PCB design guidelines are created as benchmarks for circuit design engineers to meet the industry standards. Following these guidelines will ensure better manufacturability and robust product performance.
Design guidelines are outlined to improve the testability and manufacturability of a product. They feature suggestions to improve signal integrity and electromagnetic compliance (EMC) of the PCB and thus enhance the overall performance of a product.
This article will outline various PCB design guidelines to improve the signal integrity of a PCB. Following these guidelines will help engineers in robust PCB manufacturing.
Signal integrity in PCBs
Signal integrity is the capability of an electrical signal to transmit from the driver to the receiver through a transmission line without any distortion. A distorted signal induces noise to adjacent signals on the board and reduces the overall efficiency of the circuit operation. The signal distortion becomes significant in the case of high-speed circuits and can impair the overall performance of the PCB.
Therefore, it’s important to design a PCB that is compliant with the necessary regulatory standards. A well-designed PCB with robust signal integrity will avoid any signal degradation due to attenuation, ground bounce, and impedance disruptions.
If the circuit design includes only low-speed signals, then there is hardly a signal integrity issues to manage. But in a high-speed design, signals get distorted due to shorter rise time requirements. So, there are certain signal integrity issues we need to understand to appreciate the recommended PCB design guidelines.
The process in which a part of the signal power transmitted by the source to the sink gets reflected back to the source over the trace is known as reflection. It causes oscillation and hence signal distortion. Whenever there is an impedance change in a circuit, there will be a reflection effect in the signal trace. That, in turn, adds overshoot and undershoot issues.
- Ringing, overshoot and undershoot
Ringing is a process in which an undesired oscillation of either voltage or current signal occurs due to the signal reflection in a PCB trace. If the value of the transmitted signal is more than the actual value in an ascending signal, then overshoot occurs. Similarly, when the transmitted signal is lower than the actual value in a descending signal, then undershoot occurs. All these processes distort the transmitted signal in a PCB.
In high-speed designs, adjacent signals that are closely routed can influence each other inadvertently, resulting in signal distortion. This distortion is mainly due to the coupling of either electric or magnetic fields in the PCB. Crosstalk can also occur between signals routed in adjacent layers of the board.
- Signal attenuation
A signal transmitted from a source to the load through a PCB conductor experiences a signal attenuation or energy loss due to the trace resistance and dielectric loss of the PCB. At high frequencies, signal attenuation is much higher and needs prior design considerations to handle the issue.
- Propagation delay and signal skew
The propagation delay of a signal on a PCB trace is the time taken for that particular signal to travel from source to load. It depends on the PCB dielectric constant and the trace geometry. Signal skew occurs in a group of signals when there are delay mismatches. It significantly affects performance in the case of clock and data signals in a circuit design.
- Ground bounce or simultaneous switching noise
When multiple components are switching between high and low states on a PCB simultaneously, voltage drops in power and ground paths. This results in a decrease in the voltage across power and ground pins of the components. There will also be a reduction in the noise margin and this may lead to false switching of the circuit.
PCB guidelines for signal integrity
Discontinuity in the line impedance results in a majority of signal integrity issues mentioned above. This impedance discontinuity occurs while routing the signals in situations like the branching of traces, splits in return signal path, and vias or stubs in the paths.
The guidelines to reduce signal distortion due to impedance mismatches are as follows:
- Provide correct terminating resistors at the source.
- Use smaller micro-vias to significantly reduce the signal distortions caused by vias and stubs.
- Keep minimum trace length for stubs.
- Avoid branching of traces and use appropriate routing topologies.
Design guidelines to reduce crosstalk effects are as follows:
- Using differential signals in applicable design sections will nullify the crosstalk effects.
- Minimize the length of parallel-routed signals.
- Space out the adjacent signals over a maximum distance allowed as per the routing guidelines.
- Ensure that the transmission line is close enough to the ground plane to avoid any unnecessary coupling of adjacent signals.
- Orthogonal routing of signals in adjacent planes can help avoid crosstalk to a great extent.
Below are design guidelines to reduce signal attenuation issues:
- Select low-loss dielectric material and optimum resistance traces to reduce signal attenuation errors.
- Usage of amplifiers and repeaters in the design can help boost the signal strength.
Following are design guidelines to reduce propagation delays and signal skew:
- Avoid substrates with a larger dielectric constant to help reduce the propagation delay of signals.
- By correct trace length matching, the skew of a signal bus can be minimized.
Design guidelines to reduce ground bounce and switching noise issues are listed below:
- Place power and ground planes close to each other while deciding the board layers’ stack-up.
- Decoupling capacitors are not optional and have to be implemented on the local ground.
- Better to place the decoupling caps near the component pins and use device packages with short leads if possible.
- Add necessary current-limiting resistors to avoid short circuits or overload.
The article has demonstrated how PCB design guidelines can help in improving the signal integrity of a circuit board. It involves a sequence of steps like substrate selection, stack-up design, component considerations, and layout design. Also, multiple simulation tools are available to analyze the signal integrity issues of a PCB. Thus, following all these guidelines meticulously will result in a PCB with improved signal integrity and a long-lasting performance.
Ken Ghadia is a sales engineer at Technotronix.
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