Role of Impedance in PCB Design

Impedance is a complex parameter which plays a critical role in Printed Circuit Board (PCB) design, particularly in high-speed and high-frequency circuits. 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.

It directly impacts signal integrity, power delivery, and overall system performance. Understanding and managing impedance effectively is crucial for creating reliable and efficient PCB designs. In this article, we will explore the concept of impedance, its importance, factors affecting it, and techniques to control impedance in PCB design.

What is Impedance?

Impedance (Z) represents the total opposition to current flow in an AC circuit. It is composed of both resistance (R) and reactance (X). Resistance opposes current while reactance stores and releases energy. Together, they impede current in an AC system.

• Resistance (R): The opposition to direct current (DC).
• Reactance (X): The opposition due to inductance and capacitance in an AC circuit.

The total impedance is expressed as:

Z = R + jX

Where:

R is the resistive component

X is the reactive component

j is the imaginary unit

While resistance is a fixed value, reactance changes with frequency in capacitors and inductors. So impedance is frequency dependent in AC circuits. An impedance calculator can determine impedance based on the R and X values at a given frequency. The impedance formula reveals that impedance has both magnitude and phase components.

The magnitude (|Z|) is found by: |Z| = √(R2 + X2)

The phase angle (θ) is: θ = arctan(X/R)

This phase relationship between voltage and current is important in AC power systems. Components like transformers and motors depend on proper phase angles. The phase also impacts the power factor. So both magnitude and phase are essential impedance considerations. In PCB design, impedance is particularly important for signal traces carrying high-frequency signals, as these traces behave like transmission lines rather than simple conductors.

Why to Control Impedance on PCBs?

When a signal needs a specific impedance to operate properly, controlled impedance should be preferred. In high frequency applications, keeping impedance constant on the complete electronic board is essential to protect the transferred data from damage and to maintain the clarity of the signal. The longer the trace or the higher the frequency, the more adaptation is needed. Any lack of rigour at this stage can increase the switching time for an electronic device or circuit and cause unexpected errors.

Uncontrolled impedance is difficult to analyse once the components are mounted on the circuit. Components have different tolerance capacities depending on their batch. Furthermore, their specifications are impacted by temperature variations which can lead to malfunctions. In such cases, replacing the component may seem to be the solution at first when, as a matter of fact, it is the unsuitable trace impedance that is the cause of the problem.

This is why trace impedances and their tolerances must be checked early on in the PCB design. Designers must work hand in hand with the manufacturer to guarantee the compliance of component values.

Impedance Matching in PCB design:

To maximize power transfer, complex conjugate matching is used. This matches the load impedance to the complex conjugate of the source impedance. For example, if the source is 50 + j25 ohms, the load would need to be 50 - j25 ohms.

Passive matching networks with capacitors and inductors can be designed to transform impedances. This ensures minimal reflections from mismatches in RF and communication systems. Understanding impedance concepts is vital to effectively applying them to optimize electrical and electronic system designs across a diverse range of applications and frequencies.

How Impedance Mismatch Impacts PCB signal Quality:

• Reflection: 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.
• Jitter:  Variable impedance modulation introduces phase noise as jitter.
• Distortion: High/low impedances attenuate/amplify signal harmonics.

How is impedance determined?

Usually, trace impedance is between 25 and 125 Ohms and depends on the following factors:

• Copper trace width and thickness
• Passage of the signal through vias
• Core thickness or prepreg material thickness on each side of the trace
• Core and prepreg material dielectric constant
• Distance from the reference copper plane
• Presence or not of solder resist

The board designer must therefore make sure that for high frequency signals, their choice of trace and stackup makes it possible to achieve a target impedance value, with a certain tolerance. The most advanced electronics CAD design tools calculate this automatically.

Impedance Control Techniques in PCB Design:

1. Impedance Matching

Matching the impedance of the source, transmission line, and load minimizes signal reflections. This is essential for high-speed interfaces like HDMI, USB, and Ethernet.

2. Controlled Impedance Traces

Designing traces with specific impedance values ensures signal integrity. Controlled impedance is achieved by:

• Calculating trace dimensions using tools or equations.
• Using consistent dielectric materials.

3. Microstrip and Stripline Configurations

These are transmission line structures used to control impedance:

• Microstrip: A trace on the outer PCB layer, with a reference plane beneath.
• Stripline: A trace sandwiched between two reference planes, offering better shielding.

4. Differential Pair Routing

Differential pairs are two traces carrying equal and opposite signals. Maintaining controlled impedance and consistent spacing between the pairs ensures noise immunity and signal integrity.

5. Simulation and Testing

Using simulation tools like SPICE or PCB-specific software (e.g., Altium Designer, EasyEDA) helps model and verify impedance characteristics before manufacturing.

What is Skin Effect:

At elevated frequencies, alternating current gracefully chooses to predominantly journey along the outer surface of a conductor. This well-documented phenomenon, often referred to as the skin effect, orchestrates a captivating ballet where electrons prefer to waltz near the surface. The outcome of this mesmerizing performance is an increase in effective resistance as current gracefully confines itself to a more confined cross-sectional realm near the surface.

The skin effect is a result of inductance in the wire creating reactance that opposes current flow towards the center. Litz wire strands can help mitigate skin effect issues in circuits like RF transformers. See our detailed guide on different types of via's in PCB design.

Conclusion:

To sum it up, this comprehensive article has thoughtfully provided an insightful overview of impedance and the vital considerations that concern electrical engineers. Acquiring a robust grasp of impedance is unquestionably pivotal in guaranteeing the dependable performance of printed circuit boards (PCBs) across a diverse spectrum of applications. By understanding the factors influencing impedance and employing proper control techniques, designers can create reliable and efficient PCBs. Whether working on high-speed digital circuits or RF designs, impedance management is a key to success in modern electronics.