High Speed PCB Design Practical Tutorial

When we discuss high speed PCB, this term is used to distinguish them from traditional circuit boards. A high speed PCB is a specialized circuit board designed for handling high-speed signal transmission. These high-speed signals typically have very short rise and fall times, making the design process of high speed PCB complex and challenging. It involves addressing various intricate issues such as integrity design and impedance matching.

In this article, TechSparks will provide a high-speed PCB design guide to assist you in achieving precise results in your circuit design projects. Let’s get started!

High speed PCB circuit diagram

Table of Contents

High Speed PCB Design Guidelines

Choice of Grounding Methods

The transmission speed of a PCB is influenced by the frequency of electrical signals. Therefore, considering the working frequency of signals is a crucial aspect of your work in high speed PCB design. Different grounding strategies should be employed based on different frequencies:

Single-point grounding and multi-point grounding

  • <1MHz: One of the factors affecting inductance in PCB is the geometric shape of the circuit. When the wavelength is longer, the impact of inductance on routing and components is relatively small. In situations where the working frequency of a high speed PCB is less than 1MHz, it is advisable to use a single-point grounding approach to simplify the design.
  • >10MHz: As the signal wavelength significantly shortens, the length and impedance of the ground lines become critical considerations. In such cases, adopting a multipoint grounding approach can help reduce the total impedance of ground lines, minimizing reflections and interference to ensure signal integrity.
  • 1~10MHz: The choice should be based on the proportion between signal wavelength and ground line length. If the ground line length is relatively long compared to the wavelength, a single-point grounding approach is suitable. However, if the ground line length is close to or exceeds a certain proportion of the wavelength, a multipoint grounding approach may be more appropriate.

Cross Pad Design

Cross flower pad

When dealing with the connection of component pins to large ground or power conductors, it’s essential to consider both electrical performance and manufacturing processes. From an electrical performance perspective, ideally, connecting the component pin pads to large copper conductors is desirable. However, this approach poses certain issues in terms of component soldering and assembly, such as:

  • Requires high-power heaters to complete soldering.
  • Tends to create solder voids.

To balance the requirements of electrical performance and manufacturing processes, a common practice in high speed PCB design is to use a design featuring cross-shaped solder pads, often referred to as thermal isolation or thermal pads. This design significantly reduces the likelihood of solder voids due to uneven heat distribution during soldering.

Reducing Through-Holes

via Selection in High Speed PCB

For high-density PCB designs, through-holes may not be the most suitable option and should be replaced with blind vias or buried vias, especially in high-speed data transmission lines.

Firstly, high speed PCB board applications often demand more advanced products, which may have stricter requirements for size and weight. Through-hole designs imply the use of larger components, which can contradict the initial design intent.

More importantly, through-holes need to traverse the entire PCB, meaning they are longer and have larger diameters. This poses a risk of inductance forming around the hole. Inductance impedes the flow of high-speed signals as they pass through the through-hole. Additionally, on either side of the hole are metal layers isolated by dielectric materials, creating a capacitor. This leads to changes in signal impedance, phase delays, and ultimately affects high-speed signal transmission.

In contrast, blind vias and buried vias are designed specifically for high-speed applications. They utilize special structures and manufacturing processes to minimize signal transmission losses, reflections, and crosstalk. While they may introduce complexity and increased costs, they are often considered worthwhile.

High Speed PCB Design Rules

Rule 1: Trace Shielding

Trace Shielding

In high-speed circuit, certain critical signal lines, especially clock lines, are susceptible to EMI interference, which can result in signal leakage or disturbance. To mitigate this interference, it’s essential to shield these signal lines. This shielding involves adding a shielding layer or cover around the signal lines. This can be achieved using shielding sleeves, shielding layers, or other specialized shielding materials. TechSparks recommends a design approach that involves puncturing the shielding layer at intervals of 1000 mils and connecting these holes to the ground. This grounding connects the shielding layer to the ground potential, further enhancing the shielding effectiveness.

Rule 2: Avoiding Closed Loop Routing

Routing closed-loop rules

When aiming for high-speed performance, circuit boards often end up with higher component density, which can introduce challenges in the routing layout. One particular concern is the formation of closed loops within the high-speed signal network, creating a ring-like structure where the signal path returns to its starting point. Such closed loops can lead to what’s known as the “loop antenna effect” within a circuit.

When a signal line forms a closed loop, it essentially creates an antenna structure on the PCB. This unintended loop acts as an antenna that can amplify the radiated electromagnetic interference (EMI). In simpler terms, it causes the circuit to unintentionally emit electromagnetic noise into its surroundings, potentially interfering with other components or systems.

Rule 3: Star Symmetrical Topology

Not suitable for daisy-chain topology of high-speed PCB

Topology pertains to how electronic components are physically interconnected, determining the signal flow path within a circuit. The chosen topology plays a crucial role in maintaining the integrity of high-speed signals. It’s important to select the appropriate connection scheme—whether it’s series, parallel, feedback, or bridge—for your specific project. To aid in this process, it’s highly recommended to leverage modern electronic design software for topology analysis and optimization during the design phase.

The above figure shows the daisy-chain topology, which is more commonly used, but it is often more suitable for circuits with frequencies within 10MHz. However, to preserve the integrity of high speed circuit board signals, it’s advisable to employ a star-shaped symmetrical structure. This topology gathers multiple signal lines and routes them to a central point or node. By opting for a star-symmetrical topology, you ensure that high-speed signals converge at a central hub, minimizing signal degradation and interference.

Rule 4: Mindful Wiring Orientation

Routing direction rules for high-speed PCB design

It’s easy to overlook the significance of wiring direction, as many beginners believe that ensuring correct connections suffices. However, paying attention to the direction of your traces can yield substantial benefits. Vertical routing, for instance, concentrates the direction of the current flow, which is critical in high speed PCB layout techniques. Additionally, this orientation minimizes physical proximity, reducing the likelihood of crosstalk—an interference issue common in densely routed boards.

Complex or heavily curved routing can introduce complications like signal foldback, delay, and distortion. Hence, it’s crucial to balance efficient routing and minimizing signal path complexity.

5 Aspects Affecting High Speed PCB Signal Quality

  • Overshoot: Arises from sudden, rapid voltage changes during signal transitions. Regardless of the outcome, it introduces unwanted interference to the circuit, ranging from noise to potential harm to sensitive components.

Overshoot

  • Glitch: Stem from abrupt changes in signal levels within the circuit. They can be caused by logic hazards, crosstalk, ground bounce, and more. Glitches can lead to issues such as false triggering, errors in control signals, and phase errors in clock signals, negatively impacting overall correctness.

Glitch

  • Edge Rate: Slow edge rates can result from factors like excessive load capacitance at the output, signal line length, or insufficient current capability. These factors extend the rise or fall time of the signal, producing unexpected waveforms. This, in turn, can lead to data sampling errors and timing problems.

Edge Rate

  • Backlash: Occurs due to signal line mismatches or reflections, among other factors. It leads to signal instability in high speed PCB board, resulting in additional oscillations or fluctuations, often occurring momentarily.
Backlash
  • Voltage Levels: May originate from three-state buses or bus conflicts, causing the signal’s amplitude to deviate from the expected value. This can result in device output errors or incorrect signal interpretation.

Voltage Levels

Related Articles:

Impact of Via Stub on High-Speed PCB Signal Transmission

Why Is Impedance Matching Essential for High-Speed PCB

High Speed PCB Electromagnetic Interference Analysis

How to Avoid Parasitic Effects in High-Speed PCB Via Design

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