Controlled Impedance PCB
Your Valued Controlled Impedance PCB Supplier
Controlled Impedance PCB (also refers to Impedance Control printed circuit board), Controlled impedance is the characteristic impedance of a transmission line formed by PCB conductors. In controlled impedance PCB, Impedance can not be confused with resistance, although they both measured in Ohms( Ω ), because resistance is a DC characteristic, while impedance is an AC characteristic.
Today PCB designers are driven by the pressure of fast speeding up signal switch, corresponding to the shorter signal transmission times and higher clock rates of modern digital circuits, PCB traces are no longer simple connections, but transmission lines. It is very important for PCB design engineers to understand how to control the impedance of PCB traces.
Venture provides free stack-up and impedance control calculations suggestions to customers. Our experienced engineers are ready to help you 24/7. We can work with your team at the conceptual stage of the controlled impedance PCB design to help you obtain the best results in controlling impedance, though selecting the proper material and layer stack up.
|Aluminium||105||T-Lam 6061+ 1KA10||Laird|
|Aluminium||140||T-Lam 5052 + 1KA04||Laird|
|Aluminium||105||T-Lam – Alco 6061+1KA04||Laird|
|Aluminium||105||TLam SS 1KA06||Laird|
|FR-4||170||TU-862 HF||Taiwan Union|
|FR-4 + BT Epoxy Resin||180||G200||Isola|
|FR-4||130||Tlam SS 1KA||Laird|
|FR-4||135||H140-1 / FR-4-74||HuaZheng|
|FR-4||140||FR-402 / IS402||Isola|
|FR-4||150||TU-742 HF||Taiwan Union|
|FR-4||150||TU-747 HF||Taiwan Union|
|FR-4||175||EM-827/ EM-827B||Elite Material|
|FR-4||176||R5725 Megtron 4||Panasonic|
|FR-4||200||TU-872 LK||Taiwan Union|
Your Leading Controlled Impedance PCB Supplier in China
We control impedance by varying the dimensions and spacing of the trace or laminate and perform a test to make sure that we achieved the requested impedance using TDR coupons. We at Venture are experiencing increasing demands for multilayers PCB with controlled impedance requirement. An estimated 60 % of the multilayer PCB’s with six layers or more are controlled impedance PCB. We believe in the near future, all PCB’s will likely include at least some impedance requirements.
Venture manufactures controlled impedance PCB(printed circuit board) by using the latest materials and technology, we have full strict Incoming Inspection(IQC) for all laminates, pre-preg, and copper foil, since raw materials’ thickness variances are one of the main challenges for producing controlled impedance PCBs.
We also use LDI (laser direct imaging) equipment eliminates the variances in trace width, once a controlled impedance board is imaged, it must go into the ether. The purpose is to develop a configuration on an etcher to minimize undercut. With our 10 years experiences in the controlled impedance PCB industry, Venture understands how to manage the etch process to make sure that we meet the requested impedance tolerance data.
Venture have worked together with thousands of electronic engineers to get their controlled impedance PCB into a final product, from single layer board to 32 layer board, from flex PCB to rigid flex PCB, Venture can offer the complete controlled impedance PCB solution.
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 best-controlled impedance PCB manufacturer & supplier in China. At Venture we can answer any controlled impedance PCB questions that you may have, pls feel free to contact us anytime.
Controlled Impedance PCB – The Ultimate Guide
Controlled impedance in PCB design and fabrication is quite a broad and complex topic.
Understanding it can be quite tricky.
So whether you are new to Controlled Impedance PCB’s or if you are looking for a refresher on the topic, this guide is for you.
It will help you understand all the whats, whys, and hows of impedance control.
Therefore read on.
Controlled Impedance Printed Circuit Board Basics
As I’ve said, the purpose of this guide is helping you understand all there is regarding Controlled Impedance PCBs.
A great way to achieve this is first to understand what controlled impedance is as well as its significance in PCB design and fabrication.
Also, you need to know and understand the difference between impedance and resistance in PCB’s. The two go hand in hand and are often confused by many.
Let’s get right to it.
- What is Controlled Impedance?
- Characteristics of Impedance in Printed Circuit Boards (PCB)
- Why You Need Controlled Impedance PCB
- Testing and measuring Controlled Impedance PCB
- Calculating Controlled Impedance in Printed Circuit Boards
- How to Specify PCB Impedance
- Levels of Impedance Control in PCBs
- Control Impedance Manufacturing Process
- Types of Controlled Impedance PCB
- How to Choose Controlled Impedance PCB Manufacturer
- FAQs on Controlled Impedance PCB
What is Controlled Impedance?
But to get to know what controlled impedance is, we need first to define impedance.
Impedance simply is the degree of opposition to the flow of energy in an electrical circuit, or transmission line.
It is denoted as Z and is measured in Ohms.
And, it is the result of summing the resistance (R) and the reactance(X) of an electrical circuit:
Impedance triangle – courtesy of electronics tutorials
The reactance, in this case, is the consequence of two effects namely;
- Inductance (L) which is the induction of voltages in conductors due to the magnetic fields of currents
- Capacitance (C) which is electrostatic charges’ storing due to the voltages among conductors.
Relationship between current and voltage – Photo courtesy: Wikimedia
At DC, there usually is no reactance, and the resistance of copper conductors is typically insignificant.
Here, “impedance” is merely formed by the resistance.
But for high-speed AC circuits, the reactance and therefore the impedance becomes very significant.
Impedance, in this case, can become critical to a design’s functionality.
This is because changes in the impedance along the signal path from transmitter to receiver can lead to glitches and reduction in a system’s performance.
You see, while an electrical circuit’s speed is often expressed as the frequency of the waveform: the critical concern is the speed at which the voltage and current are required to change.
This is where control comes in, to prevent impedance mismatches.
So back to our earlier question, what is controlled impedance?
Well from the explanation above,
We can define controlled impedance as a design technique that assures impedance mismatches in a circuit is within tolerable limits.
Controlled impedance PCB
Based on the above explanations, controlled impedance is a design feature.
Thus, a Controlled Impedance PCB is a circuit board design with features that can control impedance mismatches.
That is, a PCB manufactured to tight tolerances in dimensions so that impedances of transmission lines on the PCB can be accurate.
And as we’ve said, the existence of impedance in a circuit can become an issue, especially if it is a high-frequency circuit system.
The good thing is that you already know it is possible to control impedance.
What we don’t know is precisely how and why you should control it. But not for long as that is exactly what we’re going to learn in a short while.
For now, lets us learn the difference between impedance and resistance.
Difference between Impedance and Resistance
Photo courtesy: Stack Exchange
More often than not, people use resistance to generalize impedance.
You see, both of these terms represent similar ideas.
They both represent how a component opposes or fights against the flow of current.
Their difference nonetheless is that resistance is attributed to DC currents.
Impedance, on the other hand, is attributed to the AC equivalent in a circuit.
So in layman’s terms;
Resistance is the measure of a material by which it opposes the flow of DC through it.
Impedance refers to the measure of a material by which it opposes the flow of AC through it.
Resistance, note, is a measure of voltage divided by the resistance in a resistor.
And, as I mentioned earlier, Impedance is the generalized notion of voltage divided by the current for anything.
We can talk about the impedance of any component (R, L, or C).
In the case of a resistor, we use the term “resistance” instead of impedance.
For inductors and capacitors, we use the term impedance.
Although, this impedance, in this case, has the same general meaning, which is the ratio of voltage to current.
Also, in this case, impedance equations are written a lot like Ohm’s law for resistors.
Please note that Ohm’s law is the first equation.
As you already know, the traditional variable used for impedance is Z.
R for a resistor is the same for any voltage, any current, and any frequency.
For inductors and capacitors, it is more interesting.
Z depends on the frequency of the signal being applied (an extra feature a resistor does not have).
For an inductor, ZL increases as frequency (f) goes up. |ZL|=2pfL.
An inductor has a high impedance at high frequency.
For a capacitor, ZC goes down as frequency goes up. |ZC|=1/2pfC.
A capacitor has very low impedance at high frequency.
As a matter of fact, a capacitor is essentially a short circuit at high f.
Impedence calculation – Photo courtesy: Electronics tutorials
To put it simply,
- Impedance is a measure of opposition to an alternating current while resistance usually refers to direct current (DC).
- Resistance is quite simple. Impedance, on the other hand, depends on reactance and resistance.
- In most cases, impedance considers the overall circuit, unlike resistance, which doesn’t.
Hopefully, this helps you to understand the difference between resistance and impedance principally.
Characteristics of Impedance in Printed Circuit Boards (PCB)
A PCB trace has several characteristics to consider regarding impedance;
- distance between the track and other copper features (including copper layers beneath or on top of the signal layer containing controlled impedance)
- the dielectric constant
- PCB fabrication tolerances/limits etc.
All of these are characteristics to consider when calculating impedance and manufacturing Controlled Impedance PCBs.
Why You Need Controlled Impedance PCB
In the recent past, we have witnessed a continual increase in device switching speeds.
Devices have generally become faster and complicated.
Signal Integrity (SI) issues, for instance, have become more recurrent with the increase in device operational speeds.
Meaning, devices today must be able to deal with any SI issues.
You, therefore, cannot continue to treat PCB traces as simple point-to-point connections.
Instead, start considering them as transmission lines.
And also, understand the necessity and importance of impedance matching in lessening or eliminating impact on SI.
You should know that by following good design practice and approaches, you can quickly avert potential signal integrity issues.
Controlled impedance, in this case, can help you avert or mitigate SI issues.
Controlled impedance PCB
Other reasons for needing Controlled Impedance PCB are;
·Need for More Signal Power
You see, the function of a PCB trace is to transfer the signal power from a driver device to a receiving device.
Power, in this case, needs to be transmitted throughout the length of the trace.
However, you can only achieve maximum signal power with matching impedances on the PCB.
That is why you need a Controlled Impedance PCB. This kind has impedance matching that allows as much power from the device driver to end up at the receiver.
If you’re looking for a PCB that will guarantee quality device performance, then a Controlled Impedance PCB is your best bet.
A reason why most devices fail in terms of signal power and integrity is due to poor PCB design and layout.
The layout stage in PCB manufacturing is usually very critical.
If special care is not taken here, the chances are that high-speed signals will degrade as they propagate from the driver to the receiver.
If you were to view the result of this on an eye diagram, you’d notice high levels of signal distortion.
signal distortion in the transmission line
Also, you’ll realize a massive difference in power levels as the signal propagates from the start to the end.
What am I trying to say?
Devices controlled by Controlled Impedance PCBs perform faster and tend to use less energy.
This kind of PCBs enables devices to perform better for longer, thus improving their value and control reliability.
·Control energy flow
I’ve mentioned that Controlled Impedance PCBs use less energy, that’s true.
But also, if you need to controlled energy flow in your projects, Controlled Impedance PCB is a great choice.
You see, controlled impedance is vital during transitions from a lower Ohm to a higher Ohm environment where there’s impedance.
Such kind of transitions can lead to energy reflection in the form of powerful pulses. These pulses, note, are highly capable of disrupting energy flow.
So if your application involves high-powered digital devices like in RF applications, a Controlled Impedance PCB is a necessity.
·Need to manage electromagnetic interference (ELI)
If you’re worried about circuit disruptions due to electromagnetic interference, invest in Controlled Impedance PCB/s.
In the PCB world, a single pulse of reflection energy can entirely disrupt circuits.
That disruption often bleeds over to neighboring components.
And, it has the potential to interrupt energy flow and product operation fail critically
Trust me; you wouldn’t want any of these problems to occur amid a critical operation.
This exactly why you need a quality PCB that is specially designed with impedance requirements built in during manufacturing.
Testing and measuring Controlled Impedance PCB
Most Controlled Impedance PCBs undergo testing before they are put to use in a project.
When I talk of testing, I mean 100% testing to ensure they are suitable for use in a specific project.
Note, however, that it is not uncommon for the actual PCB traces to be inaccessible for testing.
Moreover, traces may be too short for accurate measurement.
They may also include branches and vias that can hinder the correct measurement of impedance.
Generally, PCB vias for test measurements would affect performance.
They would also occupy board space.
Due to this, testing is not usually done on the PCB itself but one or two test coupons integrated into the PCB panel.
Typical production panel
Note that the coupon is of the same layer and trace construction as the main PCB.
It also includes traces with precisely the same impedance as those on the main PCB.
As such, testing the coupon affords a high degree of confidence that the board impedances will be correct.
Does this mean that you can’t test the actual board traces?
The answer is no.
You can very well test your board on the actual controlled impedance trace if you must.
Just remember that while you can test the actual board, it usually easier to verify a board by probing traces on itself.
But like I mentioned earlier, onboard traces are, in most instances, often inaccessible.
And sometimes lack proper test pads too.
This and the above reasons are why it is advisable to test coupons for controlled impedance.
How do you do this?
For starters, you’ll need to match the routing.
That is, ensure that the test coupon routing matches that of the board, including trace width and spacing rules.
This should also include ground traces.
Generally, signal traces ought to be straight and open-ended with suitable signal and ground pads for probing.
Meaning, you’ll need pads for every reference plane for stripline.
Another thing that you should be keen on is to serial number boards and their associated coupons.
This way, it becomes easier to track them after separation.
Note that coupons can often be standard in shape, size, and probe pinout, among other things.
Standardization, in this case, is to allow the fabricator to build test fixtures that will facilitate-and accentuate-testing.
During the design stage, you can have coupons as part of the main board.
Usually, however, coupons are put at one or more locations on the panel.
You can use a variety of styles for this, depending on probes and associated test equipment.
The land pattern matching the test probes in use will also dictate the style you use.
To test, the coupons are inspected to ensure proper layer alignment, electrical conductivity, and cross-sectioned to examine internal structures.
If you need an accurate test for impedance, you can ask your manufacturer to design a test coupon separately.
Or, ask them to place the coupon on your working panels.
And as I’ve said, coupons integrated into working panels are put at different locations, mostly at the edges of the panels.
Signal coupling – Photo courtesy: EDN
Impedance is then tested by using a TDR (time-domain reflectometer).
To measure impedance, the TDR applies a fast voltage step to the test coupon via controlled impedance cable and probe.
Any reflections that occur on the waveform will show on the TDR, including the value of discontinuity.
Discontinuity, in this case, refers to a change in impedance value.
So if there is a discontinuity, the TDR will display its location and magnitude.
Once done, a report is generated to indicate if the impedance is at par with specifications or not.
Controlled impedance test system
The overall performance and EMC behavior of electronic equipment are not just determined by the circuitry and geometry of the layout.
But also, by the power distribution network.
In this case, you need to pay careful attention to;
- The choice of decoupling capacitors and quantity required and routing loops
- The plane capacitance required by different voltages to accommodate noise limits
- Reference plane continuing and return current paths
- Inductances caused by poor component packaging
Apart from using controlled impedance TDR techniques, you can also measure impedance using a network analyzer or a laboratory TDR.
Nonetheless, both network analyzers and laboratory TDRs are highly complex and sophisticated.
Laboratory instruments, for instance, need to be operated with great care and by a skilled engineer.
Thus, the controlled impedance test system remains the best option for measuring controlled impedance on PCBs.
Calculating Controlled Impedance in Printed Circuit Boards
To guarantee signal integrity in high-speed PCB designs, you need excellent impedance characteristics in the conductor trace connections.
You can only determine these after calculating the controlled impedance of the PCB based on the layer buildup, layout, and impedance specifications.
The outcome of these is conceivably slight modifications of the stackup as well as the relevant conductor geometries.
Understand that the impedance of a PCB is primarily influenced by;
- the distance of the signal layer
- conductor geometries
- trace width
- copper thickness
- Permittivity er
Now, you can calculate controlled impedance using simple equations to obtain nominal values of trace dimensions for a particular impedance.
Controlled impedance PCB
These equations are useful for line widths and spacing above 15mil.
However, these simple equations are only approximations.
They don’t usually give accurate results for line widths used on current technology PCBs.
Besides, the equations often require very complex mathematics.
Due to these, I recommend that you use a PCB impedance calculator to calculate PCB controlled impedances.
There are several of these calculators online.
You just need to find one that you can work with and ensure that you input the correct values for calculation.
Note that the results you get from these calculators are purely for approximation and rough estimation.
The final values of impedance and corresponding layer construction ought to be calculated by the manufacturer.
How to Specify PCB Impedance
Usually, PCB manufacturers will offer a standard stack up.
Aluminum PCB stackup
Aluminum PCB stackup
You will use this to calculate, either by hand or software, what the trace dimension should be based on this stack up.
If you find the results plausible, use that.
If not, you’ll need to specify a stack up that will work for you.
How do you do this?
Well, first of all, start with a trace thickness that is practical for routing, spacing, manufacturability, etc.
Then, calculate the dielectric thickness, given material with specific dielectric constant, for the impedance you need.
From the real options of core thicknesses and pre-peg sheets and materials, choose the closest one.
Once you do this, recalculate the trace dimensions you need.
If using software and it allows for it, simulate your critical lines, and ensure your SI is okay.
Note that this requires driver model, trace dimensions, stackup specifications (distance to references plane/s and dielectric value).
Also, it’ll require any vias that you might be using and their dimensions.
Correct these accordingly, and there you have it, the stackup and trace dimensions for the impedance that you need.
Now all you need to do next is to convey this information to your manufacturer.
To do this, simply draw on the Gerber a representation of the same, specifying thicknesses, etc.
Add some notes specifying desired dielectric constant and material.
Levels of Impedance Control in PCBs
Now that you know the basics of impedance control, it is essential also to know the different levels of impedance.
This knowledge is handy when deciding what kind of impedance control service you need for your PCBs.
With that in mind, there are three levels of impedance control.
Levels of impedance control
Impedance control popularly applies to high-end designs with tight tolerances or unusual configurations.
It is best used if your design has tight impedance tolerances, as I’ve said, that could be tough to hit the first time around.
As you will learn later in this guide, there are different types of controlled impedance.
There is characteristic impedance which is the most common, and then there are;
- Wave impedance
- Image impedance
- Input impedance
In the case of impedance control, your manufacturer will build the board.
Then he will test it via TDR to see if it meets initial impedance specifications.
II. Impedance Watching
Impedance watching refers to a situation where the impedance control trace is indicated on the design.
Here, the designer will just outline the impedance control trace.
The PCB supplier will then adjust the trace width and dielectric height as needed.
Upon approval of the complete specifications, the manufacturer can begin to build the board.
If your manufacturer allows, you can request for a TDR test to confirm the impedance for a small fee.
III.No impedance Control
If your design does not have tight tolerances, a no impedance control service would be ideal.
In this case, you won’t need any extra design elements to ensure correct impedance.
Instead, you can achieve correct impedance by conforming to standard specifications without impedance control.
Your manufacturer can provide accurate impedance without special measures, which make this the more cost-effective option.
Control Impedance Manufacturing Process
PCB Manufacturing process
Like we mentioned at the beginning of this guide, the technology of PCB manufacturing has become more advanced.
This is especially with the increasing need to control signal integrity issues.
Now, manufacturing high quality and functional Controlled Impedance PCB involves a lot of steps
These include etching, engraving photos, multilayer processing, and drilling masking, finishing and finally testing.
Unfortunately, we can’t discuss all of these steps independently.
So for the scope of this guide, I’ll discuss the manufacturing process in at most three categorical stages.
Stage 1: Controlled Impedance PCB Design and Layout
Before fabricating a PCB, a manufacturer must first define its design vaguely.
And being an intricate design, the first step would be first to define how to control impedance.
How to Control Impedance
As you can tell, fabricating a Controlled Impedance PCB is a task.
This is because it requires a high level of care to achieve consistently accurate results.
So you should know that the design is only the first challenge.
Fabrication, in its entirety, must be completed with a well-understood process.
Etching, for instance, must be accomplished without under or over-etching.
The substrate is also the dielectric in this case. Thus, it must be held to a reasonable tolerance to assure the expected impedance.
Now, when controlling impedance, you must make sure the impedance is constant at each point along the trace.
For this, you’ll need to control three key features of the circuit’s geometry.
These are the trace width, the spacing between the signal return path and the signal trace.
It also includes the dielectric coefficient of the material surrounding the trace as well as the trace thickness.
You can change these features and still retain controlled impedance.
This is as long as you change other features as necessary, so the relationship between these aspects does not change, and impedance remains constant.
Controlled Impedance Design Considerations
As I’ve said, controlling impedance means maintaining a constant trace impedance at every point of a PCB trace.
Meaning, wherever the trace travels, even if it changes layers, the impedance should stay the same throughout.
That is, from start to end.
In Controlled Impedance PCB manufacturing, there is usually little control over the impedance in a device driver or the load.
Nonetheless, you can control the impedance on the PCB.
As such, you have to match the circuitry on the PCB to the impedance of the source and load.
This way, you can ensure a consistent appearance throughout the entire path of a signal.
In this case, essential design elements that you have to consider alongside proper design techniques are:
Choice of Materials
Before, FR4 was commonly used in PCB fabrication. But with the advent of high-speed designs, correct laminates must be used.
Here, you’ll need to specify the use of a material with a lower dielectric constant.
This will help ensure best signal performance as well as minimize any cases of signal distortion or phase jitters.
Another thing that you need to consider in this is the loss tangent, which is also known as the dissipation factor.
It refers to the measure of signal loss as the signal propagations down the transmission line on the PCB.
For high-speed designs, you may want to choose the lowest loss material.
Note that different laminate materials have different loss tangents.
As such, you need to choose the material that is most suitable for your application and inform your manufacturer accordingly.
Additionally, you need to take into account the weave pattern when choosing the PCB laminate material.
You see, typical PCB core and pre-peg substrates are built from various woven fiberglass fabrics.
These are bound together with epoxy resin.
The fiberglass and epoxy each have distinctive dielectric constant values.
This prompts an inhomogeneous mechanism for signal propagation.
A loose weave pattern will, in general produce less uniform dielectric constants in a PCB overlay. This can result in trace impedance variations, and propagation skews.
And, the higher the speed, the more evident this problem will be.
On the other hand, a tighter weave pattern means a more uniform dielectric constant.
Therefore, it is crucial to choose a tighter weave pattern for the signal to be able to move over more glass.
This will result in a highly consistent dielectric constant throughout the board.
PCB design and power plane
Power islands are one of the most critical elements in a controlled impedance design.
A controlled impedance board with inaccurate power-planning can be very unstable.
Earlier on, you could route power tracks a little wider than signal tracks, and treat them like regular connections.
But today, the story is different.
If you use high-speed processors, you should know that a significant number of flip-flops are switching at any given movement in the circuit.
The switching tends to force significant amounts of current back and forth through their power and ground pins
In this case, the ground pins can create ground bounce if the amount of current is high.
Faraday’s law V=L.di/dt (Delta-Voltage equals inductance x current rate) proves this.
You just can’t use a track, for instance, to route ground signal as this may give different voltages on each side of a track.
It will be quite odd to have +0.5V on one side of your ground, and -1V on the other side.
This can cause a complete system failure.
The worst part is that discovering this issue can be difficult.
And even if you do, you will be forced to create another prototype.
The same rule applies to the other power plane.
It is quite easy to have drops in certain tracks if you don’t plane your PCB.
In this case, the circuit won’t have any power islands to support the voltage.
So if you’re designing a high-speed circuit, it is important that you use lots of decoupling capacitors.
As you do this, remember to give special care to the RF and power supply switching sections’ grounding planes.
For these, you’ll need to isolate their islands from the system ground plane.
You must also include track connections switching island to system ground.
That is, the tracks ought to be huge enough to have close to zero DC resistance, yet not more.
The reason for doing this is to avoid switching RF sections. This can create waves on the ground plane that can create ground bounce on the system ground.
You can read this article on PCB power-supply design if you need more explanation on this subject.
Other things that you should keep in mind when designing a Controlled Impedance PCB include;
- Maintaining shorter trace lines
- Avoid routing stubs and discontinuities
- Maintain same lengths on signal pairs for differential pair routing
- Use back drilling to remove unwanted copper
- Use immersion silver as a surface finish instead of ENIG
- Use smaller antipads on plane layers
- Always specify the solder mask thickens
These are the main factors that you need to consider when designing a Controlled Impedance PCB.
Note that you should integrate these during the design and layout stage.
When done, check to verify if the design is okay and as per your requirements or not and rectify where necessary.
Remember that it is always easier and cheaper to rectify a PCB design before the board is manufactured.
Common Controlled Impedance PCB Design Mistakes
During design analysis, the following mistakes are often encountered;
- Traces are crossing split lanes. Signals should always be routed on solid ground reference planes and not across a split plane or void in the reference plane.
- Traces without a reference ground plane. Impedance is often high if there are no adjacent layers. As such, it is advisable to route high-speed signals on the top or bottom layer of the board.
- Mismatches in length. This can lead to signal distortions and an increase in bit error rate.
- It is thus advisable to length-match differential pairs +/_5 mils of each other if possible.
- Use of too much pre-pegs. It is inadvisable to use more than three different types of pre-pegs in a stack up.
- Wide impedance trace space. The spacing between two traces of a differential pair should never exceed twice the width of the traces.
If you’re intent on designing a functional Controlled Impedance PCB, avoid these mistakes at all costs.
How to Design Controlled Impedance PCB
Schematic capture in PCB design
The first step in PCB design is usually the schematic capture.
Basically, you draw a conceptual circuit just like you would on paper.
Then, for each symbol on the circuit, you assign a footprint to it.
A footprint, note, is the physical package of the component.
For instance, a resistor can through-hole, various SMD sizes, etc.
After assigning the footprint, the CAD program you’re using will generate something called a netlist for you.
A netlist is a machine-readable flat representation of your schematic.
It fundamentally gives each pin of each footprint a unique ID.
Furthermore, it creates a rundown of all the connections between these nodes, given your schematics.
At that point, you get the chance to do the actual design layout.
For this, you’ll have to initially choose how big your circuit is and what number of layers you need.
Generally, you need your circuit board to be as small as could reasonably be.
At the same time, you want the option to suit every one of the connections you need at the widths you need them.
In this case, you’ll need wider traces as it is a high-speed circuit.
Also, you want the PCB to be able to satisfy all your signal integrity requirements.
For the most part, using more layers implies you can make the signal clearer, and the PCB progressively smaller.
Be that as it may, more layers equal to increased costs, so there is a tradeoff.
In most cases, the size of the PCB is insignificant.
For instance, it might be clear that the PCB should be installed in a large device that needs to be large for different reasons at any rate.
In other cases, you just want the PCB to be as small as would be prudent at practically any expense.
But this is a discussion for another day.
So when you’ve figured out the layering, you can proceed to place components starting with those with physical constraints.
For instance, if your board has switches, LEDs, or connects, they need to be in a position to match holes cut out on the box.
Here, you would go on and place the large and complicated ICs in a way that can minimize the traces required.
At that point, you can put the various components supporting those things above.
Then lastly, you can create traces (wires) that connect pins/pads that should be connected.
CAD programs help tremendously in this part since they can check for mistakes using netlist.
This is to such an extent that if your circuit doesn’t concur with your schematics, it will let you know.
It is for this reason that we make schematics in any case, even though it’s not strictly necessary.
Creating a schematics capture, as some hobbyists do, is like building a house without a plan.
Fine, you may do everything correctly, and the structure works just fine and even saves you time.
But then, this won’t work as perfectly in reality, especially for a reasonably complex design (PCB or building), like in this case.
So, after completing the traces and electric rule check passes, you can run the design rule check.
Here, the system checks for things that are disregarding what your PCB fabrication machine can do.
You need to tell the CAD program these constraints.
For example specific, if you want the traces too thin, gaps between traces too thin, holes too large, holes too small and so on.
Then you export your design to Gerber.
You then send these files to the PCB manufacturer to make the PCBs for you.
Note, however, that this is just a typical workflow.
The actual process may differ slightly between different CAD programs.
Stage 2: Controlled Impedance PCB Prototyping
Controlled Impedance PCB prototypes
After the design and layout of a Controlled Impedance PCB, a prototype is made.
Again, this is before the actual fabrication of the commercial board.
The prototype in this stage plays a key role in the creations of a PCB design.
It facilitates the manufacturer to foresee if anything needs to be resolved in the PCB design.
In case of any failure in the prototype, a new prototype is created and is kept under custody until it performs well.
Stage 3: Controlled Impedance PCB Assembly
Once approved, the prototype proceeds to the next stage, which is actual fabrication and assembly.
The board is first fabricated to specifications.
It is then connected with electronic parts and components as specified in the layout stage.
We call this PCB assembly.
To connect these components to the circuit board, several techniques are used.
The two main ones are the surface mount technique and through-the-hole technique.
The surface mount technology is the most complex and relies on soldering small leads on the board.
It is the most efficient option and creates a lightweight board with high speed and the ability to perform multiple functions.
The through-hole technology is slightly less efficient and is based on passing tiny wires through the holes on the board to connect the different components.
However, in most PCBs, both of these techniques are combined to achieve maximum efficiency in the design and performance of the PCB.
Some other manufacturers employ highly skilled technicians to solder minute or small parts using microscopes.
They also use equipment like tweezers, soldering tips, etc.
After all, this is done, the PCB is tested for actual functionality before being put to use in a project.
From this, you can see that for a PCB to be complete; every stage is as important as the equipment used.
Meaning, everything needs proper care.
So while choosing your manufacturer, do thorough research about the company and its background.
This will help you get genuine and high-quality services in return.
We will be discussing how to choose a quality Controlled Impedance PCB manufacturer shortly.
But before that, let us look at the various types of this kind of circuit boards.
Types of Controlled Impedance PCB
types of Controlled Impedance PCB
Controlled Impedance PCB is constructed in several ways to match its intended use.
Here are the three mains types of controlled impedance circuit boards:
1.Single sided Controlled Impedance PCB
In PCB design and fabrication, single-sided types are the least complex.
These are built with all electrical parts attached to one side of the base material, and with other side coated with copper traces.
Copper, in this case, is the preferred type of metal because it is a very effective electrical conductor.
A special solder mask often protects the copper layer.
Also, a silkscreen coating is a further feature to help mark the different components on the board.
Single-ended controlled impedance boards are preferred for the most basic electronic items.
They’re also often the first type of boards used by at-the home hobbyists.
Note that although single-ended boards are the most cost-effective to the manufacturer, they aren’t the most commonly used.
This is because of their design and uses limitations.
2.Double-sided Controlled Impedance PCB
Double-sided controlled impedance boards are a standard choice for a wide range of applications.
They’re built with components, and parts mounted to both sides of the base material.
This type of board is designed with plenty of holes to make it possible for the circuits on each side to connect.
The wires are soldered in place to give a strong and reliable hold.
A further option to connect the two sides is through-hole technology.
This technology can create devices that run at faster speeds and less weight.
They use small leads that are permanently soldered to the board instead of using separate wires.
3.Multi-layered Controlled Impedance PCB
Multi layer PCB design
The multi-layer controlled impedance boards are made up of several base materials with each separated by insulation.
Standard sizes for this board type include regions with 4, 6, 8, and 10 layers.
However, it is possible to manufacture huge boards with up to 42 layers or even more.
Such large sizes are mostly preferred for more complex applications.
The different boards in multilayer PCB designs are connected using wires passing through the individual holes.
They help minimize issues with space and weight.
Applications for this type of board range from handheld devices, space probe equipment to medical machinery, and servers.
Controlled impedance circuit board manufacturing can be carried out for a limited number of circuits.
Or, for large volume production.
It is therefore essential that you choose a Controlled Impedance PCB manufacturer with a proven track record.
Read on to find out how.
How to Choose Controlled Impedance PCB Manufacturer
Having learned all that you have in this guide, it should not be difficult to find a quality Controlled Impedance PCB for your project.
You know what to look for, what to ask for, and what to reject, etc.
The problem is, who you can trust to build you a great board for your application?
You see, there are several manufacturers worldwide who have expertise in designing and producing Controlled Impedance PCBs.
Controlled impedance PCB manufacturer
But, not all of them can guarantee you the quality you’re looking for.
Sadly too, only a handful can execute your ideas and specifications into an ideal PCB for your needs.
For these reasons, you have to find the right manufacturer for your needs-one that will deliver the promised product at the right time.
Luckily, the internet has it possible for people to connect globally.
In this case, you can search for manufacturers online and choose the one that is suitable for you.
As you do this, you can also enlist the companies providing cheaper and good quality circuit boards.
These manufacturers, note, take care of several features, including the design of the PCBs as well as the materials used to create them.
So what are the aspects to be taken care of when selecting a controlled impedance PCB manufacturer?
·Use of Modern Technology
The approach of new innovation has refreshed everything, and nobody would need obsolete and old software design.
In that capacity, it is smarter to choose a manufacturer that use the latest designs to make the circuit work.
·Experience and Reputation
It is exceedingly vital to choose a manufacturer with great experience in producing Controlled Impedance PCBs.
It is essential to check if the manufacturer has a skilled team of designers and engineers who are familiar with the intricate details of controlled impedances sheets.
It is better to choose a manufacturer that boasts of at least 10 years’ experience in this area.
·Receptiveness to Customization
An unbending nature in design is the last things any sane buyer would acknowledge.
Therefore, it is crucial to work with a manufacturer that is ready to tweak and work as per requirements.
Time is of the essence in any business transaction.
Thus, the manufacturer you contract should be able to meet deadlines and deliver your order on time.
Repairing circuit boards is relatively easy.
Nonetheless, it is nearly impossible for everyone to know how to do it.
In such cases, the manufacturers should be ready to provide apt servicing on the off chance that you need it.
Adherence to quality is paramount when it comes to Controlled Impedance PCBs.
So choose a manufacturer who can produce quality PCBs that meet industry standards.
And, that adhere to all your specifications.
In this regard, you may want to verify if the manufacturer’s quality control practices meet your requirements.
For instance, if a company is willing to stand behind their products with an accurate PCB test, then they are worthy.
This is because an accurate PCB test will assure you of the quality control that you need when it comes to your controlled impedance boards.
These are the aspects that if taken into account, can give you satisfactory results when you decide to work with a Controlled Impedance PCB manufacturer.
Quality controlled impedance PCB
FAQs on Controlled Impedance PCB
In as much as I’ve tried to make this guide as in-depth as possible, I understand that some of you still have questions.
That is questions about the topic that you need answers to or more clarification.
So in light of this, I will try to answer some of the commonly asked questions on Controlled Impedance PCB.
I hope you find your answer/s here.
1)How is Controlled Impedance PCB manufacturing verified?
First of all, impedance on its own is verified after a PCB is manufactured.
As you’ve read in this guide, test coupons are used to test the quality of a PCB fabrication process.
We mentioned that the coupons are fabricated on the same panel as the PCBs.
They are then probed to ensure proper layer stack-up, electrical connectivity, and cross-sectioning.
If you need to verify the quality of your PCB, you can ask your manufacturer to design a test coupon or ask them to place the coupon on your working panels.
Either way, you’ll need to use a TDR (time-domain reflector) to test the impedance.
Apart from impedance, overall board quality is also verified to ensure its performance, durability, and conformance to industry standards.
For this, testing is done visually, unlike when verifying impedance.
Remember, PCB quality checks must be done before assembly.
Otherwise, you won’t be able to rectify failure and/or defects if found.
2)What is a high-frequency circuit/signal?
A high-frequency or high-speed circuit refers to when the rise and fall time of a signal is fast enough to change from one logic state to another in less time than it takes to travel the length of the conductor and back.
If the signal propagates from the source to the receiver and back, and rise and fall time is faster than that, then you’re dealing with a high-speed signal.
In this case, you may have to start considering high-speed issues.
3)What is characteristic impedance?
Characteristic impedance defines the resistance in parallel circuit and power planes to the flow of AC.
It is often denoted as Z0.
It is categorized into two: single-ended and differential characteristic impedance.
4)Single-ended vs. differential impedance: what is the difference?
These are the two types of characteristic impedance.
Differential impedance is purely the impedance seen by a purely differential signal over a differential pair.
Single-ended impedance is the impedance seen when testing a single line that is not coupled by an adjacent line.
It is in most cases half of the differential impedance.
Differential impedance is also split into three; odd mode, even mode and common mode.
5)What is the difference between impedance and the characteristic impedance of a line?
The impedance of a line is simply the series of the impedance of the line consisting of resistance and inductive reactance.
It causes a voltage drop in a line as well as real and reactive power losses.
The value of impedance, note, controls the real and reactive power transfers and fault currents.
Also, it is used for distance relay settings, among other things.
For characteristic impedance, this one as we had defined earlier refers to the impedance as seen by a surge in a line.
6)What is reactance?
Reactance is simply a wattless quantity.
Here, energy is stored in the form of energy and can be used again.
Therefore, the loss is 0.
7)What is the difference between reactance, resistance, and impedance?
You already know what resistance and impedance are and their difference.
For this question, nonetheless, resistance simply is the measure of the opposition to the current flow offered by a material.
Unlike reactance, resistance is a wattful current.
So, reactance is the resistance offered to AC currents by inductors and capacitors only.
Impedance, in this regard, is the sum of the resistance and reactance of a circuit.
8)Some of the factors that affect a PCB’s impedance quality include?
Factors that affect board impedance include;
- Width of the PCB trace
- The thickness of the trace
- The dielectric thickness of the core
- Pre-peg or solder mask around the trace
Other elements that can also affect impedance quality are model reliability, measurement integrity, process control, and capability.
Also, defined work instructions and lab analysis are some of the things you should look into when verifying impedance.
9)Can impedance and resistance be the same?
Theoretically, yes, they can.
But practically, it will require very precise equipment.
Understand that resistance is usually due to pure resistor while impedance is due to the resistance of resistor, capacitor, and inductor.
Meaning, the pure resistor is possible in theory.
In practice, every conductor has some minimum resistance, capacitance, and inductance.
So if you can avoid capacitance and inductance, impedance and resistance can be the same.
Otherwise, it will be impossible.
10)What are high impedance states?
High impedance or high Z is a state when the inputs do not drive the output.
High-Z is often used in buses when you want to transfer more than one signal through the same wire without losing data.
You can also use High impedance state in multiplexing.
11)Why should a circuit have low output impedance and high input impedance?
Well, a circuit will usually have a high input impedance if a voltage requirement drives it.
And, a low input impedance means that a circuit has to supply a larger current, which is a good thing for a voltage current.
That is, if the input is high compared to the source impedance, the voltage level will not drop too much due to the divider effect.
Take this for instance;
You want to amplify a voltage signal. It will require that you get a proper input voltage at the input of your amplifier.
In this case, if your amplifier has a high input impedance, the voltage will not drop at its input.
Thus, your input voltage will remain at the desired level.
Note, however, that if your amplifier has less input impedance and a voltage drop occurs at the input, you may end up getting smaller voltage at the input.
This happening may end up amplifying a wrong signal.
So generally, the low input impedance is desired to suck out maximum current from a circuit.
It is, however, not a must for a circuit to have both high and low input impedance.
All this depends on the application.
12)What is surge impedance loading?
Surge impedance loading (SIL) refers to the load at the receiving end of a circuit which is equal to root.
The SIL give an approximate loading of a transmission line.
If the load on the line is such that the reactive power produced by the line is equal to reactive power absorb, that is surge impedance.
The line in this regard is said to have natural load or unit surge impedance load.
13)What is even and odd mode impedances? How do they relate to differential and common mode impedance?
Even, odd and common modes are parts of differential impedance.
Odd mode impedance, in this case, refers to the impedance of a single transmission line.
This is when two lines in a pair are driven differently, with signals of the same amplitude and opposite polarity.
Even mode refers to the impedance of a single transmission line when two lines in a pair are driven with a common mode signal.
That is, with the same amplitude and polarity.
Differential impedance, in this case, is the impedance between the two lines when the line pair is driven differentially.
Common mode impedance is the impedance between the two lines when the pair is driven with common mode stimulus.
This ultimately makes common mode impedance to be equal to half the even mode impedance.
And, the differential mode impedance equal to twice the odd mode of impedance.
14)Which software is best for controlled impedance PCB design?
Well, there are several PCB design software in the market, both free and paid for.
Popular ones include;
15)What are the applications of controlled impedance?
Controlled impedance should be considered for PCBs used in fast digital applications.
– Computing 100MHz and above
– High-Quality Analog Video
– Signal Processing
– RF Communication
I hope that this guide has helped answer some of your questions regarding Controlled Impedance PCBs.
Like I said earlier, understanding impedance and issues surrounding controlled impedance are not so easy.
Nonetheless, I have tried to include every useful information in this guide to enable you to understand this topic better.
At this point, I believe that you are an expert on controlled impedance boards.
If you have any more questions on this topic or inquiries on Controlled Impedance PCBs, just ask on the comments section below- we will be glad to help.