Memory and Storage

6 ways to check PCB design quality

18 August 2020
There are many ways to determine if a PCB is not the quality electronic device makers want it to be.

A printed circuit board, or PCB, that is poorly designed will never satisfy the quality required of commercial production. The ability to judge the quality of PCB design is of great importance. It requires experience and knowledge of PCB designing to do a full design review. However, there are a few ways to quickly judge the quality of a PCB design.

A schematic diagram might be sufficient to tell about components and how they are connected for a given functionality. However, the information provided by the schematic about the actual placement and connection of the components for a given operation is very limited. It implies that even if the PCB is designed by carefully implementing all the component connections of a fully working schematic, there is a possibility that the end-product will not work as intended. To quickly check the quality of PCB design, consider the following:

1. PCB traces

The visible traces of the PCB are covered with a solder mask that helps protect the copper traces from shorts and oxidation. There are different colors available for it, but the most commonly used color is green. Note that it is very difficult to see traces due to the white color of the solder mask. In many cases, we can see only the top and bottom layers. The internal layers are not visible when the PCB has more than two layers. However, it is very easy to judge the quality of the design, by seeing just the external layers.

During design review, check the traces to confirm that there are no sharp bends, and they all run in straight lines. Sharp bends should be avoided because some high frequency or high power traces can cause trouble. Avoid them entirely as they are the ultimate signal that the quality of design is poor.

2. Decoupling capacitors

To filter out any high-frequency noise that may negatively impact the chip, decoupling capacitors are positioned very close to the power pins. Generally, if the chip contains more than one voltage drain-to-drain (VDD) pin, then one decoupling capacitor is required for each such pin and sometimes more.

Decoupling capacitors ought to be set very close to the pins meant to decouple. The effect of decoupling capacitors is greatly reduced if they are not kept close to the pins. If the decoupling capacitors are not placed right next to the pins on most of the microchips, then it is another indication that the PCB was not designed properly.

3. PCB traces length equalization

For multiple signals to have precisely timed relationships, PCB trace lengths must be matched in design. Matching the trace lengths guarantees the arrival of all signals with an equal delay at their destination and helps to maintain the relationship between the signal edges. Access to the schematics is needed to know if any set of signal lines require precise timing relationships. The traces can be followed to check whether any trace length equalization, otherwise called delay lines, have been applied. Most of the time these delay lines look like squiggly lines.

It is worth noting that additional delays are caused by the vias in the signal path. If it is impossible to avoid vias, then it is important to make sure that all trace sets have an equal number of vias with precise timing relationships. Or, the delay caused by the vias can be compensated by the use of delay lines.

4. Component placement

Although inductors have the ability to produce magnetic fields, engineers should make sure that inductors are not placed close to each other if they are being used in the circuit. An unwanted coupling can be produced between inductors if they are placed close to each other, especially end-to-end. Current can be induced in the large metallic objects due to the magnetic field produced by the inductors. So, they must be placed at a distance from metallic objects, otherwise this may cause a change in the value of inductors. Unwanted mutual couplings can be reduced even when inductors are placed close together by just setting them perpendicular to each other.

The impact of the heat on other components needs to be considered if the PCB has power resistors or any other heat-generating component. For instance, if temperature compensation capacitors or thermostats are used in the circuit, then they should not be placed near the power resistors or any component that generates heat.

A section must be dedicated on the PCB for on-board switching regulator and components associated with it. This section must be set as far away as possible from the section that handles the small signals. There must be a separate section on the PCB for the AC side if AC mains are directly applied to the PCB. There would be an issue in the quality of the PCB design if the components are not sectioned out as suggested above.

5. Trace width

Engineers should take extra care to appropriately size the traces that carry high current. There is a possibility of noise pickup issues if the traces that carry fast-changing or digital signals run parallel to traces that carry small analog signals. Traces that connect to inductors have the ability to act as antennas and they can cause unwanted radio frequency emissions. To avoid this, these traces should not be wider.

6. Grounds and ground planes

The ground plane must be split if the PCB has both sections, digital and analog, and are joined at a common point only, generally the negative power terminal. This can help to avoid adverse effects on the analog section from the digital section caused by ground current spikes. The ground return trace of sub-circuits (if the PCB has only two layers) need to be separated and then must join at the negative power terminal. It is highly recommended that for a moderately complex PCB to have a four-layer board minimum, two internal layers are required for supply and ground plane.

Conclusion

It is important for engineers to have enough expertise in PCB design to judge the quality of a design or one designed by an employee. Yet, engineers without expertise can review the above-mentioned methods. It is always a good idea to get the quality of PCB design checked from an expert before jumping to prototyping, especially if designing a startup product.

To contact the author of this article, email engineering360editors@globalspec.com


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