.....take a look at NEW PCB Train services....8 layers (1.6 mm thick).....PCB Train Express 0.8mm service.......2 layer PTH 2.4 mm thick service.......
Start yourorder now

PCB
Fabrication

Start yourorder now

Express Laser
Stencil Service

Start yourorder now

PCB
Assembly

Start yourorder now

Laser Cut Solder
Paste Stencil

Online storeStart shopping

PCB Train
Online Store

Controlled Impedance
Guidelines

Contact Us

What is Controlled Impedance?

Controlled impedance is an advancement of Ohm's law and relates specifically to alternating current circuits. Alternating currents are more complex and require higher levels of precision on the behalf of the PCB manufacturer. With direct current, only the resistance needs to be known to determine the relationship between voltage and current flow. With alternating current, voltages will be self induced by associated current generated magnetic fields; that is to say, inductance. In addition, the voltage difference between conductors will create an electrostatic charge; that is to say, capacitance. Each individual track on a PCB will have a specific impedance value in relation to its neighbouring tracks or ground planes.

With high speed digital signals it can be crucial to predict and match the impedance of the transmission line (relevant tracks and ground plane) with the input and output impedance to minimise losses and reflections. Impedance factorsa are increasingly important as frequencies increase.

Manufacturing custom-made controlled impedance PCBs can be expensive. PCB Train's circuit boards are fabricated as consistently as possible, and we use our technical knowledge to advise what feature widths (track widths and gaps) to use to achieve commonly required impedances. Because of this, our clients can receive the benefits of controlled impedance circuits without suffering the additional costs.

The number of possible transmission line structures is infinitely vast. Variables influencing the impedance are track width, insulation gaps, both in the “x,y” axis and “z” axis, the geometry of the transmission line structure, and the dielectric constant of the insulation material. We use intelligent software to design a test coupon of the same structure and impedance as that required within the final circuit. A time domain reflectometer (TDR) is used to measure the impedance of the test structure which is manufactured on the same sheet as the PCB. A digital microscope is used to measure the geometry of the structure. If the impedance of the test coupon structure agrees with the predicted value, then the reasonable assumption is that the similar structure in the body of the PCB will also have the required impedance.

Controlled Impedance Data

This impedance information is published to assist users of the PCB Train service who are looking for a track impedance calculator to predict track impedance.

These guidelines relate only to PCBs fabricated on the PCB Train 4 Layer M/L 1.6 mm FR4 1 oz copper service. View the multi-layer build here.

Customers use this information at their own risk.

Layer descriptions are defined as follows:

  • Layer (1) = top copper
  • Layer (2) = inner-layer top
  • Layer (3) = inner-layer bottom
  • Layer (4) = bottom copper

Our calculations and measurements have established the following:

  • A layer (1) single free standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
  • A layer (1) single free standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)
  • A layer (2) single free standing copper track (a micro-strip) of 1.64 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (3)
  • A layer (2) single free standing copper track (a micro-strip) of 0.68 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (3)
  • A layer (2) single free standing copper track (a micro-strip) of 0.26 mm width will have an impedance of 100 ohms with respect to a solid copper ground plane on layer (3)

These guidelines relate only to PCBs fabricated on the PCB Train 4 Layer M/L 0.8 mm FR4 1 oz copper service. View the multi-layer build here.

  • A layer (1) single free standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
  • A layer (1) single free standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)

These guidelines relate only to PCBs fabricated on the PCB Train 6 Layer M/L 1.6 mm FR4 1 oz copper service. View the multi-layer build here.

  • Layer (1) = top copper
  • Layer (2) = inner-layer top
  • Layer (3) = inner-layer 2
  • Layer (4) = inner-layer 3
  • Layer (5) = inner-layer bottom
  • Layer (6) = bottom copper
  • A layer (1) single free standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
  • A layer (1) single free standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)

All tolerances + / - 10%

If you require controlled impedance PCBs complete with measured impedance test coupons, please ask us to quote.