An Introduction to Controlled Impedance for PCBs


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 formula

 

Electrical engineers and PCB manufacturers alike consider these key factors to determine impedance: Resistance, Inductance, and Capacitance.

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

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

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

 

Elements in PCB Manufacturing that can influence the Controlled Impedance

  • PCB Material: Also called dielectric, the fiberglass material makeup will influence the impedance. This can vary greatly in the materials and is why some material choices are specified by the designer in PCB designs.
  • Lamination Characteristics: The process of lamination and the prepreg used in that lamination process 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 circuit’s 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 things, most PCB manufacturers will provide you a controlled stackup or controlled dielectric which will allow you to verify the impedance required.  By specifying the thickness of the material in the design you can achieve the desired impedance.

Here are some thoughts on controlled impedance from Bay Area Circuit’s Engineering Manager:

Often PCB 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 impedance modeling and calculations for both trace/width size as well as material stackups.

 

 

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