Suspended Substrates and Their Applications in High Speed Communications

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Suspended Substrates and Their Applications in
High Speed Communications
By Ron Miller,   Consultant, Signal Integrity
Design, GHz Data, Newark California
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Suspended Substrate(SS) embedded in PCBs

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

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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

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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

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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

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• 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.)

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• 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.

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• 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.

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• 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.

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Manufacturing Suspended Substrate PCBs

• 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.
• PCBs may combine Standard and Suspended
  Substrate
• layers

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• 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.

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• 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.
•  

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• 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.

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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.

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Cost Savings

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

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

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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
•  

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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.

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• 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.

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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

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Summary of (SS) in PCBs

• 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