Controlled impedance constitutes one of the most crucial considerations in modern high-speed printed circuit board (PCB) design. Simply, impedance represents the opposition to electrical current flow within the board. Controlled impedance tracks ensure the PCB environment presents consistent impedance to critical signals.
Maintaining proper impedance matching along transmission paths allows clean signaling critical for high frequency applications. Uncontrolled impedance introduces reflections and losses degrading signal integrity. This article details what impedance entails, why control ranks so necessary in circuits along with methods to master impedance in layouts.
What is Impedance?
In PCB design, impedance refers to the apparent resistance presented to electric current flow along a conductive trace. It encompasses the comprehensive opposition considering capacitive, inductive and resistive properties of the materials comprising the circuit path.
Impedance changes with operating frequency as reactive elements take effect. It defines the voltage / current ratio at a given frequency:
Impedance (Z) = Voltage / Current
The impedance depends on inductance (L), capacitance (C), resistance (R) inherent to the trace and surrounding environment:
Z = √(R + jωL)2 + (jωC)2
R – Sheet resistance of copper trace
L – Self-inductance along trace
C – Capacitance to reference planes
At low frequencies, resistive terms dominate. At higher frequencies, inductive and capacitive elements impose impedance. Controlling factors like dielectric constants and trace dimensions dictate the impedance response a signal encounters in a PCB channel.
Why Control Impedance?
Uncontrolled impedance causes signal reflection issues leading to overshoot, ringing artifacts and data loss that impedes reliable operation at Increased operating frequencies. Control supports signal integrity.
1. Prevent Signal Reflections
High-speed signals require matched source, transmission line and load impedances to absorb the full signal. Mismatched impedance causes partial wave reflections interfering with the original signal.
Even minor impedance deviations lead to disproportionate reflectance with more noise disrupting faster, higher frequency signals. Reflections degrade rise times and jitter degrading bit error rates. Controlled tracks prevent reflections.
2. Facilitate Impedance Matching
Source and load components assume designed impedance values for interconnects – typically 50 Ω or 100 Ω differential. Traces must match anticipated impedance ensuring necessary termination resistors properly damp signals.
Mismatched lines over/under load termination networks hampering absorption of fast transient edges. Controlled lines present expected impedance for matching components.
3. Reduce EMI Radiation
Uncontrolled impedance increases high frequency return loss and ringing which worsens electromagnetic interference (EMI). Controlled lines help confine signals avoiding unintended radiation that risks coupling into nearby susceptible circuits.
In summary, controlled impedance traces counteract reflections, absorb signals cleanly, and reduce interference – critical factors for reliable high-speed PCB functionality.
How to Achieve Controlled Impedance?
Adjusting physical PCB parameters allows tuning impedance to target values along traces mitigating unchecked variations:
1. Trace Width
The main conductor width sets baseline impedance value. Wider lines decrease; narrower traces increase impedance assuming uniform thickness.
2. Dielectric Stackup
Dielectric material type and thickness surrounding traces heavily influences impedance based on inherent capacitance. Tighter dielectrics raise values.
3. Reference Planes
Closeby continuous reference planes augment capacitance lowering impedance. Spread planes raise impedance by reducing coupling.
4. Trace Separation
Spacing between signal and surrounding traces, pads or void areas impacts coupling determining impedance.
By tuning parameters appropriately, target impedance achieves matching. Advanced fabricators carefully model stackup dimensions and material electrical properties to yield controlled impedance across routing layers.
Controlled Impedance Target Values
While impedance ranges widely, two target values dominate modern PCB layouts:
50 Ω Impedance
- Standard controlled impedance used for single-ended signals
- Matches source/load termination resistance
- Requires narrower traces with tighter reference planes
100 Ω Differential Impedance
- Typical value for differential pair routing
- Wider trace spacing balances capacitive coupling
- Closer planes increase capacitance
Many factors determine choice of target impedance for a design but these two levels handle majority of applications. Control resolution within +/- 10% desired.
Traces get clearly marked in layouts meeting controlled spec after extensive modeling. Strict dimensional tolerances maintained throughout fabrication for consistency.
How Accurately to Control Impedance?
Extremely tightly controlled impedance better maximizes signal quality but demands much higher process capabilities driving up cost. Reasonable tolerances work for most applications.
A useful way to consider controlled impedance accuracy:
| Impedance Control | Application |
|---|---|
| +/- 5-10 Ω | Highest speed RF boards >5 Gbps |
| +/- 10% | Fast digital 2-5 Gbps |
| +/- 15% | Slower bandwidth below 2 Gbps |
Realistically, impedance depends on many interrelated factors that individually influence values as much as +/- 20% even on controlled lines:
- Copper thickness
- Dielectric composition
- Trace locate position
- Layer registration
- Fabrication accuracy
Well characterized PCB processes keep deviations reasonably constrained. But extensive modeling best informs expectations when pushing higher frequencies.
Controlling Impedance in PCB Layout
Beyond smart stackup, layout practices greatly aid achieving impedance control and timing across routing:
- Minimize length variance between differential pairs
- Match lengths through vias and layers
- Eliminate acute angle turns on traces
- Avoid impedance discontinuities
- Validate with impedance analysis
Careful layout adhering to controlled parameters ensures PCB environments present consistent, matched impedance to facilitate high-speed signaling.
When are Controlled Impedance Lines Necessary?
Controlled impedance traces help maximize signal quality and reliability but come with fabrication difficulty and cost impacts. Reserved only for necessary nets, some key considerations around usage include:
Clock Signals
High frequency clocks over 100 MHz propagate fast rise times requiring impedance control to prevent skew between board sections. Route clock trees through controlled dielectric layers.
SerDes Data Lines
High speed serializer/deserializer data channels demand matched impedance environments to limit reflections between source and destination components.
Analog Lines
Mixing uncontrolled digital routing can introduce noise into analog signals. Maintain impedance through analog channel length including shield layers for isolation.
Memory Buses
Address/command/data buses to RAM devices with clock/strobe signals benefit from consistent impedance routing to time signals reaching memory IC pads.
In summary, controlled impedance applies to nets with high frequency components or susceptibility. Matching impedance ensures quality signaling over traces, through vias across layers in the PCB stackup.
Implementing Impedance Control
Achieving impedance requires coordination across design stages:
Model anticipated stackup with projected dielectric constants and trace dimensions to calculate expected impedance. Assess termination schemes and line lengths.
Stackup Planning
Specify laminate materials, prepreg, copper weights, and layer sequencing to achieve impedance targets while meeting isolation needs.
Layout
Carefully route wide/narrow traces in controlled dielectrics with referenced return planes maintaining consistent geometry.
Fabrication
Precisely process board layers under strict dimensional tolerances keeping registered alignment. Test coupons validate continuity.
Verification
Employ impedance analysis to identify undesired variations exceeding limits needing layout adjustments or review of process assumptions.
While controlled impedance layout ability steadily improves in PCB tools, fabricators also enhance dimensional accuracy and stackup repeatability easing the challenge of impedance control for designers.
Overview of Controlled Impedance Advantages
In summary, controlled impedance tracks bring major high-frequency signaling benefits:
- Prevent reflections that distort signal rise times
- Enable impedance matching to properly terminate lines
- Reduce EMI from uncontrolled ringing
- Maintain fast switching speeds and data integrity
- Ensure high bandwidth with low bit error rates
- Critical for multi-gigabit SERDES channels
As data rates continue increasing, controlled impedance applies to an expanding range of chip-to-chip nets. Constraint driven trace width, spacing and layer planning facilitate quality high-speed signaling.
FAQ
Why is impedance matching important?
Impedance matching means source, transmission line and load all share common impedance preventing signal reflection allowing full absorption of wavefronts. Clean signaling relies on matched impedance environments.
