what is Best practices and considerations when designing your PCBPosted on 2022-02-24
When designing a PCB, we usually rely on previous experiences and tips that we typically find online. Each PCB design can be optimized for a specific application, and usually, its design rules apply only to the target application. For example, an analog to digital converter PCB rules would not be suitable for an RF PCB or vice versa. However, some guidelines can be seen as universal for any PCB design. Here, in this tutorial, we will go through fundamental problems and tips that significantly improve PCB designs.
Power and Signal Distribution
Power distribution is a key element in any electrical design. All your components depend on power to exert their function. Depending on your design, some elements can have the best power connection, and on the same board, some of them could have the worst. For example, if you have all your components powered by a single trace, each component will observe a different impedance, leading to multiple ground references. For example, if you have two ADC circuits, one in the beginning and another one at the end, and both ADCs read an external voltage, each analog circuitry will read a different potential relative to their own.
We can summarize the Power distribution in 3 possible ways: Single point source, star source, and multipoint source.
(a) Single point source: each component has its power and ground trace separated from the others. The power traces of all the components only meet at a single reference point. A single point is considered the best for power. However, it is infeasible for complex or large/medium projects.
(b) Star source: Star-like source can be seen as an improvement of a single point source. It differs due to its key feature: the same trace length between components. Star connection is often used for complex high-speed signal boards with various clocks. In a high-speed signal PCB, the signal usually comes from the edge and then goes to the center. From the center, all the signals can go to any region of the board with minimal delay between the areas.
(c) Multipoint source: It is considered the worst in any case. Nevertheless, it is the easiest to be used in any circuitry. The multipoint source can create differences in the reference between the component, as well as common impedance coupling. This design style also allows high switching ICs, clocks, and RF circuits to introduce noise in nearby circuits sharing the connection.
Of course, in our daily lives, we will not always be able to have a single type of distribution. The best trade-off we can have is mixing single point source with multipoint source. Basically, you should make your Analog sensible device and high-speed/RF systems in a single point while letting all the other less sensitive peripherals in multipoint.
Have you ever wondered if you should use Power Planes? Well, the answer will be a resounded YEAH YOU SHOULD. Power planes are one of the best ways to deliver power and decrease noise from any circuit. Power planes shorten the ground paths, reduce the inductance, improving the Electromagnetic Compatibility (EMC) performance. It is also attributed that power planes on both sides also create a parallel plate decoupling capacitor, preventing noise propagation.
Power planes also have a clear advantage: due to a larger area, it permits the flow of a much larger current, increasing the operating temperature range of your PCB. But pay attention: the power plane improves the operating temperature, but the traces have to be considered as well. The rules for the traces are given by IPC-2221 and IPC-9592
For PCBs with RF sources (or any high-speed signal applications), you must have a complete ground plane to improve your board’s performance. The signals have to be on different planes, and to achieve both requirements at the same time are nearly impossible using a two layers board. If you are designing antennas or any low complexity RF board, you can achieve it using two layers. The figure below shows an illustration of how your PCB could better use the planes.
In mixed-signals design, manufacturers often recommend splitting the analog ground from the digital ground. Sensitive analog circuitry is easily affected by high-speed switching and signals. If you make two different analog and digital ground, you will have a split the ground plane. However, a split ground comes with its own challenges to overcome. We should be mindful of crosstalk and loops areas for split ground, mainly created by the ground plane’s discontinuities. The figure below shows two examples of split ground planes. On the left, the return current can’t take the direct way along the signal trace, and so a loop area occurs, while the design on your right loop areas will not happen. Also, every ground plane must have its own path to the common ground, this reduces even further the noise. Please note that this paragraph is best for mixed signals designs only, and some chips do not benefit from split ground planes (some manufactures, like Texas Instrument, say that if your chip does not require it, you should opt for a single plane). So, always check the datasheet of your device in order to know which ground plane rule works best for you.
Electromagnetic Compatibility and Electromagnetic interference (EMI)
EMI can be a considerable drawback for high-frequency design, such as RF systems. The ground planes previously discussed help mitigate EMI, but ground planes can pose a different problem depending on your PCB. In a stack-up layers board, with four or more layers, the planes’ distance has the utmost importance. When interplane capacitance is small, an electric field will extend itself over the board. Simultaneously, the impedance between the planes reduces, allowing the return current to flow to the signal plane. This will generate EMI for any high-frequency signal crossing the planes.
Easy solutions to avoid generating EMI are: Prevent high-speed signals from crossing many layers; Add decoupling capacitors; and place ground vias around the signal trace. The figure below shows a good PCB design with a high-frequency signal.
Bypass capacitors and ferrite beads are the ones used for filtering noise produced by any component. Basically, any I/O pin can be a source of noise if used in any high-speed application. For better use of those, we will have to pay attention to few things:
Always put the ferrite beads and bypass capacitors as near as possible to the source of the noise. When we use auto-place and autoroute, they usually don’t know each component’s function in your circuit, so you should review it having in mind the distance.
Avoid vias and any other trace between your filter and your component.
If you have a ground plane, use more than one via to ground it properly.
I hope this tutorial will be useful for you and clear some of your doubts. If you have any questions, I will be happy to answer.