The use of highly complex and intricate high speed PCBs is becoming more and more common and widespread across a variety of applications and fields. But while making high speed PCBs may have become a frequent activity in recent days, designing highly successful ones is still a feat that eludes many.
There are many elements in a high speed PCB layout that have the potential to disturb the performance of the signal and impair the signal transmission and propagation due to internal interference. These include the likes of crosstalk, electromagnetic interference, mismatched impedance, signal reflection, and ground bounce among others.
In this article, we will help you familiarize yourself with these persistent high speed PCB issues and give you some handy tips on how to deal with them effectively and successfully.
The process of building a good high speed PCB layout starts with the designing process. You have to be careful when drawing up the schematic and take many different possibilities into consideration. Add the details and maintain a smooth flow of the signal throughout the circuit. You want to highlight already in the schematics phase the critical and high speed signals and attached attributes such as trace lengths, add the value ranges, plot the terminations, show the impedance lines, and identify any potential areas that may produce noise. Some tools will show those attributes along the design phase and help support the layout phase.
The Right Material
Once the schematic design is ready to go, you need to select the right material for your high speed PCB. Each material or substrate has a different dielectric constant which has an effect on the impedance of a given transmission line. A lower relative dielectric constant means faster signal propagation in that material. Another factor to consider is the loss tangent which has the capacity to affect high frequency signal propagation. FR-4 and GETEK are two of the most commonly used substrates or materials for high speed PCB layouts.
When building a PCB, you will not get a guideline that states exactly how many layers you can add to it. Ideally though, you may be using a four layer PCB. it should have two signal layers accompanied by a GND and a VCC plane. This is in case of microstrip traces. If stripline traces are being used for a more complex circuit, then a six layer stackup is also recommended to reduce propagation delay and characteristic impedance. In a high speed PCB setup, you need to have at least one reference ground plane for the microstrip traces with sensitive signals. You should contact your PCB manufacturer to get some guidelines for more than 8 layers stackup.
Power and Ground Plane
Ideally, your high speed PCB should have a complete ground plane as well as a complete power plane. You should have a separate layer and ground plane for every regulated voltage you are using in your design. Instead of piling on the layers, some people choose to go with split ground planes. Unfortunately, these can produce radiations, create loop areas and block the flow of recurrent current, and give rise to crosstalk between signal lines.
Using Traces and Vias
Traces and vias are important components of a high speed PCB layout.
If you are using right angles in a trace, the capacitance in the corner of the angle increases which changes the impedance and produces reflections of current which can disrupt the signal transmission. Following are some methods you can use to prevent such disturbances:
On the other hand, the use of vias can increase the trace length and it can also cause impedance changes because of added inductance and capacitance. Improper placement of vias can also create pockets of loop areas for current.
Maintaining Signal Integrity
When traces are located right next to each other, they can unnecessarily and automatically be coupled in induction and capacitance. This sort of noise between parallel traces can interfere with other operations in the circuit which is why they need to be diminished or eliminated to maintain the integrity of the signal.
The key to reducing crosstalk between parallel traces is to increase the distance between them. Here are some techniques you can use to accomplish this:
Reducing Ground Bounce
Ground bounce is a board level phenomena. These are essentially higher transient currents that are produced in the outputs by the discharge of load capacitances of the devices. The high current change on the signal causes fluctuations in the voltage. This issue is a consequence of faster devices that have decreased output switching time. In FPGAs, there are a number of different factors that impact the level and intensity of ground bounce produced in the chip, including the load capacitance of the devices, the inductance of the socket, as well as the amount of switching outputs used. Since the origin of ground bounce is multifactorial, it is difficult to predict its magnitude in the given PCB and only a relative approximation of the impact of each parameter can be made.
Following are some methods that can be used to reduce the amount of ground bounce produced in your high speed PCB:
Reflection of signals in the lines due to mismatched impedance is another issue that can occur with high speed PCBs. This causes a ringing at the load receiver and also reduces its dynamic range. To prevent this from happening, we need to reduce or eliminate the mismatch in the impedance, meaning the source and trace impedance must be equal.
The impedance mismatch can be eliminated by adopting certain best practices in your termination scheme. Here are some tips to follow depending on which type of termination scheme you are using on your PCB:
PCB level transmission lines can also cause trouble at higher frequencies due to mismatched impedance.
This impedance depends on the width and thickness of the trace, the dielectric constant of the material used for the PCB, and the height between the reference plane and the trace. Between the two types of layouts- stripline and microstrip- a typical stripline layout PCB has a slower rate of change with trace height above GND. It also has a signal that is bound between two layers of FR-4 material, while a microstrip layout PCB has a single conductor in the open air. Since a stripline layout has a higher and more effective dielectric constant, the dielectric span needs to be greater here as compared to microstrip layout PCBs to achieve the same impedance.
When it comes to the click distribution, it is possible to create a number of signal reflections that disrupt the optimum functioning of the high speed PCB because of the branches and trace lengths. Differing trace lengths can produce delays which also impairs the functioning of the board.