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Impedance Control PCBs

As electronic circuits' operating speed has increased, so needs for PCBs to have controlled characteristic impedances, and the majority of PCB manufacturers are producing impedance control PCB for many applications.

What is Impedance?

Impedance, measured in Ohms (symbol Ω), is somewhat different than resistance. Impedance is an AC characteristic, while resistance is a DC characteristic. Impedance becomes essential as the signal frequency increases, typically becoming critical for PCB traces at signal components of two or three hundred MHz and above.

What is Controlled Impedance?

At high frequencies, PCB traces do not behave like simple connections. When PCB board traces carry signals containing high frequencies, The PCB manufacturer must pay attention to to design traces that match the impedance of the driver and receiver devices. The longer the trace, or the greater the frequencies involved, the greater the need to control the trace impedance. Controlled impedance helps us ensure that signals are not degraded as they route around a PCB. The PCB manufacturer controls the impedance by varying the dimensions and spacing of the particular trace or laminate.

Understanding Characteristic Impedance in Your Circuits and PCBs

A PCB trace impedance comes with several features to look into concerning impedance. The features of a 50 ohm PCB trace PCB board design impedance include: dielectric constant, length, width, height, PCB fabrication limits/tolerances, and the distance between the track & other copper features.

These are the characteristics to look into when manufacturing controlled impedance pcb and when calculating it.

Why is impedance control pcb necessary?

When a signal needs a specific impedance to operate properly, controlled impedance should be preferred. In high frequency applications, keeping impedance constant on the complete electronic board is essential to protect the transferred data from damage and to maintain the clarity of the signal. The longer the trace or the higher the frequency, the more adaptation is needed. Any lack of rigour at this stage can increase the switching time for an electronic device or circuit and cause unexpected errors.

Uncontrolled impedance is difficult to analyse once the components are mounted on the circuit. Components have different tolerance capacities depending on their batch. Furthermore, their specifications are impacted by temperature variations which can lead to malfunctions. In such cases, replacing the component may seem to be the solution at first when, as a matter of fact, it is the unsuitable trace impedance that is the cause of the problem.

This is why trace impedances and their tolerances must be checked early on in the PCB design. Designers must work hand in hand with the manufacturer to guarantee the compliance of component values.

Factors That Affect Impedance Control

There are a few factors that affect the printed circuit board controlled impedance include: dielectric thickness, trace width, copper thickness, dielectric constant Er of the material selected for the stack, and thickness of the solder mask.

Trace width: The larger the trace width is, the lower the impedance will be. The thinner the trace width, the more impedance is offered. Increasing the board thickness increases the impedance while reducing it will decrease the impedance.

dielectric thickness: dielectric thickness also affects impedance. The dielectric strength of a material is a measure of the electrical strength of an insulator. It is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed in terms of Volts per unit thickness.

Copper thickness: The copper thickness is also considered while calculating the trace impedance in high speed and RF digital circuits.

thickness of the solder mask: Solder mask can impact PCBs with RF circuitry on the outer layers, which can lessen high-frequency electrical performance.

Dielectric constant: The dielectric constant is the ratio of the electric permittivity of a material to the electric permittivity found in a vacuum. In a PCB, dielectric constant tends to vary inversely with frequency. A PCB with a low, stable dielectric constant is suitable for high frequencies and controlled impedance. A more difficult dielectric constant can often affect impedance in unpredictable ways.

Other Design Considerations

Trace lines should be kept as short as possible and reduce lengths wherever possible. If the trace lengths are fairly long, terminations should be used to prevent reflections.

Routing stubs and discontinuities, which add to the reflections and degradation of the signal quality, should be avoided.

For differential pair routing, try and ensure that the signal pairs have the same length.

Use of back drilling – for a thick backplane where the signal goes from the top layer to one of the inner layers, the remainder of the copper barrel of the via or the pin of the press-fit connector will be a stub, resulting in reflection. Back drilling removes the unwanted copper. It is a technique used to remove the unused portion, or stub, of the copper barrel from a thru-hole in a printed circuit board.

Consider using immersion silver as a surface finish rather than ENIG. Immersion silver results in less insertion loss (lossy) than ENIG purely because the nickel content in ENIG is very lossy and due to the skin effect, it is not very good for high-speed designs. The flatness of the pad is just as good as ENIG and it is more workable than ENIG.

Reduce the size of antipads on plane layers. Antipads are where pads are removed, or copper is removed on plane layers where the pad should not or does not connect to that plane. Sometimes the anti-pad size is too large creating unnecessary voids in the plane. By making the anti-pad a little bit smaller allows for more plane continuity resulting in a cleaner signal and return path.

How to calculate differential impedance?

To ensure signal integrity in PCB designs with high-speed, there’s a need for great impedance characteristics in the connections of the conductor trace.

These can only be determined after the PCB’s controlled impedance is calculated based on the impedance specifications, layout, and layer buildup.

You can use impedance calculator online. It will help you calculate your trace widths, single-ended or differential impedances – for both microstrip and stripline models – and other parameters such as the dielectric height, the dielectric constant, and the trace thickness. The tool will also provide a guide for dielectric constant values for various PCB materials.

Also, you can contact us to get impedance calculation.

Capabilities in Manufacturing Impedance Control PCB

Monthy Capability3650 m²/month
Layer4 Layers
MaterialFR4, TG180
Finished board thickness1.6m
Min Trace Width/Space8/8mil
Min hole size0.25mm
Min copper in hole thickness1oz
Outer layer Finished copper thickness3oz
Inner layer base copper thickness3oz
Impedance Control Tolerance±10%



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