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Introduction
High speed PCB design is critical in electronics’ world of creating reliable, high performance circuits that maintain signal integrity across complicated pathways. We will look at and explore the basic building blocks, constraints and approaches of high speed PCB design. Using the fundamental layout rules and component parameters, we’ll achieve the optimum PCB efficiency.
What is High Speed PCB Design?
Today high speed PCB design is becoming more common. PCBs can easily deal with the high frequency signals through these designs. In computers, smartphones, and routers, you’ll find them. They are needed for any quick data devices. However, high speed PCBs require careful design in order to do well. These signals are high speed. EMI and signal loss can be caused by them. This is why layout and routing are an issue. The signal flow reacts to each component. With the emergence of 5G, AI, IoT etc these high speed PCBs will have to keep pace.
High Speed PCB Layout Design
Schematic
Careful and precise PCB layout planning is required for high speed signal paths. The first thing you work with is a clear PCB schematic. This is the first step in a design problem. The structure of your circuit is shown in a schematic. It makes every thing easy to follow and organized. The net, or ‘line’ is what connects specific parts. Make these nets clearly for accuracy.
PCB Libraries
Layout design becomes faster and much easier in PCB libraries. Footprints are stored for each component of your circuit. Pad placements and part features are shown in each footprint. All footprints are checked for accuracy by librarians. Each one of them is verified and reliable.
Components
Key parts such as memory and Ethernet chips are where you want to start. There are guidelines for following floorplan and datasheet. This maintains signal flow clean and uncomplicated. Also add parts like crystal oscillators, capacitors, resistors, etc.
Termination Resistors
On high speed PCBs, early termination resistors need to be placed. And these resistors control signal flow, so squeezing them in last can be a problem. To terminate in parallel, put a resistor adjacent to the receiver pin. It stabilizes signals and decreases reflections. For series termination, literally stick the resistor right after the driver pin. It avoids extra stub length as well as clear signals.
Component Orientation
And of course, be sure to line things up the same way as other similar components. Mainly it will let you wire connections easily. During assembly it also makes soldering easier. Components fit better when they are on the same axis.
SMT and THT Component Placement
All surface mount devices (SMDs) must be placed on one side. That keeps your layout clean and simple. We recommend putting all through-hole components on the top side. With this setup, you save time, and you cannot make mistakes. Your PCB design is easier to work with if your layout is clear.
Analog and Digital Circuits
To best design your PCB separate intern analog and digital circuits. That means that their ground paths should not be mixing. The signals can become weak if they do.
Ground Plane Isolation
Strictly speaking, this is important for clean signal flow, as many of the sample concatenation schemes suffer from aliasing or losses due to phase jumps at the sample delays. For use outside of the board, USB and RJ45 connectors are used. Noise is avoided if the board is kept ground separate.
Surge Suppressors
Protect your circuits using surge suppressors. And they stop voltage surges from causing damage. Put these suppressors close to the signal in connectors. It makes them ready to work and remain effective. Do not use vias to transport the suppressors. In fact, inductance in vias can be undesirable, as it can cause performance degradation.
High Speed Routing
The copper traces of a PCB are joined by routing. These traces are defined to connect components which are linked based on circuit netlist. A successful board is about good routing. It influences the design and guarantees that signals work well. PCB routing is made more difficult by high speed signals. Follow those high speed routing practices to help. Follow the guide below to get to a great design. With your components laid out, you have a simple design for routing. But you should expect that your design needs to be adjusted, anyway.
Vias and Microvias
Moreover, trace lengths and via sizes need to be considered. Size and depth of the vias are also important. Sometimes better performance can be afforded from smaller vias. They also lower signal loss and maintain integrity. Placement is something you will choose in via sizes. Causes the vias to be placed close to their pads. It helps keep extra capacitance away, which could be a problem. They can be even placed on or inside of pads.
Through-hole
They are very important in PCB design. The current goes in a hole that goes from on layer to the other. You can have two types of through-hole vias: PTH and NPTH. Plated through hole (PTH) is the term. That means there’s metal plating inside. It is inseparable from the top and the bottom layer. Non-plated through hole refers to NPTH. There is no metal plating on this type.
Blind
These holes allow for one layer to connect to an other layer. Unfortunately, they don’t go through the whole board. You can use the top layer and apply it to a middle layer. You can also connect the bottom layer to an interior layer. After making the board the other end of the hole is hidden. Hence why they are called blind vias. They save space on your PCB. Use blind vias instead of normal ones so that it is more compact.
Buried
The PCB has vias inside. The inner layers of the board connect with them. You cannot see them from outside. The vias do not reach into the outer layers. Buried vias help reduce clutter. They allow more room for traces on the surface. Violas according to IPC standards should be small. The diameters must be 6 mils or less. The size allows the board to remain intact.
Microvias
These are really tiny little holes in PCBs. Manufacturing creates them with lasers. Regular vias have much larger diameters than Microvias. High Density Interconnection (HDI) PCBs almost always use these vias. Usually, microvias are two layers deep or below. This is actually because trying to add copper plating inside its cylinders is tricky. The smaller the via size, the stronger is the requirement for plating.
Via in Pad
A via-in-pad is not like a regular vias where the trace goes away from the pad to prevent the solder paste from leaking while soldering. The pad is covered with nonconductive epoxy first and then capped and plated to leave the surrounding pad area intact. This design also shortens the signal path to minimize parasitic inductance and capacitance, letting smaller components fit onto the PCB and is ideal for BGA components. Back drilling is often used to remove all of the via that is not removed, to reduce signal reflections, which can cause the signal to lose performance.
What to Consider in Designing High Speed PCB
To design high speed, we resort to rapid digital signals. Such signals make components easily and quickly share data. These are what you should consider for a high speed PCB:
Stackup and Impedance
Layer count is one thing that you should be thinking of when designing your PCB stackup. So you need enough layers to carry all your digital signals. Some methods require math as well as experience with high speed boards. Board size can affect large ICs with BGA or LGA foot prints. In terms of BGA, you can usually get two rows per signal layer. Also do not forget to count lines for power and ground planes. In addition to shortening your routing cycle, your stackup will determine impedance. All the stackups have layers for high speed signal, power and ground.
Board Size and Net Count
So when you’re designing your high speed PCB, you want to also consider the size of the board. How big do you want it? A big difference is the size. The bigger the board the more you have to work with. It’s easy to spread your components out. Since that extra space makes things smaller it makes routing nets much easier. Next, consider the net count. How many of those do you have to connect? The more nets you have, the more connections you will have. Less layers of things help keep things neat and less layers.
Routing Density
If you have a high net count, stuff gets tricky. The small board isn’t much space to work. But you might not have much room for routing on the surface. When this happens you need more internal layers. This will render drift closer together, which makes routing harder. It also helps you push the routing density that you can achieve on a smaller board. In this case, you have to plan. Keeping everything together in one place is key. As you design, you want to avoid the problems.
Number Of Interfaces
You may just want to route one or two interfaces per layer. It all depends on your bus width and board size. As with so much with DNN applications, keeping signals on the same layer helps a lot. That brings consistent impedance across all signals. This is also important in order to reduce skew. Skew can cause timing problems in a design. So, go for fewer interfaces per layer.
Low Speed and RF Signals
These signals may occupy surface layer space. This could be a better use of this space for high speed buses, or for important components. The addition of these signals could require an extra internal layer. Of course, adding more layers can add to your board cost and complexity. So, plan carefully. Additionally, low speed signals are noisy. With a certain impact, this noise may affect high speed performance. Hence, isolating these signals properly.
Power Integrity
It makes sure you get a stable power to your circuits. Each voltage level will be used with large power and ground planes. Put these planes on layers close by. It also enhances your capacitance. Stable power delivery is enabled by high capacitance. Don’t forget to have decoupling capacitors included. They help smooth out the voltage.
PCB Material Options
Most electronics use FR4 as a materials in their PCB. But it doesn’t do such a good job at dealing with high frequency signals, especially microwaves. There are things like PTFE, which work with high frequencies, or Rogers materials. Below is which material works best for the categories.
For Normal Speed and Loss
For normal speed PCB, FR-4 is the only material that is there. This is a widely used and easy to find type of move. But it isn’t perfect. Dielectric constant changes a lot with frequency. It also has more losses. For only a few GHz applications it works well because of this. Isola 370HR is a common example of this material.
For Medium Speed, Medium Loss
They provide better results then normal speed materials. Across frequencies, the dielectric constant of their material is flatter. That means less variation and both less load and better performance. Moreover, because their dielectric loss is about half that of normal speed materials, lower power is dissipated. You can use these materials up to around 10 GHz for the signals. A good example is Nelco N7000-2 HT.
For High Speed, Low Loss
These are great materials for PCBs. Its dielectric constant curves are flat. In other words, performance at other frequencies will improve. In fact, they also have low dielectric loss, which is a plus. Less loss equals less unwanted electrical noise. So your signals remain clearer and stronger. Isola I-Speed serves as a good example.
For Very High Speed, Very Low Loss
For these applications special materials must be used. RF and microwave use these materials at the top of the line. Their dielectric constant curves are the flattest. They do well across many frequencies. They are also the least dielectric loss. That keeps your signals strong and clear. These materials can be used for up to 20 GHz applications. Isola Tachyon 100G is an example of such material.
Layer Count and Thickness
In designing a high speed PCB, layer count and thickness comes to mind. You need enough layers for all your digital signals. Some math and some experience to help, that is. However, BGA or LGA footprint ICs can be large ICs that can change board size. For BGA fanout you can fit two rows of signal layer per row. Also don’t forget about power and ground layers. Layer count can be cut down by microvias, as they save space. They also allow for more routing density. This also makes your design more efficient. They are smaller for microvias and also improve their processing power. Without those parts, your devices would be much clunkier and less innovative.
High Speed PCB Design Challenges
Trace Length Tuning
Adjusting trace lengths is needed to sync signals. So if the length is off, the interface might just fail. That is especially important at high frequencies. For parallel interfaces, consider only trace lengths. But space can be cramped for adjustments. In serial interfaces, signals are grouped in differential pairs. Tuning becomes more complex because of this. One has to be close to the other. In other words, the length differences must fall within certain limits. Signal Quality can be affected by misalignment. In fact, also keep in mind delays on CPUs or FPGAs.
You must to watch single ended and differential impedance. Individual tracks are at single ended impedance. For paired tracks differential impedance. Standard impedance values are associated with each type of interface. There are some less common types of impedance as well. Differential impedance is half odd mode impedance. If the signal is identical it is called common mode impedance. Common mode value is half the even mode value. However, it is worth noting that these values are rare, but are worth having. Signal reflections can also result from wrong impedance. Including this can damage signal quality causing noise, as high power is wired through these devices.
Track Shape and Bends
Most often you never have straight tracks from source to receiver. You often have to bend the tracks. The corner options are best, rounded. Problems arise from sharp bends. It gets time to lay down great tracks. For each track you might draw each track many times. Don’t use 90-degree bends, they cause problems. Bends change the track width. And this can affect impedance, and this can result in reflections. It improves uncoupled length for differential pairs.
Termination
One particular method is parallel termination where differential pair of tracks are placed between a resistor. It should be at the end of the line. It should be close to the receiver. This reduces signal reflections during proper terminations. This enhances quality of data transfer. For differential pairs the resistor value should equal. Low resistor value can cause signal’s over termination. It hurts the signal quality. For some ICs there is built in termination resistors. If they don’t, you don’t need external resistors. The use of them can lead to over-termination.
Grounding and Vias
There is typically no way to trace signals on a single layer, so you often need vias to connect traces across PCB layers. However, this can be an issue if the vias are not placed next to signal vias since the ground reference would then be disrupted. In addition, a too large amount of vias may cause overheating by high current density. Additionally, there is sufficient space between vias. However, if vias are not designed properly, they can introduce impairment of signal integrity through impedance problems.
Placing Components
In close quarters, it’s tricky to group the similar components. Keeping digital and analog components in separate ground areas is difficult, and they must be assured to remain that way. It also complicates placement because you have to have enough space for short traces. For another, components need to be kept away from interference sources, such as power converters. Further, the placement of high speed components too close to board edge will not suffice to provide a good signal quality.
Ground Polygon Placement
Do not route tracks over cut outs or splits. Extra noise and signal delays can be created by this. You could also find yourself plagued with integrity problems and poor signal quality. For all tracks that cross polygon splits, use ceramic stitching capacitors. It also reduces effects back on the signal.
Crosstalk Management
It can represent a problem in high speed PCB design. A signal on a track will affect neighboring tracks. That usually happens when the tracks run parallel for a long daylight, especially it’s mountains. That crosstalk is worse with the longer parallel section. But one way to combat crosstalk is to make tracks at least three times wider apart from each other than they are wide. For differential pairs, maintain a distance of five times their width apart. The distance should be 8-10 times their width if the differential pair sends a clock signal. Keep asynchronous signals away from high speed tracks, paying them a lot of attention.
High Speed PCB Design Guidelines for Component Placement
When designing high speed PCBs, limitations of standard components must be considered as well as guidelines of how to place components in them most efficiently so they perform optimally and are as reliable as possible.
Ensure Clear Signal Return Paths
As you can imagine, it is important to have clear space for ground plane signal return paths. By minimizing signal degradation, it also helps signals return efficiently to their source resulting in an improvement of overall signal integrity.
Provide Adequate Channel Spacing
Make room for dense data and memory bus signals to at least allow for routing. Proper separation helps eliminate crosstalk and interference, necessary to keep high speed data flowing.
Separate Analog and Digital Areas
Analog and digital components, should be physically separated to prevent interference. The separation created allows the digital noise to be minimized over the sensitive signals, resulting in improved performance of both circuits.
Shorten High-Speed Signal Paths and Trace Impedance
Position components strategically so high speed signal paths are shortened. Short paths help reduce trace impedance which improves signal quality, and speeds up data transfer rates.
Follow Manufacturer’s Datasheets
You always refer to a manufacturer’s datasheet for specific placement instructions and guidelines. The insights and recommendations in these documents directly relate to the components you are using and are meant to make them perform their best in your design.
High-Speed PCB Design Guidelines for Routing
In high speed PCB design, routing a high number of routes to a design can affect signal integrity and performance greatly when not done properly.
Fully Engage Design Rules
Full implementation of design rules and constraints such as line lengths, spacing, and differential pairs is important.
Establish Area Rules and Keep-Out Zones
So set up any necessary area rules and keep out zones that meet your specific routing requirements. It avoids interference from the other side and looses the disruptive signals from the other side of the board.
Prioritize Short, Direct Routing Paths
Try and work with your routing paths as direct and short as you can. Signal delay is reduced and interference is minimized, resulting in improved overall signal high-speed signal performance when using short paths.
Avoid Routing Over Ground Plane Voids
You should not pass tracks over voids or breaks in the ground plane. Such issues can generate electromagnetic interference (EMI) or impedance mismatches and kill signal quality.
Provide Extra Clearance
Provide the clock lines and differential pairs with extra clearance from other routing. The optimum line of separation (given by adequately large dielectric thickness between the signal and reference layer) is 3 times the dielectric thickness.
Assign High-Speed Transmission Lines
Route their designated layers on high speed transmission lines, then route their return paths one layer on the adjacent reference plane.
Minimize Layer Changes
In high speed lines, avoid changing layers. If layers change, then add return path vias to minimize current loops and keep return current path as direct as possible.
Avoid Parallel High-Speed Lines
Running high speed transmission lines in parallel can lead to unwanted crosstalk between the lines.
Prevent Broadside Crosstalk
Broadside crosstalk between dual stripline traces is watch out for, especially where there’s less spacing between dual stripline traces than between traces on the same layer that run side by side.
Utilize Wider Traces
This will also reduce impedance and provide help with improving signal quality. But make sure that this approach is okay in your design requirements.
Minimize Vias
To lower inductance, stay within signal integrity constraints, only use the minimum number of vias you can. Most of the time, vias are not necessary, but if they are, blind, buried, or microvias can be used to reduce signal reflections.
Avoid Dense Clusters
Dense clusters of escape vias can make it hard to clear the return paths on the ground plane, and so should be handled with caution. This keeps the overall signal integrity clean.
FAQs About High-Speed PCB Design
When Is a Printed Circuit Board Design Considered High Speed?
A high speed PCB will have connections such as HDMI, Ethernet and USB to facilitate fast data transfer. It’s high speed as well if the circuit has many sub circuits that are linked through high speed interfaces. The track is at least one third the rise time of the signal. Furthermore, if digital signal frequency is 50MHz or more, it is high speed. If the PCB is small and you can’t fit any components then it’s a high speed design.
Why Add Bypass Capacitors to Ground?
They are a path for common mode return currents to be returned to the source. Helps to complete the current loop efficiently. This is where the usefulness of mid-point grounding comes in as you won’t want to disrupt differential current flow. Some of these currents return through stray capacitance. In differential systems, the capacitors are required even to eliminate stray currents.
How To Know If You Need a High Speed Design?
To know if you are in need of a high speed design, you check a few things. Check to see if your board has high speed interfaces like DDR, HDMI. If it does you have to follow some special rules of the design. Then, you need to examine the length of your traces with respect to the same of the signal wavelength. If they are the same, most likely you need a high speed design. Finally, if you have antennas and wireless interfaces on your PCB, you’ll want to route to high speed signals.
Conclusion
The following key practices for designing high speed PCBs were covered on this PCBTok blog. It listed component placement, routing tips and clear signal paths. It also dealt with some common challenges and suggested means to enhance performance and signal integrity in high-speed design.
