Types of Controlled Impedance PCB

Types of Controlled Impedance PCB

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.

What is Controlled Impedance?

Impedance simply is the degree of opposition to the flow of energy in an electrical cirsuit or transmission line

It is denoted as Z and is measured in Ohms

And,it is the result of summing the resistance(R) and reactance(X) of an electrical circuit:

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 ehe voltages among conductors.

Characteristics of Impedance in Printed Circuit Boards (PCB)

A PCB trace has several characteristics to consider regarding impedance;

These include;

Height

Width

Length

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.

Why You Need Controlled Impedance PCB

·Need for More Signal Power

·Improved Performance

·Control energy flow

·Need to manage electromagnetic interference (EMI, ELI)

Testing and measuring Controlled Impedance PCB

Most Controlled Impedance PCBs undergo testing before they are put to use in a project.

Due to this, testing is not usually done on the PCB itself but one or two test coupons integrated

into the PCB panel.

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.

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.

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.

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.

Please note;

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(PDN).

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

Calculating Controlled Impedance in Printed Circuit Boards

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

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.

These are:

I.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;

Characteristic Impedance

Characteristic impedance (Zo) is the most important parameter for any transmission line.

From: The Circuit Designer's Companion (Fourth Edition), 2017

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

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.

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.

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.

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.