Suspended Substrates and Their Applications in High Speed Communications

By

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

• 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

Suspended Substrate(SS) embedded in PCB’s

• 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

Description of SS

• 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

Frequent Asked Questions

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

Numbers and Formulas

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

• 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

Summary of (SS) in PCB’s