Friday, December 09th
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Controlled Impedance: First of all I am not an Electrical Engineer and I do not play one on TV.  This is meant to be a very basic idea of what controlled impedance is and how it relates to printed circuit manufacturing.  I would love to have comments from electrical engineers in the comment section below!

What is Controlled Impedance?

Electrical impedance is the measure of the opposition (resistance) that a circuit presents to the passage of a current when a voltage is applied in an alternating current circuit.   It is represented by Zo.

Controlled Impedance for PCBs

Impedance

The key factors that are used in determining impedance are Capacitance, Inductance and Resistance.

Capacitance: is the ability to store energy in an electrostatic field.  This has the maximum effect on Zo.

Inductance: is the electromagnetic field that is created by the signal on the trace. This has a secondary effect on Zo.

Resistance: is the signal attenuation (loss) caused by the transmission path (in our case a copper trace).  This has the least effect on Zo.

The mathematical equation for controlled impedance is represented below.

Elements in Printed Circuit Board Manufacturing that can influence the controlled impedance.

  • The PCB Material: The fiberglass material makeup also called dielectric will influence the impedance.  This can vary greatly in the materials and why some material choices are made and called out in PCB designs.
  • Lamination Characteristics: The process of lamination and the prepreg used in that lamination will also influence the design.
  • Soldermask: The thickness variations and the dielectric properties.
  • Trace Characteristics: the length, width and thickness of the trace on the board is a key influence on the circuits impedance.

Why does it matter?

As integrated circuits have improved in their ability for processing more information it has become necessary to move more power through the circuits of the board.  This has increased the “speed” of circuits while driving down the size of components as well as the need for much smaller traces with more power going through them.

On the manufacturing side of the board most often you can get the impedance that you need from doing what is called a controlled stackup or controlled dielectric.  By calling out the thickness of the material in the design you can achieve the impedance that you need.

Here are some thoughts on controlled impedance from one of our engineers Wayne Dieck.

Often board manufacturers can get the correct trace impedance by doing what is called controlled stackup or controlled dielectric.  This is this process of defining the material thicknesses to achieve a specified impedance value.”

 “The other option is to set the specific impedance value (in ohms) for a given trace, or traces, by layer.  Common values are 50 or 100 ohm, single ended or differential.  A stackup calculator is used to determine the overall width and spacing of the trace and the material thickness for the desired impedance value.

Then to verify that those conditions have been met for controlled impedance a test can be done with a Time Domain Reflectometer (knows as a TDR Report).

Most PCB Manufacturers can provide you with impedance modeling and calculations for both trace/width size as well as material stackups.

There you have it! Questions or comments? Please put those in the space below.

To read more about the manufacturing process and how Controlled Impedance could be impacted read the following posts.

PCB 101 Series

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