Poorly designed printed circuit boards or PCBs will never meet the quality required for commercial production. The ability to judge the quality of PCB design is very important. Experience and knowledge of PCB design are required to conduct a complete design review. However, there are several ways to quickly judge the quality of the PCB design.
The schematic diagram may be sufficient to illustrate the components of a given function and how they are connected. However, the information provided by the schematics regarding the actual placement and connection of the components for a given operation is very limited. This means that even if the PCB is designed by meticulously implementing all the component connections of the complete working principle diagram, it is possible that the final product may not work as expected. To quickly check the quality of the PCB design, please consider the following:
1. PCB trace
The visible traces of the PCB are covered with solder resist, which helps protect the copper traces from short circuits and oxidation. Different colors can be used, but the most commonly used color is green. Note that it is difficult to see traces due to the white color of the solder mask. In many cases, we can only see the top and bottom layers. When the PCB has more than two layers, the inner layers are not visible. However, it is easy to judge the quality of the design just by looking at the outer layers.
During the design review process, check the traces to confirm that there are no sharp bends and that they all extend in a straight line. Avoid sharp bends, because certain high-frequency or high-power traces may cause trouble. Avoid them altogether because they are the final signal of poor design quality.
2. Decoupling capacitor
In order to filter out any high frequency noise that may negatively affect the chip, the decoupling capacitor is located very close to the power supply pin. Generally, if the chip contains more than one drain-to-drain (VDD) pin, each such pin needs a decoupling capacitor, sometimes even more.
The decoupling capacitor should be placed very close to the pin to be decoupled. If it is not placed close to the pin, the effect of the decoupling capacitor will be greatly reduced. If the decoupling capacitor is not placed next to the pins on most microchips, then this again indicates that the PCB design is incorrect.
3. PCB trace length is balanced
In order to make multiple signals have accurate timing relationships, the PCB trace length must be matched in the design. Trace length matching ensures that all signals reach their destinations with the same delay and helps maintain the relationship between signal edges. It is necessary to access the schematic diagram to know whether any set of signal lines requires precise timing relationships. These traces can be traced to check whether any trace length equalization has been applied (otherwise called delay lines). In most cases, these delay lines look like curved lines.
It is worth noting that the extra delay is caused by vias in the signal path. If vias cannot be avoided, it is important to ensure that all trace groups have an equal number of vias with precise timing relationships. Alternatively, the delay caused by the via can be compensated by using a delay line.
4. Component placement
Although inductors have the ability to generate magnetic fields, engineers should ensure that they are not placed close to each other when using inductors in a circuit. If the inductors are placed close to each other, especially end-to-end, it will create harmful coupling between the inductors. Due to the magnetic field generated by the inductor, an electric current is induced in a large metal object. Therefore, they must be placed at a certain distance from the metal object, otherwise the inductance value may change. By placing the inductors perpendicular to each other, even if the inductors are placed close together, unnecessary mutual coupling can be reduced.
If the PCB has power resistors or any other heat-generating components, you need to consider the effect of heat on other components. For example, if temperature compensation capacitors or thermostats are used in the circuit, they should not be placed near power resistors or any components that generate heat.
There must be a dedicated area on the PCB for the on-board switching regulator and its related components. This part must be set as far as possible from the part dealing with small signals. If the AC power supply is directly connected to the PCB, there must be a separate part on the AC side of the PCB. If the components are not separated according to the above recommendations, the quality of the PCB design will be problematic.
5. Trace width
Engineers should take extra care to properly determine the size of traces carrying large currents. If traces carrying rapidly changing signals or digital signals run parallel to traces carrying small analog signals, noise pickup problems may arise. The trace connected to the inductor has the ability to act as an antenna and may cause harmful radio frequency emissions. To avoid this, these marks should not be wider.
6. Ground and ground plane
If the PCB has two parts, digital and analog, and must be connected at only one common point (usually the negative power terminal), the ground plane must be separated. This can help avoid the negative impact of the digital part on the analog part caused by the ground current spike. The ground return trace of the sub-circuit (if the PCB has only two layers) needs to be separated, and then it must be connected at the negative power terminal. It is strongly recommended to have at least four layers for moderately complex PCBs, and two internal layers are required for the power and ground layers.
in conclusion
For engineers, it is very important to have sufficient professional knowledge in PCB design to judge the quality of one or one employee design. However, engineers without professional knowledge can view the above methods. Before transitioning to prototyping, especially when designing a startup product, it is always a good idea to always have an expert check the quality of the PCB design.