Detailed explanation of best practices and considerations when designing PCB

When designing PCB, we usually rely on the experience and skills we usually find on the Internet. Each PCB design can be optimized for a specific application. Generally, its design rules are only applicable to the target application. For example, the analog-to-digital converter PCB rule does not apply to RF PCB, and vice versa. However, some guidelines can be considered generic for any PCB design. Here, in this tutorial, we will introduce some basic problems and techniques that can significantly improve PCB design.

Power and signal distribution

Power distribution is a key element in any electrical design. All of your components rely on power to perform their functions. Depending on your design, some components may have the best power connection, while some components on the same board may have the worst power connection. For example, if all components are powered by a single wire, each component will observe a different impedance, resulting in multiple ground references. For example, if you have two ADC circuits, one at the beginning and the other at the end, and both ADCs read an external voltage, each analog circuit will read a different potential relative to themselves.

We can summarize the power distribution in three possible ways: single point source, star source and multipoint source.

(a) Single point power supply: the power supply and ground wire of each component are separated from each other. The power wiring of all components shall only converge at a single reference point. Single point is considered the most suitable for power. However, this is not feasible for complex or large/medium-sized projects.

circuit board

(b) Star source: Star source can be regarded as the improvement of single point source. Due to its key characteristics, it is different: the routing length between components is the same. Star connection is usually used for complex high-speed signal boards with various clocks. In high-speed signal PCB, the signal usually comes from the edge and then reaches the center. All signals can be transmitted from the center to any area of the circuit board, and the delay between areas is minimal.

(c) Multipoint source: In any case, it is considered the worst. However, it is easiest to use in any circuit. Multipoint sources may cause reference differences between components and in common impedance coupling. This design style also allows high switching ICs, clock and RF circuits to introduce noise in nearby circuits that share connections.

Of course, in our daily life, we will not always have a single type of distribution. The best compromise we can achieve is to mix single point sources with multiple point sources. Basically, analog sensitive devices and high-speed/RF systems should be placed in one point, while all other less sensitive peripherals should be placed in multiple points.

Powered aircraft

Have you ever thought about whether you should use power planes? Well, the answer should be loud. The power board is one of the best ways to transmit power and reduce the noise of any circuit. The power plane shortens the grounding path, reduces the inductance, and improves the electromagnetic compatibility (EMC) performance. It should also be attributed to the fact that a parallel plate decoupling capacitor is also generated on the power supply planes on both sides, thus preventing noise propagation.

The power board has another obvious advantage: because of its large area, it allows more current to pass through, thus increasing the operating temperature range of the PCB. However, please note that the power layer can improve the working temperature, but wiring must also be considered. The tracking rules are given in IPC-2221 and IPC-9592

For PCB with RF source (or any high-speed signal application), you must have a complete ground plane to improve the performance of the circuit board. The signals must be on different planes, and it is almost impossible to meet two requirements at the same time using two layers of boards. If you want to design an antenna or any low complexity RF board, you can use two layers.

In mixed signal design, manufacturers usually recommend separating analog from digital. Sensitive analog circuits are easily affected by high-speed switches and signals. If the analog and digital grounding are different, the grounding plane will be separated. However, differences have their own challenges to overcome. We should pay attention to the crosstalk and loop area of the divided ground caused mainly by the discontinuity of the ground plane.

Electromagnetic compatibility and electromagnetic interference (EMI)

For high-frequency designs (such as RF systems), EMI can be a major disadvantage. The ground plane discussed earlier can help reduce EMI, but depending on your PCB, the ground plane may cause other problems. In a laminate with four or more layers, the distance of the aircraft is critical. When the in-plane capacitance is small, the electric field will expand on the board. At the same time, the impedance between the two planes decreases, allowing the return current to flow to the signal plane. This will generate EMI for any high frequency signal that crosses the plane.

A simple solution to avoid EMI is to prevent high-speed signals from crossing multiple layers. Add decoupling capacitor; And place grounding vias around the signal wiring. The figure below shows a good PCB design with high frequency signals.

Filter noise

Bypass capacitors and ferrite beads are used to filter the noise generated by any component. Basically, if used in any high-speed application, any I/O pin can become a noise source. In order to make better use of these contents, we will have to pay attention to the following points:

Always place ferrite beads and bypass capacitors as close to the noise source as possible. When we use automatic placement and automatic routing, they usually do not know the function of each component in the circuit, so you should consider the distance to check. Avoid vias and any other routing between filters and components. If you have a ground plane, use multiple through holes to ground it properly. PCB processing factory shares: the best practices and precautions in PCB design are explained in detail, and each PCB design can be optimized for specific applications.