Fab Insights: A Practical Guide to Impedance Control & High-Speed Routing

Maintaining signal fidelity is crucial in high-speed digital and analog circuits. This involves a detailed understanding of impedance control and high-speed routing on printed circuit boards (PCBs). By working with MacroFab, you can ensure that these critical aspects of your design are meticulously managed, resulting in successful prototyping and manufacturing. But how do you know if your next design even needs impedance control?

What is Trace Impedance?

Trace impedance is the total of all resistance and reactance components within an electrical signal path. Ideally, the characteristic impedance of a transmission line should precisely match both the transmitter and receiver, guaranteeing the signal is fully absorbed without reflection or attenuation.

Single-Ended and Differential Impedance

For single-ended I/O signal transmission, a common impedance is 50 ohms. However, certain applications, like analog CATV video signals, may require different impedance values, such as 75 ohms. For high-speed differential signals, maintaining a consistent 100-ohm differential impedance between signal pairs is crucial. Standards like USB, HDMI, and JESD204B specify the tolerance for this impedance.

Real-World Effects

Real-world parasitic effects and mismatched materials can complicate ideal transmission lines, leading to signal loss and reflections. The characteristic impedance of a trace depends on the stack-up geometry and PCB materials. If manufactured accurately, the characteristic impedance remains constant regardless of trace length.

Determining if you need impedance control depends on your specific application and signal requirements. Here are some key considerations:

• High-Speed Signals: If your design involves high-speed digital or analog signals, impedance control is crucial to maintain signal integrity.

• Signal Integrity Requirements: Applications requiring precise signal integrity, such as high-frequency communication protocols (USB, HDMI, Ethernet), typically require controlled impedance.

• Long Signal Paths: Designs with long signal paths where transmission line effects become significant may benefit from impedance control to prevent signal degradation.

• Differential Pairs: If your design uses differential pairs for communication (e.g., LVDS, USB, HDMI), maintaining a consistent differential impedance is essential.

If your design falls into any of these categories, it's likely that you need to implement impedance control to ensure optimal performance and reliability.

Choosing the Right Dielectric

The dielectric material dictates the signal bandwidth for a PCB trace. High-speed applications may require materials beyond standard FR4. MacroFab can assist in determining if premium dielectric materials are available or need to be ordered. For detailed information on MacroFab’s standard PCB material properties and stack-ups, refer to MacroFab's knowledge base.

Prepreg Materials and Lamination

Prepregs bond core laminates within a PCB, and their final thickness depends on copper content, height, and prepreg type. High process control during lamination ensures predictable thicknesses. High-speed materials with lower dielectric constants suit applications above 500 MHz. Designers should specify impedance values, trace width, spacing, and layers for controlled impedance traces in fabrication notes.

What is High-Speed Routing?

"High-speed routing" refers to routing PCB traces to minimize parasitic effects in high-speed signals. High-speed signals have wavelengths equal to or less than four times the trace length. Proper design is essential to avoid problems, especially in complex PCB designs.

Challenges in Modern Digital Design

Modern digital designs often involve high-speed signals. Digital signals, particularly square waves, contain higher-order frequencies beyond the clock frequency, causing potential RF-like effects. These can distort signal edges and cause false transitions or signal ringing.

Impedance

Resistors are not the only electrical part that can create impedance. Capacitors and inductors create an impedance that depends on the geometric shape, the frequency of the signals passing through them, and part specifications. Copper traces also create small values of impedance.

Stray Capacitors

Capacitors can carry electrical current if it is continuously alternating. Capacitors can also conduct a variety of signal types as long as the voltage alternates. The capacitance value determines how much current can flow. This is the source of the impedance mentioned above for capacitors.

The geometry of the metal parts on a PCB will form unintended capacitors wherever two conductive surfaces face each other, with a non-conductive dielectric in between them (air, the PCB substrate, plastic, etc.,). The area of the two surfaces and the distance between them, and the non-conductive dielectric affect the capacitance that is formed.

Parasitic Capacitance and Inductance

Traces and signal paths on the PCB are susceptible to this parasitic capacitance effect. The PCB substrate plays the role of the capacitor’s non-conductive dielectric material in this case. PCB substrates are composed of fibrous materials woven into a sheet and then impregnated with a resin. The type of weave and resin employed can cause direction-dependent differences in the capacitance formed with the substrate in between.

• Stray Inductors: Inductors are conductors wound into a coil. However, even straight copper traces will have some inductance. Bends and spiral shapes in trace pathways add to this inductance, and the presence of ferromagnetic material in components also increases the inductance.

• Transmission Lines: A helpful way to model the parasitic effects of these capacitors, inductors, and resistances on traces is to treat the traces as a transmission line. Transmission line models are a standardized network of interconnected resistors, inductors, and capacitors with specific arrangements and values in accordance with the stray impedances that are typically formed as described above.

Reflections and Crosstalk

Current traveling down a transmission line is modeled as a wave of current, which is more accurate when we are in the high-speed or high-frequency range. Traveling wave phenomenon can occur in the transmission line which can create backward flowing reflections if an abrupt change in impedance is encountered by the signal as it travels along the transmission line. At higher frequencies, such reflections can completely eradicate the original signal or even damage the source driver.

• Crosstalk Interference: One of the most common issues that can occur in high-speed design is interference caused by a signal on one trace “leaking” onto another trace through an unintended parasitic capacitor that was formed between the traces. Sometimes cross-talk is caused by one trace acting as a transmitting antenna and the other trace acting as a receiving antenna. The space between the traces can be much larger in this case. Both situations create signals in places where they are not wanted, and such intruding signals are often tough to remove by secondary means such as filtering.

• Signal Attenuation and Reflections: Stray inductors formed by the conductors can sometimes be large enough to produce a significant impedance to the flow of the signal through the conductors. This impedance will limit signal current just like an ordinary resistor. Frequency is very important here, and the magnitude of a trace’s impedance can be far greater at higher frequencies.

Inconsistent Grounding and Propagation Time Delays

As mentioned earlier, a signal encountering an abrupt change in impedance can experience a transmission line reflection phenomenon which can send a portion of the signal current backward, reducing the intensity of the forward wave. Depending upon the arrangement of the network, voltages or currents can build up to unexpectedly high values, potentially causing damage to components in the circuit.

• Inconsistent Grounding: Traces that exhibit a high impedance can effectively defeat the purpose of the system ground by producing different ground potentials. Ground traces can become a chain of inductors connected in series, presenting a slightly different impedance to ground. The chain will cause each ground to be at a different voltage level than actual ground and different from any other “ground” connection. The differences produced depend on the total current flowing in the chain which can compound the problem. The circuit’s ground becomes unpredictable.

Most circuit designs assume that all connections to the ground are at the same voltage potential. Malfunctions and erratic behavior are almost guaranteed if ground connections are at different voltage levels like this. Circulating currents between grounding points at different voltage levels can also occur which creates ground loops. Inconsistent grounding can cause real havoc in circuit sections that have high gain by creating unwanted feedback paths which can stimulate unwanted oscillations.

• Propagation Time Delays: This effect becomes much more troublesome as the overall speed of the circuit increases to the point where the propagation time delays are comparable to the period of the clock signal. This makes it much more likely that individual bits will be lost due to synchronization issues.

High-speed routing and impedance control are crucial aspects of PCB design to maintain signal integrity and ensure optimal performance in high-speed digital and analog circuits. By understanding the impact of trace impedance, common challenges, and potential problems, designers can make informed decisions to mitigate these issues.

Stay tuned for next week's follow-up article, "Fab Insights: Preventing High Speed Routing Problems," which will dive deeper into formulas and practical techniques to address these challenges and achieve successful high-speed PCB design.