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In your low-speed PCB designs, selecting the right components, getting them all placed on your printed circuit board layout, wiring everything up with traces, these seem easy, since the signal transfer is in low-speed PCB design is “ideal”—signal has no loss.
However, if you want to have a high-speed PCB design, it will be a complex story, signal integrity, crosstalk and reflections,.etcall these factors matters.
High-speed PCB design refers to any PCB design that signal integrity begins to be affected by the board physical characteristics, such as layout, case, layers stack up, interconnections, etc… If you start designing a board that encounters problems like delays, reflection, crosstalk, or emissions, so congratulations, you’ve found yourself in the world of high-speed PCB design!
When it comes to high-speed PCB design and PCB layout, the high-speed application is no longer about just designing the quickest, most efficient way, or simply meeting mechanical housing requirements.
We at Venture high-speed PCB design team take the position of every processor, memory module and connector into account, and carefully determine the interconnect design between them, we make sure our high-speed PCB design is done in a streamlined and efficient manner.
Your Leading High Speed PCB Design Supplier in China
A successful highspeed PCB design requires integration of the device, PCB, and other elements into the design. To achieve high-speed operation, the design of the PCB must achieve:
- Minimum on-board noise generation
- Minimum cross-talk between traces
- Reduction of ground bounce effect
- Impedance matching by the proper setting
Venture high-speed PCB Design team has the industry leading design engineers and 10 years’ high speed PCB layout experiences, from material development and production, circuit fabrication to final component assembly.
Venture has a full selection of high speed PCB design software, such as Cadence Allegro\ORCAD, Mentor WG\PADS, Protel99\ Altium Designer (AD)..etc. Please let us know which EDA tools you want to use in your project, our most frequently used software are Eagle, Altium Designer, Protel(Protel PCB design), PADS.
No matter you are an electrical engineer, a product designer, a system integrator or a maker looking for a high speed PCB design, Venture design & Layout team will be here to help you develop your high speed PCB design projects.
Through our 2 hours rapid response services from our 24/7 sales and tech support team, and excellent after-sales service, we will be your expert high speed PCB Design partner in China. At Venture we can answer any high speed PCB Design questions that you may have, pls feel free to contact us anytime.
High-Speed PCB Design – The Ultimate Guide
Let’s say on various occasions you struggled to achieve practical specificiations for your high speed PCB design.
You formulated a plan, transferred it to layout, follow all the necessary protocols; unfortunately, it fails the ultimate test.
That sounds so annoying.
What could be the problem?
Solutions to that scenario are an overview of what you’ll come by in this guide. What’s more, at the end of this guide, you will be an expert in High-speed PCB design.
Now that’s indeed a collection of vital information you shall ever find in a single guide.
We should get going.
High-Speed PCB Basics
Under this segment, I will take you through the following concepts: definitions, classes of high-speed PCB, and finally, parts of the high-speed PCB.
So What is a High-Speed Printed Circuit Board?
First, I have to mention that there isn’t a specific definition of what a high-speed PCB is. However, the good news is that we can tell what they are by considering some perspectives, especially in the PCB designing process.
Even with such considerations, there’re a couple of reasonings put together to identify what it takes to achieve a high-speed PCB.
High speed PCB
We shall briefly look at some of them.
So what distinguishes a High-speed PCB from any other type?
To understand what makes a PCB to be ‘high speed,’ we look at some three fundamental factors.
Case 1 of High Speed Printed Citcuit Board Definition
First, you will know you’re dealing with a high-speed PCB from the level of frequency involved. Note that, the frequency often affects the performance and the signal integrity of the circuit. Consequently, frequency is a critical aspect of such a PCB.
So, therefore, mostly any circuit with an excess of 100 MHz up to 1000MHz rightfully belongs to a high-speed class although those of high values belong to ultra-high speed.
Any other lower values belong to low speed ( ˂ 25MHz ), moderate (≤ 100 MHz).
Case 2 of High Speed Printed Circuit Board Definition
The Second aspect is the traces guideline.
Traces in PCB – Photo courtesy: MacroFab
How do you put in the traces to notice such circuits?
The precise way to do so is to analyse the sizes of your traces. As a result, a high-speed PCB would have a trace which exceeds a third the rise time of the switching speed for the device you’re working.
Case 3 of High Speed Printed Circuit Board Definition
Finally, we look at the separation distance. During the designing, our target PCB class should be a large unit circuit but subdivided.
Above all, you’ll notice that each segment works distinctively.
Now that’s an honest thought for definition.
We can still look at another one.
For this case, it considers a high-speed PCB to be one that experiences signal strength issues.
Now, with those considerations, it’s interesting to realise that this circuitry also exists in some classes. Next, we look at these sub-classifications.
Types of High Speed Printed Circuit Board
A variety of high-speed PCBs is available in the market, though they all serve the same function.
Some fundamental differences would be in the manufacturing process, together with their structural design.
Therefore, such forms of PCBs are as follows:
- Single sided circuit boards
- Double-sided circuit boards
- Multilayer boards
Taking a close look at those classifications, you notice one thing; it’s the difference in terms of several layers. That’s right.
So before indulging in the analysis of each, it’s worth mentioning that each PCB has got some primary sections which in turn differentiates it from the rest of the group.
- The conducting layer, i.e., the conductive material.
- Dielectric film.
- Insulator material.
- Plated/ Non-plated through holes. Etc.
As we get into discussing each type of high-speed printed circuit board, you should be capable of spotting the difference among them based on those parts we just mentioned.
·Single Sided Circuit Boards
Single sided PCB is undisputably the purest form. Why?
It only has a single layer of conducting material on the dielectric film.
The unit side has got a share of its consequence; the designing process entails fixing the components of the boards on one side.
Elsewhere, the circuits lie on the other side. It was conventionally the earliest form initially used in the old generation PCBs.
Due to their structure, these types have got non-plated through holes/vias on their boards.
Non plated through hole in PCB – Photo courtesy: Research Gate
The main limitation of this circuit is probably because you only have got to execute all essential functions from one side precisely due to the one conductor layer. Thus it’s non-cross permitted.
You could easily design this kind of circuit by screen printing or even network printing.
Single sided PCB
Importantly, for your fabrication needs of this form of a board, you will require FR4 Fiberglass laminates, aluminium, copper, among others.
·Double Sided Circuit Boards
This type, however, has got double layers of conductive materials. Most often copper makes the conductive material for such circuits.
So basically, this form of PCB has copper embedded onto both sides of the laminates.
Also, between the conducting layers, there is a fixed insulator for separating them.
Plated through holes also complement their structures to connect the two sides of conducting layers.
Double-sided PCBs are popular in circuitry industry since they are very suitable for designing interconnections.
Double sided PCB
A few customisations are in order when designing this kind of circuit to suit customers preferences. Such include: eliminating the holes, covering both/ none/ single sides, etc.
·Multi-Layer Circuit Boards
The design of the Multi-layer circuit boards incorporates the inclusion of more than three conductive material layers.
In between these conductive layers, insulating sheets separate them appropriately.
Notably, the outer covering layers is optional here depending on your fabrication.
Most manufacturers fabricate multi-layer boards by combining both single-sided circuit boards with double-sided circuit boards into a unit circuit shield.
Multi layer PCB – Photo courtesy: The Engineering Projects
Each of these several layers requires firm attaching by fitting with plated through holes. Laminating the circuit is also another option.
The multi-layer boards are increasingly gaining popularity due to the several benefits associated with their usage. Such advantages are limiting crossovers, reducing crosstalk, and regulating shielding.
With these three major types of high-speed PCBs, you must be wondering now what comprises them. Indeed, they all have several segments that work together to bring out the right circuitry.
Next, we look at these parts.
Parts of High Speed Printed Circuit Board
The manufacturing of the high-speed PCB involves the utilisation of several materials ranging from simple to complex ones.
This process results in the establishment of several parts of the boards.
The main parts of the circuit boards are as follows.
- Conductor layer
- Solder mask
Note that I don’t dispute the existence of any other part not in the list above. The reason is that to date, most manufacturers have got the tendency to customizing their PCBs to meet a specific requirement.
Let’s briefly discuss these parts.
i.High Speed PCB Substrate
The substrate is a general term that we can use to refer to all the materials and structures that cover the conductor layer.
High speed PCB substrate
Now the substrate material is as essential as any other part of the circuit for apparent reasons. First thing you have to know is that copper material is an excellent electric conductor. Nevertheless, it readily reacts with oxygen by oxidisation process resulting in the formation of a thin brown layer of material on its surface.
For that matter, to prevent such reaction, any exposed copper surface should be fully covered. The appropriate covering material for use here is the substrates and cover overlay.
Secondly, the substrate material is also responsible for either the flexibility or the rigidity of the PCB.
Rigid and flexible PCB
To achieve such a coating layer or the covering, you will require gold or solder.
These two substances, i.e., gold and solder, are highly preferred due to two significant reasons.
One, they are all good conductors of electricity besides being highly durable and being capable of withstanding harsh environmental conditions such as rusting, high temperature, etc.
The common substrate materials include fiberglass, FR4, amongst others.
ii.Conductor Layer in High Speed PCB
Before you decide on the number of the conductor layers for your PCB, you should know that your choice will depend on quite a few factors.
Your Type of applications, the costs involved, and the amount of space for your circuit are some of the factors that would influence your number of conductor layers.
In most cases, the user chooses the layers of the printed circuit board that is suitable for them. Hence, the need for construction of various numbers of layers.
Furthermore, the critical conductor material layer extensively used is copper.
Copper is the number one recommended material because it’s cost-effective, thereby offering you value for your money.
PCB – Photo courtesy: Science Direct
The size and the respective thickness of copper may vary, i.e., 9 µm, 12 µm, 18 µm to 356 µm accordingly.
Besides, the typical copper, other varied forms of copper are also appropriate; some of these forms include beryllium copper, half-hard, rolled-annealed, and electro-deposited.
The simplest of the layers in terms of construction, designing and for frequent use being the single layer PCB, which in turn brings about a single conductor layer.
On the other hand, enormous and complex circuits would require the use of either double layer PCB or multilayer PCB in comparison to single sided ones.
In fact, with the advancement in technology, it’s possible to connect several conductor layers up to 10 to 12 layers.
The most critical thing is effective communication among the components of the several layers.
Otherwise, there exist other conductor materials apart from pure copper material or other forms of copper.
The top materials on this second priority list are carbon, silver ink, Inconel, constantan, and aluminum.
iii.Solder Mask in High Speed PCB
The solder mask is also another additional material layer that forms an outer cover for the copper layer.
Solder mask coating – Photo courtesy: Wikimedia
For most applications, it’s green in color although it can be of any other color.
The solder mask also serves very significant functions such as providing an insulation layer to the copper traces against other conductors. Specifically, the insulating layer shields the pads from coming into contact with other conductors.
Now you can see that solder mask is a must-have a layer.
iv.Silkscreen in High Speed PCB
The silkscreen is the last layer that you will find on the circuit board.
The silk layer is also critical since it’s on this layer that the characters, writings, and symbols appear.
All these forms of art help the user in identifying the role of every component of the board.
Usually, epoxy ink is worthy for screen printing.
Importantly, you can optionally use the silk layer for the top and bottom respective layers of the PCB, but according to the user preferences.
In the case of the bottom and the top layers, it’s referred to as silkscreen bottom and silkscreen top respectively.
Pad is a piece of copper material protruding on the surface of the circuit board.
It’s on this pads that provide a port for mounting the component leads and eventually soldering.
So technically, the pads provide mechanical support to all the PCB components.
The beauty of the PCB owes to the fact that it eliminates the use of conventional complicated wire systems connections.
PCB Trace – Photo courtesy: PCB Calculators
As such, in PCBs, components aren’t connected with the use of wires but with the help of conducting materials, specifically copper.s
Apparently, the copper in use, in this case, is smooth and appears in the form of a line on the circuit board surface. This type of copper is the trace.
The top layer of the PCB is typically the outer layer you quickly see. It’s on this layer that you mount the PCB components.
PCB Layers – Photo courtesy: PCB Way
Additionally, this layer is often colored green.
Unlike the top layer, the bottom one is identifiable because of the holes and pads on it. For that reason, soldering of the PCB components takes place through the hole.
It’s worthwhile to stipulate that you could coat the top layer or/ and bottom layer of the PCB green.
There you have it — the parts of a high-speed PCB.
With those background information about the high-speed PCB, we move into a more industrial segment of our discussion.
You can’t afford to miss our next discussion.
High Speed Circuits Design Basics
In this section, we shall focus on the designing phase of these circuits. We shall peg our discussion on several issues about the designing process.
To kick off our evaluation, let’s begin from the very basic concepts.
What does high-speed PCB design entail?
Basically, a high-speed PCB design is any designing process in which the:
- Physical characteristics
- Structure, and
- Configuration of your circuit boards affect the integrity of the signals.
High speed PCB design
The physical characteristics of the circuit boards we are talking about here may include the PCB layout, interconnections, packaging, layers, etc.
Besides, the kind of issues that affects signal integrity in high-speed PCBs are:
Now, all these factors are the ones you will come across in the world of high-speed PCB design.
Significantly, it’s due to all these circumstances that make high-speed PCB design a unique type of venture.
Actually, you would readily realise that a standard PCB design doesn’t involve consideration of such little issues but solely revolves around component placing and routing.
The difference is quite evident when dealing with a high-speed PCB design.
You will devote all your energy and resources in making an accurate analysis of the:
- Proper location of the traces
- Width of the traces
- Proximity to other signals
- Types of parts they form a complete connection
Now the bottom line for the high-speed PCB design is the full inclusion and considerations of the signal integrity issues.
The fancy part of high-speed PCBs is that all of them have to transfer signals along the copper traces to a pre-determined destination.
High speed PCB design – Photo courtesy: Autodesk
Of course, the signals could be an analog signal or digital signal.
Analog signal vs digital signal
Briefly, I will differentiate the two.
A digital signal is only limited to digital systems, and it has two modes of representation of values, ie, high point and low point, or on and off.
Commonly, a digital signal is otherwise known as a clock signal or a square wave.
On the contrary, an analog signal has got a distinguishable range of both positive and negative values. Implying we can describe analog signals variedly based on the strength and frequency of the signal.
What does it take to achieve a high-speed PCB design?
Your focus should be on the following:
- Minimum on-board noise generation
- Minimum cross-talk between traces
- The decrease in ground bounce effect
- Concurrent switching noise under operation
- Correct signal line termination
- Impedance matching
To achieve all these, you need to focus on the following:
1)High Speed PCB Board Material Type
The kind of material for constructing the PCB always play a significant role. Specifically, the dielectric material determines the quantity of noise and cross-talk emanating from the rapid switching I/O signals.
Each dielectric material has a unique value of dielectric constant. Therefore, every PCB substrate would, in turn, have a distinct relative dielectric constant.
The dielectric constant is a particular parameter that any PCB engineer must get right. This because it’s this setting that affects the impedance of a transmission line.
Consequently, signals can propagate quickly in materials with lower dielectric constant.
There yet exist other factors that affect signal performance and noise separation; such considerations are transmission line effect and modeling. The typical transmission line layouts are microstrip and stripline.
2)Crosstalk in High Speed PCB Design
Crosstalk is the troublesome circumstance in which the coupling of signals between parallel traces occurs.
Cross talk in PCB Design
You can minimise crosstalk by properly routing and staking-up layers through the microstrip and stripline layouts.
3)Filtering the Power Noise in High Speed PCB
In some cases, the power supply may produce some low-frequency noise, usually of 1 kHz and below.
You could reduce such kind of noise by filtering it at the section whereby the power connects to the PCB as well to every gadget.
With all that information, we move onto the next part of our discussion.
High-Speed PCB Design Guidelines
When it comes to high-speed printed circuit board design, you will consider the following steps:
1.Make a schematic plan for your High-speed Design
A schematic is typically one of the stages that most people often forget about yet very critical in the design.
Schematics is a conventional way of communicating connectivity techniques and issues into the circuit board. So without it, then you could potentially be in for a rude shock together with frequent setbacks during the designing process.
And that’s where the schematics come in; the plan offers you a suitable way of organising and bringing forth a viable high-speed design.
Some of the parameters and information vital at this stage that you’re likely to require include:
- Signal speed
- Power supply for all the ICs
- Sensitive signals
- Correct system configuration amongst others
To exhaust all the possible information necessary for developing a complete plan, you’ll need to work with your manufacturer.
By doing so, you get to understand their minimum tolerance allowances together with any other new modification.
Also, Onboard noise is quite a significant issue that you should begin thinking about at this earlier stage. Therefore, together with your manufacturing partner, you may come up with a strategy for reducing it.
The first technique you could use to approach decreasing the noise is by utilising the two routing methods, ie. Microstrip and stripline traces.
2.Keep an Updated Document for Your Board Stackup
With a correct plan, next, you will have to keep track of all the information regarding the layer stackup for your application.
Involving a manufacturer in this phase is essential because the document needs to be comprehensive enough to accommodate information about:
- Specific and detail aspects for the design rules
- Necessary materials for manufacturing.
Since materials are quite critical in this procedure, it’s worth identifying and pointing out some of the materials you’ll likely encounter.
FR4 PCB Material
- FR-4. Electrical engineers highly recommend this type for applications that would require clock speeds of less than 5 Gbps. For that matter, it’s always grouped as a low-speed kind of material.
- FR-4 comes with two significant advantages, i.e., it brings about the ability to control impedance as well as being cost effective.
- Nelco, SI, or Megatron. Each one can easily attain a speed of 5 – 25 Gbps clock speed.
- Rogers – in case your application requires exceptionally high speed, then the Rogers material perfectly fits your demands. This type records an ultra-speed of up to 56Gbps. It’s thus evident that it’s a high-frequency material with a proper impedance besides being capable of withstanding high temperature.
The only limitation of this form of material is its expensive cost of production.
Aside from the materials involved, you also need to understand some of the few-layer stackup techniques.
- The first thing you have to do is to ensure that you place the signal layer adjacent to a plane layer. By doing so, your signals would have a valid return path.
- Rout all the high-speed signals on to the inner layers of the board, especially between planes, thus providing a shielding zone against any potential radiation.
- You may use several ground planes in your layer stack up.
3.Floor Planning and Organisation
This procedure purely entails the details about effectively organising your board. The vital decisions that await you here include deciding the exact points for placing all of your sub-circuits within the surface of the larger design.
Such kind of organisation is quite evident during the designing phase whereby you’ll need to separate the digital section from the analog part. Furthermore, it’s crucial also to keep apart the power segment from the digital and analog sections.
4.Appropriately Use the Ground and Power Planes
You initially need to ensure that the ground plane is complete, i.e., never split your ground plane with a routed signal.
Creating a split in the plane only results in the eruption of more problems such as electromagnetic induction and problems in signal timing.
The appropriate way of splitting a ground plane is by ensuring that you add a 0-ohm resistor beside the signal trace.
5.Optimally Decrease the Sizes of Your Land Patterns
Electrical engineers recommend leaving out a reduced oversize dimension for all the pads at 0 -5% of the proportion of the component pins.
Unlike the standard PCBs that set aside 30 %, the little spacing for the high-speed PCBs is for good reasons.
First, it dramatically increases the mechanical support.
Second, it effectively decreases needless capacitance. Finally, by limiting the space for your pads, you, in turn, increases the spacing for your differential pairs, vias and ICs.
6.Route your Signals
The high-frequency emission typical with these designs always leads to electromagnetic radiation, thereby potentially causing signal interference.
So to eradicate such problems to increase the shielding benefits for your signal, you may do the following.
- Maintain long size parallel signals on your circuit board at an optimal minimum.
- Keep a maximum distance between the signal traces. You may optionally route signals on a separate layer if they’re noisy.
- Route the signals orthogonally, especially when dealing with different layers.
7.Set Aside a Current Return Path
It’s mandatory for the signals in high-speed designs to require a return path once transmitted. The way also has to be of the least impedance.
So to ensure a smooth transition of the signals, you will have to use vias. The vias need not be ordinary but extraordinary too. You have to make sure they’re coupled and have got a matching impedance.
8.Apply the 3W Rule
The 3W rule will help you determine the amount of separation you need to give the traces to decrease the coupling effect.
The 3W rule states that the separation distance between traces have to be thrice the width of that of a single trace when you measure it from the centre to centre.
You never have any limits with the separation; in fact, you can extend it from the stated three times to up to ten times in a situation whereby you’re experiencing a high degree of coupling or crosstalk.
9.Apply the 20H Rule
Your potential problems don’t end with coupling and crosstalk but come to plane coupling, ie. The coupling effect likely to be between the power and ground planes in the layout.
Fringing is the possible effect of such coupling.
One way to avoid such complication is by the application of the 20 H rule. The rule states that you need to make your power plane 20 times lesser than the dielectric thickness between the neighbouring power and ground planes.
The final tweaks you have to do is the routing procedures. There are way many techniques for doing the routing, and that’s why you should collaborate with an expert.
Let’s have a look at some common ones, though.
- 90 degree – avoid using the right angled bends in the traces as this may lead to reflections of the signal.
- Differential pairs – for you to achieve complete elimination of electromagnetic field, ensure the signals in your differential pairs have an equal length and gap.
- Transmission lines – here you can use the microstrip and stripline traces in designing the transmission lines.
Those are a few tips on how to go about designing your high-speed PCB.
Material Considerations for High-Speed PCB Design
You will undoubtedly agree with me that no matter which kind of PCB you’re working on, starting with the right equipment and material is a plus in the proper direction.
A number of factors play a role in choosing PCB material such as pricing, material properties, laminate material thickness, material alternatives, etc.
Properties of High Speed PCB Material
Some of the highly regarded properties include:
- Dissipation factor
- Signal decay
- Dielectric constant
- Controlled impedance
We shall briefly analyse them to get the sense.
i.Dissipation factor / loss target
Dissipation factor refers to the degree of the tendency of the dielectric material to receive energy from the waveform that passes via the material.
Therefore, PCB materials have got low or high dissipation factor. The DF values are essential when investigating the number of losses exhibited by the high-speed circuits from which you fabricate them.
It thus means that that high-speed circuits constructed from PCB material with low Df values emit a reduced amount of losses. This kind of shortfalls includes conductor loss and dielectric losses.
Dissipation factor – Photo courtesy: Intel
On the other side of the spectrum, materials with high Df values subsequently leads to the production of circuit boards with high losses.
Despite the significant reduction in losses when working with PCB materials of low Df values, such related circuits materials are relatively costly compared to their counterpart materials of high Df values.
Now that’s a very challenging scenario that requires checks and balances. No wonder most engineers get stuck between going for costs or performance.
Signal decay, otherwise known as exponential decay, refers to the decrease of the strength of the signal over a period, and it depends upon the type of signal, form, and frequency.
Decay in signal transmission – Phot courtesy: University of Minnesota
When selecting a material with sufficient signal decay, your focus should be on time constant, because, one with more time relative constant would take a longer time for a drop in signal strength.
iii.Dielectric Constant (Dk)
The dielectric constant of the material significantly affects the impedance of the circuits fabricated on it.
The material’s Dk is always sensitive hence changes rapidly under the influence of frequency, temperature together with other reasons.
The corresponding impact thereby translates to adverse effects on the performance of the high-speed PCBs since it alters the impedance of transmission lines, which in turn hampers the signal integrity.
Dielectric constant in PCB
So what’s the recommended Dk values?
Pick the PCB material with the Dk values, which are low and also stable in equal measure about the frequency and the temperature.
Your design should aim at maintaining a consistent impedance over the entire signal path and have a uniform impedance for differential pairs.
So to regulate the impacts of the signal integrity issues, use the characteristics impedance (Z0) to find outboard properties such as dielectric constant, trace widths, and layer thicknesses.
Now we move onto the central part of our segment.
Material Selection for High Speed Printed Circuit Board
We already know that PCB materials are available in several forms but with varying levels of performance. As we earlier mentioned, the real challenge lies between prioritising cost or performance.
But since we know that the performance of the material depends on some parameters and most importantly, the dielectric constant.
Consequently, before you choose your preferred material, determine the dark and Z0 for the transmission lines you’re designing. To simplify your load, use a PCB design software, Dk chart or an impedance calculator.
When through now follow this guideline.
- First, choose a suitable high-frequency PCB board material. Look out for the cores, prepreg, and substrate materials.
- Specify the board material properties. Often engineers would incorporate sizes, thickness, and dielectric constants when specifying a particular PCB material. A high-speed circuit design would require channels with closely matched phase or amplitude without sparing tight performance tolerances.
Since you initially calculated your Dk and Z0, use those values to pick on the appropriate thickness and copper weights for your PCB.
With the materials, the high-speed PCB layout follows. Nonetheless, you’re likely to experience problems during the layout.
How do you deal with the errors?
Sources of Errors in High-Speed PCB Layout
There a number of factors that can contribute to errors you may experience in high speed PCB layout.
Some of these errors may include including onboard noise and electromagnetic interferences.
That’s why you have to take care when undertaking such procedures.
Let’s look at som of the main causes of errors in high speed PCB layout.
·Electromagnetic Interference (EMI) and Electromagnetic Compatibility
Electromagnetic radiations originate from radio frequency noise and subsequently causes interference during the operation of the PCB.
Electromegnetic inteference – Photo courtesy: Electromagnetic Compatibility Lab
Elsewhere, electromagnetic compatibility is the designing technique that aims at reducing the severity of the electromagnetic interference emitted by a device.
It’s debatably impossible to achieve a zero-rated electromagnetic radiation device. Therefore, your aim as an engineer should be to reduce the electromagnetic interference and sustainability to tolerable levels.
Always refer to the FCC and CISPR standards for the acceptable EMI levels for every device.
Crosstalk occurs whenever two or more adjacent traces couple in such a manner that they affect the operation of each other, resulting in the production of some noise.
Separating the traces wide enough is the most basic way of limiting crosstalks. You can as well use power planes.
Clock signals have got the tendency of forming a nominal clock frequency and harmonic frequencies above the clock frequencies. As a result, you must consider the EMI at the clock frequency and the harmonics higher than the nominal clock frequency.
When the PCB operates at higher frequencies, then the effects associated with the transmission line develop rapidly on the board.
Controlling the impedance of a PCB level transmission line shouldn’t be that stressful. You could opt to either use a microstrip or a stripline.
A circuit trace routed onto the outside layer of the circuit board with a plane underneath is the microstrip, while, a stripline is a circuit trace routed onto the inner layer of the circuit board between two low-voltage plane makes a stripline.
Differential signals hugely help in reducing the amount of noise in a communication path. Also, the differential signals have the same magnitude and are opposite in potential. Nonetheless, differential signals aren’t much affected by crosstalk or any emitted noise.
·Return Current and Loop Sections
High-speed layout designs often demand allocation of a valid return path for a signal. However, the return path does vary in comparison to the source of your voltage. For example, for a DC circuit, your return path will be that of the lowest possible resistance.
Elsewhere, for an AC signal, the path is that of the least impedance. The difference in the return path can be a potential problem whenever the signal trace breaks the ground plane.
Furthermore, in such a case, it leads to the formation of a loop. Unfortunately, you must avoid any loop since it may comprise of EMI radiators, which in turn affects the designs electromagnetic compatibility.
So those are ideally the sources of error in the high-speed PCB. With that information, we can proceed to discuss some of the conventional PCB layout techniques.
Let’s keep on moving together.
High-Speed PCB Layout Techniques
This part of the guide will address some of the critical aspects that you need to always have in mind during layout.
This extensive section aims to enable you to be capable of avoiding any additional measure that might affect the practicability of your design.
The layout techniques we shall be discussing are as follows:
Working with the Current Density
The current density is the intensity of the current flowing via a conductor. Discussion about the current concentration must also revolve around the return currents.
Yes, current flow to a specific point has also to have another return path.
When designing your circuit, the return current flow should be directly under the signal trace it came. In this way, the path will be of least impedance.
Besides, you have to allocate another solid return path under the same signal trace to maintain a uniform nature of current density.
High-Frequency Input Current Path
A high-frequency current path at all times should follow the passage with a minimum impedance. Moreover, for your layout, place the receiver part nearest as possible to the input section.
Such a connection creates a very minimal loop area, thereby significantly reducing the delays in the signal path.
The beauty of this method is: it creates the reference ground point for other circuits non-reactive. Additionally, it gives you freedom from having to abide by stringent strip-line techniques.
High-frequency Output Current Path
Similar to the input current flow, the output current path must also have a return path. The only difference being that the return current for the output current path must flow via the bypass capacitors and exists back into the power supply lines.
As a result, place the capacitors near the driver power supply pins and the output trace.
High-frequency Output Current Path
You can go about this by minimising the current flow area as much as possible. The more straightforward way to do this is by putting the connector and the driver at proximity.
High-frequency Output Current Path Differential from a Fully Differential Amplifier
Here you need to reduce the loop area specifically on the driver side. You can as well utilise the single capacitor that’s between the opposite amplifier supplies.
Note that, a renowned function of a fully differential amplifier is in the driving of the ADC. During this process, keep the current flow path around the amplifier and the ADC symmetrical enough.
Adding Bypass Capacitors to Ground
Such a technique enables the common-mode return currents a passage back to the transmitter to complete the loop. Also, it may not disrupt the differential current flow.
Routing Differential Traces
You need to route the differential traces side by side. Hence it keeps noise insertion as a common-mode signal which gets attenuated into the differential system. Besides, you can route together all the differential traces around the obstacles, but you shouldn’t separate them at all costs.
Finally, maintain the trace lengths equally to keep the delays the same.
Those are quite a few helpful layout techniques. However, I don’t dispute the fact that more of them exist; it’s only not consistent with the scope of this guide.
Continue reading to understand the necessary procedures in the PCB design.
Step-by-step in High-Speed PCB Design Process
Many procedures well inform the success of a high-speed PCB design process.
Let’s head straight into the procedure.
I.Specify Design Requirements for High-Speed PCB
Now, the initial step you have to think of is to define a proper circuit board requirements by considering the components you source for placing on the board.
Several factors potentially affect the performance of electronic circuits, especially the high-speed PCB and interestingly inability to define the board requirements come top on the list accurately.
So defining the requirements entails matching the board dimensions, mounting hole locations specifically to your prototype. You also have to consider and intensely plan for noise and shielding requirements.
Furthermore, it would be reasonable to decide on the type of the component mounting technology you’ll likely apply.
Is it SMT, THT or any other?
It then boils to the fact that you would specify the right trace width together with trace spacing at this earlier stage.
This is to evade some possible PCB layout errors such as capacitive, electromagnetic interference and inductive.
Those errors if not well planned for, are likely to come up later in the design.
Eventually, define appropriately the vias, the number of ground planes, and set out the routing layers. By doing all these, you enhance the next stages in the design.
II.Choose High-Speed PCB Design Software
For accurate development of electronics like high-speed PCB, you have to comprehend circuit board modalities, including using computer-aided design software (CAD).
Remember, all boards that you fabricate should be by the regulations of the electric membership corporation (EMC).
PCB design software inteface
With that, let’s now underline the various software. After going through all of the software am sure you shall find the perfect match for your project.
However, note that these design softwares are several and thus we cant exhaust all of them in this guide. The software alone deserves a comprehensive discussion in a whole book.
We shall analyse a few.
PADS is arguably among the most affordable PCB design software. The tool would suit independent electrical engineers.
Furthermore, it’s popular in small and medium organisations or companies which don’t have too much pressure for delivery.
Besides the low costs involved, it’s as well simple to use, suitable for beginners, and produces high-quality designs compared to other low costly ones.
But, most engineers discourage the use of PADS for more demanding multi-layer circuit boards.
Allegro Cadence, on the other hand, has a wide range of features; it comprises of several PCB design utilities. The software smoothly works in up to low memory machines of up to 512 MB of RAM. So you could even use it on a one-cored computer.
The advantages of the software are: it’s ideal for; low-performance hardware, complex projects, and large organisations.
The only shortcoming lies in the complexity of the user interface.
The OrCAD software is available in three versions, ie. Free, standard, and professional mode. The free version only gives you the necessary capabilities and functions. While the standard version avails a complete functionality while the expert mode enables you to design very complex PCB efficiently.
The software has got a friendly user interface, and it’s easy to use. However, it doesn’t satisfy the demands of large enterprises and complex projects.
Most engineers highly regard the Altium Design software since it’s efficient, easy to use and serves the needs of a professionally upfront project.
Above all, it works on one or double layer boards, and it’s capable of achieving rapid digitalisation.
Unfortunately, it may be tiresome when you’re working on a multi-layer board.
Other notable high-speed PCB design software includes:
III.Design High-Speed PCB Schematics
Arguably, a good layout would start with a good schematic. That’s why you have all the reasons to be utmost keen and careful when drawing a schematic. You need to figure about your signal paths in the circuit.
Remember, a schematic should have a natural and steady flow from the left side to the right. In this way; it will translate the same to the actual board.
It would be perfect and make designing easy if you can incorporate every useful detail information on the schematic.
Therefore, what kind of information should you include on the schematics? Reference designators, power dissipations, tolerances, waveforms, trace lengths are some of the must-have info.
In case you’re looking forward to delivering a super schematic then you will appreciate the following additional information: tuning information, thermal information, controlled impedance lines, notes, component value ranges, circuit operating descriptions, etc.
IV.Perform Simulations on High-Speed PCB for Signal and Power Analysis
To avail a fully functional PCB and meet the time demands your projects, detecting, analysing, and solving signal integrity issues during the design stage will help.
When you discover and correct the problems at an earlier stage, you lower the final production costs.
With the help of your selected design software, you should perform analog simulation, signal integrity analysis, thermal analysis, and design for manufacturability (DFM) Analysis.
Analog simulation covers all board-related issues and design yields. With signal integrity analysis, you avoid any unnecessary configuration errors and un-routable constraints.
Elsewhere, thermal analysis helps you analyse all board-related thermal problems such as board and component overheating at earlier design stages.
Finally, the DFM analysis though optional at this stage is significant as well.
It reduces the production related issues, limits the number of revision spins in every design, and lastly saves time for your release schedule
V.SaveHigh-Speed PCB in Required File Format
Storing data is one of the things that most people often go wrong at doing without knowing the impending danger they’re exposing their projects.
What you need to do is to manage a proper archive because it’s essential in backing up and also organising your design data, thereby increasing the productivity of your project.
Again, you don’t have only to store data in the vault. You should create, index, and restore archives within your vault.
In that way, you will be able to conduct a quick search for any content within the vault, add any upgrades such as comments. This could aid in differentiating different versions as well as comparing schematic and layout files.
Additionally, managing an efficient vault saves you the stress of losing design data.
You retain your schematic and layout files data for quite a long period and maybe use it for your future projects.
So how does the design software helps you save data?
Take, for example, PADS, KiCard or Altium, all of them keeps the design information in a secure vault on your personal computer or network.
Not only that but also they add interactive tools for modifying and improving design reviews. What’s more?
You can simply create several backups of your design data to easily retrieve them later on for an upcoming analysis and modifications.
Finally, when it comes to generating your files for your contract manufacturer, you won’t worry much. Most of them prefer Gerber files, while other CAD file formats are enough.
VI.Perform Design for Manufacture ( DFM)
DFM analysis is yet another step you can take in identifying, analysing, and solving problems in your design projects.
Using DFM software enables you to detect faults that may be difficult to .know yet devastating in the same measures.
Some of these issues that may pass undetected include starved thermals, acid traps, silvers, insufficient annular ring amongst others.
Besides the error identification, the DFM aims at reducing the general production costs by involving the application of set standards to minimise.
Cycle time for the development of products.
- To complement the importance of the DFM, you should follow set rules. The rules include: limiting the number of parts in a design
- use of standard components
- ease of manufacturing.
- Use more of multifunctional components.
- Make the most of component placement acceptance.
- Reduce positioning and handling.
- Design modules for incorporation in several products.
- Always develop a modular design.
VII.Forward the High-speed PCB Design for Fabrication
As you can see, the design task is a bit complicated and involves some nitty-gritty.
Once you already through with all the other problems, you eventually come to the last step where you will have to work with another team ( fabrication team).
But which kind of team should you have? Work with a professional, competent, reliable, and supportive manufacturer that would make your design work more manageable and productive.
High-Speed PCB Fabrication Process
Before we get into building the high-speed PCB, do you remember the types of the PCB we earlier on mentioned? Ie. Single sided, double sided and multi-layer?
Now the following procedure is suitable for any the PCB you’re looking forward to building.
Below are the steps you need to follow during the high-speed PCB fabrication.
1.Printing the High-Speed PCB Design
Without mentioning, you already know that you have to design your PCB first.
Luckily we talked about all what it takes to create a PCB including a schematic plan, related software, and of course the proper checks. Please refer above for comprehensive coverage.
With the design in hand, we can proceed to print. A plotter printer is the best gadget for printing the plan because it employs an accurate printing technology which gives out a clear and detailed copy of the PCB design.
A plotter printer produces a photo-negative film of the actual PCB on a plastic sheet. The film exhibits a color system on the inner layers. The black color covers the conductive segments, i.e. the copper parts of the PCB.
The other sections of the board remain clear, and in this case, clear ink shows regions of non-conductive material.
Section of plotter printer – Photo coutesy: Robin
On the other hand, the outer layers exhibit the contrary pattern: clear coloring for copper while black for the sections where you will finally etch away the copper. Keep the film securely far from any contact with the external object.
The plotter instantly develops a film for every layer of the PCB and the solder mask.
Essentially it means, a three-layer PCB gets six sheets: three for the layers and the other three for the solder mask. The films must all be uniform so that they map out the PCB alignment.
After that, punch registration holes through all the films. The holes keep the films entirely in line as we shall see later on in the process.
2.Printing the Copper on High-Speed PCB
Print the PCB design onto a laminate then pre-bond the copper on to both sides of the laminate. Before proceeding, you have to understand that in this step, we aim to whittle away the copper to reveal the design from the film.
Copper printed on PCB
After that, cover the laminate panel using a resist. Take precaution at this stage and avoid any contact with dust particles.
Next, line up the film, and the board by the help of the punched holes previously and expose them to UV light.
The light hardens the resist on the copper. The already painted black ink will prevent light from falling onto areas that shouldn’t harden so that you remove those regions.
Next, pass the board through a series of washing to eliminate any trapped unhardened photoresist and other unwanted particles on the surface.
With that, you now have got the product covered with the resist only covering the copper areas we need. The resist left actually should depict only those copper that will eventually emerge in the final PCB.
Finally, it examines the board to verify that no error occurred then proceed to the next step.
Note: this procedure must only apply to circuit boards with more than two layers.
3.Inspect Layer Alignment in the High-Speed PCB
Clean all the sheets to make them ready. Align the outer and inner layers of the PCB by the aid of a visual punch.
Next, use another machine precisely AOI to conduct an optical inspection of the layers to ensure no defects still exists.
What the tool does is to compare the inspected PCB with the extended Gerber design.
Upon the completion of the inspection, the board moves to the next stage.
4.Laminate the High -Speed PCB Layers
The objective of this step would be to join the separate layers. i.e., fusing the outer layers with the substrate. We do this process in two substeps: layer -up and bonding step.
Begin by Placing the prepreg layer on top of the alignment basin, then place a substrate layer on top of the prepreg follow with another layer of copper. Add more sheets of prepreg onto the copper foil and lastly a press plate.
Ensure the copper press plate is intact.
When done, transfer the system to a mechanical press. The machine presses and fixes the pins down through the stack of layers intermittently.
Transfer the stack to a laminating press.
Here the machine applies both heat and pressure to the layers. The heat melts the epoxy off the prepreg while the pressure fuses the layers.
When done, now remove the top press plate and pins from the stuck leaving the PCB free.
5.Drilling and Platting High-Speed PCB
It’s time to bore holes into the stacks.
But first, use an X-ray machine to find out the drill spots. Then drill guide holes after which by the help of extended Gerber design as a guide use an electric drill to make holes.
Plating follows right after completion of drilling, and it involves using chemicals to join different layers. Clean the panel and dip it into chemical baths. A thin layer of copper about one-micron thick forms onto the panel surface and into the earlier holes drilled.
The whole process of dipping, removal, and procession should be automatic.
6.Plating and Imaging Outer Layer of High-Speed PCB
Apply the photoresist to the outer layer of the panel and image it. Using the correct technique for plating the inner layers in the previous step, do the same to the outer coated layers.
The only difference is the use of tin to cover the copper from the outer layers.
7.Laser etching High-Speed PCB
The non-useful copper and copper below the left resist layer undergo etching. The chemical solution aids in the removal of the excessive copper while the tin protects the necessary copper during this procedure.
Finally, we have the PCB connections. Solder masking then follows.
8.Apply Solder Mask on High-Speed PCB
Clean the panels and apply them with an epoxy solder mask ink. Pass the boards through a UV light to reveal portions of unwanted solder mask for removal.
Lastly, pass the board into an oven to cure the solder mask.
9.Silkscreen High-Speed PCB
Now take the board through ink-jet writing to print all the necessary information on the surface. The PCB then undergoes the last stages.
10.Testing High-Speed PCB
After the completion of all the steps take the PCB through a couple of tests on various sections to verify its functionality according to the original design.
Testing lab for PCB
The two main tests are circuit continuity and isolation tests. The circuit continuity looks out for any potential disconnections in the PCB.
The isolation test elsewhere checks for the isolation values of the different parts of the PCB to confirm the existence of any faults.
The tests confirm the consistency of the fabrication process with the design.
That’s the complete guide on fabricating the high-speed PCB.
High-Speed Printed Circuit Board Assembly
Before you engage in the assembly process, it’s vital to source for your products and have them ready. And that takes us to the next subsection.
Prepare Bill of Materials (BOM) for High-speed PCB.
the BOM comprises of a list of the necessary raw materials, subcomponents, parts, intermediate assemblies, sub-assemblies and the quantities of each that you require for the manufacture of the high-speed PCB.
Therefore, your BOM would roughly include the following items, i.e., electronics and consumables.
- Electronic components
- Soldering materials
- Soldering flux
- Soldering equipment
With all the above equipment and materials, you can proceed to PCB Assembly.
BOM – Photo courtesy: PCB Cart
Mount Components on High Speed Printed Circuit Board
With everything ready, you can proceed to placing elements on the surface of the circuit board. ,
So technically, you can place the components by use of three techniques as below.
·Surface Mount Assembly Technique
The SMT is a process for the SMD electronic components.
SMT applies to both single and double side mounting.
These SMD components have no legs nor heads. The technique involves the following.
- Solder paste printing
- Components mounting with the use of an automated machine that picks the parts such as the ICs, and places them on the pre-programmed locations on the board.
- Reflow soldering- under this, the board passes through an oven of high temperature which melts the solder paste. A cooling chamber follows in which the solder paste solidifies and sticks the parts firmly.
·Through Hole Assembly Technique
The through hole assembly is for use with parts that have got leads and protruding wires, e.g. capacitors and coils. You need to plug them via the holes on the PCB for soldering.
The process includes wave soldering and hand soldering.
Wave soldering is quite direct involving:
- Inserting components.
- Application of flux
- Wave solder at high temperatures
hand soldering, on the other hand, is suitable in fabricating units with minimal workload/ repairs.
·High-Speed PCB Mixed Assembly Technique
The mixed assembly technique utilizes both the SMT and THT to mount another particular type of components such as special connectors and capacitors.
as a result, some forgeries combine both the two techniques on the same PCB.
PCBA that utilizes mixed technologies should happen only in the following situations: single side mixed assembly, one-sided SMT and one side THT, double side mixed assembly.
Applications for High-Speed PCB
I am sure you interact with high-speed PCBs every day. You can find them in several industrial settings.
Besides, they are irreplaceable and play a significant role in our lives. That’s why everyday electrical engineers strive to upgrade and crack the shortcomings we experience during their application.
Some of the applications and industries that heavily utilizes the high-speed PCBs include:
- Audio and video devices
- Gps trackers
- Monitoring equipment
- Consumer electronics such as mobile devices, desktops, laptops etc
- Telecommunication industry
- Medical industry
- Automotive industry
- Broadcasting applications
- Security applications
Clearly, High-speed PCB designs aren’t complicated.
What you need to remember is to always begin with a schematic. Besides, coordinate closely with your circuit designer and layout designer.
Topics you need to consider are: High-speed design guidelines, layout techniques, design process, and the fabrication process.