Impedance Control on PCBs

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Sep 29, 2021|Product Innovations and Design

Impedance controls for PCBs ensure the board can transmit signals with little to no loss. Because these boards work on alternating current, you need impedance parameters to construct the board and then test it for verification.

While impedance is a well-known physical property in the electronics field, the steps that go into its control aren’t so commonly known. Here is the information you should consider when constructing a PCB.

What is Impedance?

Impedance is the measurement of the opposition by the circuit when alternating current runs through it. In other words, impedance is a combination of capacitance and the induction of high-frequency electrical current.

Although impedance is measured in Ohms, it isn’t the same thing as standard electrical resistance. Resistance, as a property, only comes into play when using direct current. Alternating current, because of the variation in current frequency, impedance becomes the property of concern.

To create the best current transmission, the impedance between two conductors should be equal. Sending a current between two conductors of different impedance values decreases the effectiveness of the current. This loss of effectiveness is measured through reflections and attenuations found in the current.

What is Impedance Control and Why is It Important?

Impedance control is the set of controls that exist to allow for impedance matching. Impedance matching allows for the optimal signal to be achieved along the trace. Some PCB designs have a range of impedance differentials that they allow, but that specification comes from the customer or designer.

These impedance parameters set by designers matter for the overall function of the PCB. PCBs are meant to link the different electrical components of the device together. Without proper signal transmittance, the device won’t function at its peak performance. The lost signal can result in slowdowns, inefficient power use, or a total loss of function for the device.

However, impedance control is an expensive test to run. Determining what current values are needed and where they are needed can take a long time to study. Still, these controls become more important over time as more and more of our tools are made electronic.

Here’s a list of some of the devices where PCB impedance controls are needed:

• Digital communication devices like smartphones
• Video signal processors
• TV, GPS, and video game consoles
• Automobile motor controls

Without proper impedance controls in place, these devices wouldn’t provide the work and convenience that we expect from them.

What Affects Impedance Matching?

Impedance matching happens by altering some of the properties of the trace you are looking at. The changeable properties on a PCB are the material makeup of the PCB, the layout of the PCB, and the width and length of copper used to make the trace.

The non-conductive material that makes up most of the PCB is called the substrate. This substrate is where the copper traces are installed onto the board. Once the board is built to the right dimensions, the copper traces are installed. Specific properties of this substrate and its construction affect your impedance matching.

For example, the uniformity of the board affects impedance. If your substrate is not even, the current won’t flow as it should through the PCB.

Environmental properties like heat affect the size of your board, too. As the board heats up, the substrate material expands from the increased energy. The change in height can increase impedance if the substrate’s coefficient of thermal expansion is not considered when designing the board.

The core and prepreg materials are factors in impedance matching, too. Choosing materials with the correct conduciveness and thermal expansion coefficients here will affect your ability to match impedances.

The copper trace lines will need to be considered, as well. Depending on the length and thickness of the copper trace, your impedance value will change. This change occurs due to the copper accepting different amounts of current depending on its length and width.

Finally, review the power plate and ground plate of the board. Regardless of which one you use, placing the plate on the board so that traces can route back to their origin helps with impedance matching. Return traces will need to be of a similar length as the original trace. So, keep the PCB dimensions in mind when designing the board.

Why Control Impedance on PCBs?

Different signals will have a specific impedance that they operate best at. Impedance controlled boards handle the signal transmittance optimally. With an optimal signal, you ensure that the transfer of data stays intact and lossless.

Also, it’s hard to check for impedance controls once other components are attached to the board. These components will have different tolerances and signal transmittance rates that they can accept and output. Once these components are attached, they introduce too many variables to run an impedance control on the PCB itself.

Impedance Control Verification

Test coupons are used when determining the impedance controls for a new PCB. Test coupons are smaller PCBs designed and constructed to test for the different specifications needed for impedance control. They are installed at the edges of the board and used to verify properties such as signal strength, conductivity, and signal loss.

Test coupons are either custom-made for the PCB in question or selected from a range of offerings from a vendor. Using a time-Domain reflectometer (TDR) alongside these test coupons, impedance can be measured and rendered as test data. If this data matches the specification range needed for the board, the PCB is certified in its impedance controls.

Conclusion

Impedance control for PCBs starts with the construction of the board. The substrate, traces, and the construction of the board change the impedance value for the circuits.

A combination of calculating these values ahead of time and running a TDR verification test on the board ensures that you are building a custom PCB with impedance control in mind. Without these controls, the electronics that we use in our daily lives wouldn’t operate at the level we have come to expect from them.