Layout Design Rules for Rigid-Flex PCB

12 June 2023
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As we all know, the layout of rigid-flex PCB is essential. To make the board more reliable and have an excellent electrical performance, we need to pay attention to some principles in the design process. You need to know some layout design rules for rigid-flex PCB.

Keep the Trace Width As Small As Possible

Regarding layout design, one of the most important things to keep in mind is the trace width. This is because the width of the trace will directly affect the circuit's electrical performance. If the trace width is too small, it will result in high resistance and voltage drop across the trace, leading to signal integrity issues.

On the other hand, if the trace width is too large, it will result in excess power consumption and heat generation. Therefore, it is important to keep the trace width as small as possible while ensuring that the circuit can function properly.

Factors to Determine Optimal Trace Width

A few factors need to be considered when determining the optimal trace width for a circuit.

The first is the current-carrying capacity of the trace. This is directly related to the cross-sectional area of the trace, which is determined by the width and thickness.
The second factor is the voltage drop across the trace. This is determined by the resistance of the trace, which is in turn determined by the width of the trace.
The third factor is the skin effect, which occurs when current flows through a conductor.

The skin effect causes the current to flow on the conductor's surface rather than through the entire cross-sectional area. This reduces the effective cross-sectional area of the trace and increases the trace's resistance.

It is generally best to keep the trace width as small as possible. However, there are some cases where it may be necessary to increase the trace width to meet the circuit's electrical requirements.

For example, if the current-carrying capacity of the trace needs to be improved, then the trace width will need to be increased. Similarly, if the voltage drop across the trace needs to be reduced, the trace width will need to be increased.

Ways to Reduce the Trace Width

There are a few different ways to reduce the trace width:

The first is to use thinner traces. This will reduce the cross-sectional area of the trace and therefore reduce the trace's resistance.
The second way to reduce the trace width is to use narrower traces. This will reduce the width of the trace and the skin effect.
The third way to reduce the trace width is to use multiple layers. This will increase the effective cross-sectional area of the trace and therefore reduce the resistance of the trace.

Keep the Trace Length As Short As Possible

Layout design is critical for printed circuit board (PCB) manufacturing. The trace length is one of the most important factors in the layout. If the trace length is too long, it will cause signal integrity issues, such as crosstalk and impedance mismatch. In addition, the long trace will also increase the inductance and resistance of the signal, which will degrade the circuit's performance.

Therefore, it is important to keep the trace length as short as possible during the layout design. In general, the trace length should be less than 5 inches.

Use Vias Sparingly

Vias are generally used to connect different layers of the PCB. However, too many vias will increase the inductance of the trace. Therefore, it is necessary to use vias sparingly. Generally, only one via should be used for every 2 inches of trace length.

Vias are small holes that connect different layers of a PCB. They're necessary for routing signals between other parts of the board but can also take up a lot of space. By using vias sparingly, you can save space on your PCB while ensuring that all of the necessary signals are routed correctly.

Use Ground Planes Whenever Possible

Ground planes are beneficial because they help reduce electromagnetic interference (EMI). They also provide a low impedance return path for currents flowing on the surface of the PCB. This helps to improve the electrical performance of the circuit. Therefore, it is necessary to use ground planes whenever possible.

Things to Keep in Mind While Using Ground Planes

When using ground planes, it is essential to keep the following points in mind:

Make sure that all signal traces are routed over a ground plane.
Be aware of the different types of ground planes (e.g., power, ground, control) and how they can be used to improve the performance of your circuit.
Ensure all vias are correctly placed, so there is no impedance mismatch between different planes.
Use proper decoupling techniques to reduce noise and improve power delivery to your circuit.

Keep the Trace Spacing As Large As Possible

Trace spacing is one of the most important things to consider when designing a rigid-flex PCB. Trace spacing is the distance between two conductive traces on a PCB. It is important to keep the trace spacing large enough to prevent electrical shorts and ensure reliable signal integrity.

The trace spacing is also crucial for the electrical performance of the PCB. To reduce the crosstalk between different traces, keeping the trace spacing as large as possible is necessary. In general, the trace spacing should be greater than ten mils.

Things to Consider While Determining Appropriate Trace Spacing

There are a few things to consider when determining the appropriate trace spacing for your rigid-flex PCB.

The first is the thickness of the copper foil. The thicker the copper foil, the larger the trace spacing should be.
The second is the width of the traces. Wider traces require more space between them to prevent crosstalk and ensure reliable signal integrity.
Finally, you need to consider the environment in which the PCB will be used. You will need to use a larger trace spacing if the PCB is exposed to harsh environments, such as extreme temperatures or chemicals.

Final Words

Flexible circuits, also known as Rigid-Flex PCBs, offer several advantages over traditional rigid printed circuit boards. They are ideal when there is a need for rigid and flexible circuitry in the same assembly. It is important to follow some basic layout design rules to get the most out of your Rigid-Flex PCBs.