Impedance represents the total opposition offered by a printed circuit board (PCB) trace to alternating current (AC) signals transmitted along its length. Impedance profoundly impacts signal quality in high-speed PCBs.
Unlike ideal wires having zero impedance, real-world PCB traces with finite dimensions positioned over reference planes demonstrate measurable impedance arising from the combined effects of resistance, inductance and capacitance distributed along the conductor’s length.
This article provides an in-depth examination of the impedance phenomenon within PCB signal traces including modeling methods, performance impacts, control techniques and frequently asked questions to clarify this critical electrical parameter for signal integrity engineers.
Origins of Impedance
Impedance constitutes the vector sum of resistive, inductive and capacitive elements extracted from Maxwell’s equations governing electromagnetic wave propagation along metallic transmission structures like PCB traces:
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Z = R + jωL + \frac{1}{jωC}
Where:
Z – Trace impedance (Ω)
R – DC trace resistance (Ω)
L – Equivalent inductance (H)
C – Equivalent capacitance (F)
ω – Signal angular frequency (rad/s)
j – Imaginary unit
The resistive factor arises from copper losses along the trace. Inductance gets contributed by current loop self-inductance, ground return paths and decoupling capacitors. Capacitance accrues from electric fields between the trace and underlying reference planes.
Impact of Impedance on Signals
Impedance profoundly impacts PCB signal quality:
Reflections – Impedance mismatches reflecting signals back to drivers causing over/undershoots
Crosstalk – Impedance differences couple signals from aggressors into victims
Ringing – Improper source/load terminations trigger resonances
Timing Margins – Mismatched impedances slow edge rates stealing timing budgets
Jitter – Variable impedance modulation introduces phase noise as jitter
Distortion – High/low impedances attenuate/amplify signal harmonics
Power Integrity – Impedance differences cause ground bounce noise on supplies
Therefore impedance control proves vital for signal integrity especially in multi-Gbps serial links.
Modeling PCB Trace Impedance
The PCB trace impedance gets modeled by an equivalent electrical transmission line with distributed resistance, inductance, conductance and capacitance along its length:
Distributed Transmission Line Model
Solving the telegrapher’s equations provides the characteristic trace impedance value:
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Z0 = \sqrt{\frac{R + jωL}{G + jωC}}
Where:
R – Conductor resistance (/length)
L – Conductor inductance (/length)
G – Conductance (/length)
C – Capacitance (/length)
Z0 – Characteristic impedance (Ω)
For typical FR-4 PCB dielectrics with low conductance:
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Z0 ≈ \sqrt{\frac{L}{C}}
Here the impedance gets dictated primarily by the trace inductance and capacitance.
The inductance relates to the magnetic flux linkages generated by current flow through the trace. The capacitance accrues from charge separation between the trace and reference planes across the dielectric.
Factors Affecting Impedance
Multiple physical and electrical factors impact the characteristic impedance value:
Trace Width – Narrower traces have higher inductance and lower capacitance increasing impedance
Trace Thickness – Thicker traces lower resistance and inductance reducing impedance
Dielectric Constant – Materials with higher dielectric constants lower capacitance
Reference Planes – Closer plane spacing increases capacitance lowering impedance
Routing Layers – Inner layers have smaller plane spacings than outer layers
Frequency – Impedance reduces at higher frequencies due to penetration depth effects
Temperature – Impedance changes +/-150ppm per °C due to thermal expansion
AC Current Density – Higher frequency current crowds towards trace edges increasing internal inductance
Surface Roughness – Rougher foil surfaces increase localized capacitance
Manufacturing Tolerances – Common ±10% impedance control proves extremely challenging
Therefore accurately modeling and physically realizing the target impedance requires considering various electrical and mechanical factors related to PCB stackup, materials, lamination process, signal frequencies and fabrication tolerances.
Methods for Controlling Impedance
Careful modeling coupled with tightly controlled PCB processes enables achieving impedance targets:
1. Define Target Impedance – Set goals based on driver/receiver capabilities (Typically 50Ω – 100Ω)
2. Select Stackup – Use plane layers suiting noise, thermal needs
3. Run Simulations – Iteratively fine-tune widths/spacing until model converges
4. Specify Tolerances – Allow margins balancing performance and manufacturability
5. Review Fab Notes – Ensure stackup, material needs get clearly communicated
6. Perform Design Analysis – Use field solvers like HFSS to model complex shape impacts
7. Validate with Measurements – Test sampled PCBs to dial-in processes for production
8. Monitor Throughout Lifecycle – Run statistical analyses determining process stability
While engineers often solely focus on modeling during design, partnering closely with fabrication to underscore accuracy needs across all stages including documentation, testing and process controls proves imperative for reliable impedance control especially for cutting-edge PCBs.
Impedance Modeling From Layout Software
Modern PCB layout tools like Altium Designer or Cadence Allegro contain extensive transmission line modeling assisting controlled impedance design:
Integrated Solvers – Quickly model selected traces to visualize impedance deviations
Interactive Tuning – Slide width/gap controls allow tuning to hit targets
Cross-section Editors – Graphically define dielectric stackup details
Bulk Parameter Import – Import material characteristics like Er, Dk, T, dielectric losses etc
Constraint Driven Routing – Automatically route maintaining width/spacing rules
Real-time DRC – Impedance rule checks highlight violations
Layer Transitions – Model vias/pads assessing discontinuity impacts
Export Results – Transfer electrical reports to verify with external tools
Impedance Modeling Using PCB Layout EDA Software
Leveraging such integration during design improves confidence before fabrication while graphically assisting engineers visualize factors influencing impedance.
Dealing with Clearances
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Air Er = 1 Gap Trace
When adjoining traces approach coupling distances under 8x dielectric thickness, the lower permittivity air region increasingly influences capacitance necessitating smaller trace widths to retain target impedance since:
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Zdiff = Zo/\sqrt{εeff}
Where εeff equals the weighted average permittivity for the diffusing electric fields occupying both air and dielectric mediums:
This is counterintuitive for designers expecting thinner traces to boost impedance. Field solvers help model such differential impedance effects resulting from side-by-side routing dense boards.
Analyzing Layer Transitions
Understanding the impact of vertical discontinuities like vias proves vital for multilayer modeling:
Capacitive Steps – Conductor barrels spanning planes form parasitic capacitance
Inductive Bumps – Sudden narrowing of currents through via holes increase inductance
Resistive Drops – Barrel and landing pad interfaces add contact resistance
Reflection Coefficients – Impedance differences at transitions reflect signals
Therefore when architecting controlled impedance stackups:
- Use smaller drills without anti-pads to reduce capacitance
- Minimize reference plane gaps under vias lowering inductance jumps
- Eliminate stub traces before/after transitions
Modeling Layer Transitions Across Vias
Supplementing simulations with empirical measurements assessing test vehicles with representative layer transitions verifies analysis correlation before committing to fabrication.
Leveraging Wide Traces
At higher frequencies beyond 100MHz, current migrates towards trace edges due to skin effects.
This self-inductance increase gets offset by lowering trace thicknesses.
Etching thinner cores reduces cross-sectional area:
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Rac = Rdc/(1-e^-δ)
Where conductor depth δ equals skin depth.
Narrower widths then compensate for thinner vertical dimensions retaining target impedance.
Thereby counterintuitively, controlled impedance traces become skinnier at higher frequencies rather than wider.
Evaluating Materials
Dielectric constant consistency directly impacts impedance accuracy during fabrication since permittivity deviations translate into capacitance fluctuations degrading margin:
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ΔC/C = -Δεr/εr
Glass fabric laminates demonstrate better stability than resin-rich thermoset builds across temperature/humidity.
Smooth drilled holes with minimal resin smear ensure precision impedance control through consistent epsilon homogeneity eliminating air voids along the axis lacking dielectric resulting from tear-out or glass fiber pull-away.
Common Impedance Values
Different applications utilize various standard impedance values:
50Ω – High speed serial links, memory interfaces
**60-85Ω **– DDRx/LPDDRx addressing noise budgets
**75Ω **– Coaxial cable feeds, SATA
90-100Ω – PCIe/USB for digital systems
100-120Ω – Embedded passives integrating broadside-coupled transmission lines
So selecting suitable target impedances balancing tradeoffs proves essential right at project inception even before modeling stacks.
Evaluating Impedance Consistency
Small impedance deviations exceeding few percentage points distort signals in multi-Gbps links having low voltage swings.
Validating consistency requires analysis of multiple coupons:
- Test at slow and fast edges assessing frequency impacts
- Include ground vias/stitching effects
- Assess both /> vector directions
- Model multiple segments for statistical variation
This underscores accurately conveying margin needs to fabrication partners through extensive report documentation since most specification standards simply mandate impedance compliance levels under +/-10% whereas high performance computing applications demand under 2% consistency.
High Speed Material Choices
Dielectric performance directly impacts impedance control quality at multi-Gbps speeds:
FR-4 – Traditional glass weave laminate suffices below 5Gbps along with right cost points
Megtron 6 – Tight weaves enhance stability for 10Gbps signals
Nelco 6000 – Low loss up to 20Gbps meets HPC server backplanes
Polyimide – Thin flexible cores aid embedding passives construction
Liquid Crystal Polymer – Low moisture absorption sustains impedance in harsh environments
Therefore material selections balancing electrical, mechanical and budget factors proves imperative before even commencing stackup construction for controlled impedance interfaces.
Conclusion
This article provided comprehensive insights into the various electrical and physical factors contributing towards impedance in PCB signal traces along with modeling/evaluation steps necessary for achieving precise control to meet signal integrity requirements up to multi-Gbps transfers. Careful stackup construction coupled with extensive modeling validation through measurements and collaboration with reliable fabrication partners skilled in dimensional process controls proves vital for realizing impedance accuracy. As data rates scale faster amid inexorable integration, mastering impedance management increasingly differentiates cutting-edge PCB developer capabilities going forwards.
Frequently Asked Questions
How is impedance measured in PCBs?
Common methods for measuring PCB trace impedance includes using vector network analyzers (VNA) or time domain reflectometers (TDR) which transmit step waveforms along test traces having SMA/coaxial launch points while capturing returning signal reflections from impedance discontinuities thereafter mathematically deriving associated impedance values using optimization search algorithms against staple transmission line theory.
Why is impedance matching important in PCBs?
Achieving impedance matching minimizes signal reflection losses due to mismatched termination resistances across source/load ends of traces that otherwise incur ringing causing over/undershoots stealing timing/noise budgets. Perfect impedance matching eliminates discontinuities ensuring maximum power transfer for improved signal integrity especially vital for low-voltage multi-Gbps serial links common in contemporary computing architectures.
Can microstrip and stripline traces be used interchangeably?
No, microstrips with a single reference plane differ significantly from symmetrical striplines having upper and lower reference layers leading to differing impedance values for same widths/thickness due to substantially lower distributed shunt capacitance per unit length for an asymmetric microstrip construction since electric field lines occupy more air rather than dielectric resulting in almost 40% higher impedance thereby preventing arbitrary substitutions between both transmission line varieties.
How are irregular shape traces handled for impedance control?
Maintaining target impedance across curved or mitered traces with non-uniform widths requires extensive 3D field solvers to model complex electric/magnetic field fluxes used for shaping cross-sections along the traced path compensating for directional current density variances maintaining a homogeneous impedance profile particularly vital in high frequency links preventing reflection noise from corroding signal integrity.
Can controlled impedance layers be skipped on outer PCB layers?
Skipping controlled impedance rules on external layers interacting with components risks uncontrolled impedance discontinuities at trace ends/bends injecting reflections since surrounding air lacks referencing ground planes leading to resonance susceptibility despite enabling tighter layout spacing. Therefore extending controlled construction constraints consistently from internal signal layers up to outer pads forming component anchors proves necessary upholding signal quality.
