectric increases the trace impedance for
  • traces with the same width to height ratio by a factor of
  • approximately 2. Alternatively stated, for a given
  • impedance and trace width, the height may be reduced by
  • a factor of approximately 2.
  • Where a data-bus or non-synchronous signals share the
  • same channel, the cross-talk from signal to signal within
  • the same channel can be reduced by the use of an air
  • dielectric and by reducing the height spacing of the trace
  • to the metal plate compared to the cross-talk of a strip-line
  • transmission line with the same impedance and the same
  • spacing of traces.

Impedance Control

  • Impedance control for SS is very Accurate
  • From layer to layer
  • From piece to piece
  • From batch to batch,
  • From location to location(X and Y) on the same laminate,
  • in dielectric constant and loss factor
  • Impedance Comparison
  • Standard Impedance of 5 % vs 10% for FR4
  • Controlled Impedance of 2 % vs 7 percent for FR4.

Cost Savings

  • The cost of impedance Control is much less
  • No costly tight tolerance material needed.
  • No costly special steps or handling needed.

Industry Applications

  • ATE Loadboards
  • Personal Computers
  • High Speed serial busses
  • Backplanes and Parallel data busses

ATE Loadboards

  • ATE SS Load-boards and Test-boards saving tester time.
  • Test Time for Semiconductors is limited by the signal
  • delay in the test board.
  • SS can cut the overall test time by up to 50 %.
  • Half as many testers are needed for a given throughput.
  • Testers cost between 1 and 5 million dollars

Personal Computers High-Speed Performance

(Bench marks limited by)

  • Internal speed of the microprocessor.
  • Speed of the PCI bus (R/W storage and memory)
  • SS doubles PCI bus speed
  • Cross-talk between traces of the data bus.

High-Speed Serial Signals

  • (Fibre Channel, GB Ethernet, Infiniband etc.)
  • PCB attenuation and dispersion limit speeds to 4 GBS
  • SS can extend the speed to 10 GBS
  • SS allows longer bus lengths.

Backplanes and Parallel busses

(Mother-board and Back-plane parallel busses)

  • SS can eliminate the need for trace compensation
  • Busses can be longer and faster.
  • Crosstalk and external EMI are greatly reduced

Summary of (SS) in PCB’s

  • Reduces High Frequency Attenuation
  • Improves Dispersion of Signal Edge
  • Reduces Path Delay
  • Reduces EMI and Crosstalk
  • Reduces Eye Closure(amplitude)
  • Reduces Jitter
  • Reduces Data Errors