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With the technology advancement on the materials of integrated circuits, the operation speed of electronics devices has been significantly improved, which prompts the integrated circuits to move towards high density, small volume, lightweight. These changes require high-frequency, high-speed PCBs where impedance control becomes more and more important in the design. 

What is Impedance

The total resistance to an alternating current of known frequency flowing through it is called impedance. For PCBs, it refers to the total impedance of a circuit layer (signal layer) to its closest related layer (reference plane) under high-frequency signals.

Types of Impedance

1. Characteristic impedance（Z0）

In electronic information products such as computers and wireless communications, the energy transmitted in the circuit of the PCB is a square wave signal (called pulse) composed of voltage and time. The resistance encountered is called the characteristic impedance.

2. Differential impedance

Two identical signal waveforms with opposite polarities are input to the driving end, which are respectively transmitted by two differential traces, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance between the two traces.

3. Odd-mode impedance

Odd-mode impedance refers to the impedance of one trace to the ground in the two traces. The impedance value of the two traces is the same.

4. Even-mode impedance

Even-mode impedance is the impedance when the two same signal waveforms with the same polarity are input to the driving end and the two traces are connected.

5. Common mode impedance

Common mode impedance is similar to odd mode impedance while with a larger impedance value.

What is Impedance Control

On a circuit board, there will be various signal transmissions in the conductors. To increase its transmission rate, the frequency must be increased. However, the impedance value might change due to different factors such as etching, stack thickness, wire width, etc. of the traces, as a result, the signal will be distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called "impedance control".

Why Impedance Control is important

The requirement of impedance control is proposed to ensure the integrity of the high-speed signal on the circuit board. It plays a key factor in the normal and stable operation of a high-speed digital system where the characteristic impedance（Z0）of the key signal traces should be regarded as a key consideration. If the impedance of the key transmission trace is not matched, it may lead to signal reflection, rebound, loss, and deformation of the original signal waveform (overshoot, undershoot, ringing), which will have a direct impact on the performance and even the function of the circuit.

For high-speed signals, the trace is not as simple as directly connecting two points. The impedance factor should be considered as it would affect whether the signal is completely transmitted from one point to another point. Some people might think that the shorter the traces between two points, the less loss of the signal. However, in electronic circuits, especially high-speed signals, it is all about matching. Signal transmission takes time. The difference in timing will have a different impact on the circuit performance. If the impedance is not matched, there will be very large distortion and attenuation during the signal transmission process, which may lead to instability or reduced transmission speed of the final products, or in the worst cases, the products don’t work at all.

Factors affecting Impedance Control of PCB

Width of traces(W)

Z0 increases rapidly as the trace width narrows. The trace width change in PCB prototyping will lead to a great change in the impedance value, which requires the manufacturer to ensure that the trace width meets the design requirements in the PCB manufacturing process within the allowable tolerance. At present, the signal transmission trace width of most high-frequency lines and high-speed digital lines is 0.10 or 0.13mm. For normal electronics products, the trace width control deviation is ±20%. However, in the case of signal transmission lines with Z0 control, the Z0 deviation will exceed ±10% at this time and can no longer meet the requirement. To match Z0, the trace width must be strictly controlled.

Copper thickness(T)

The thinner the copper foil, the higher the Z0 value. And a thin copper foil makes it easier to manufacture fine traces that contributes to the control of Z0. It should be noted that during the PCB manufacturing process, the surface of the trace should be clean without residue or black trimming oil before electroplating. Otherwise, the copper might not be correctly plated, which will lead to a change in the thickness of the trace and affect the Z0 value. 

The dielectric constant of insulating material (εr)

The Z0 is inversely proportional to the εr of the insulating material. The speed of signal transmission in the dielectric material decreases with the increase of the εr. To obtain a high signal transmission speed, the εr of the material must be reduced. Meanwhile, high Z0 should be used for high signal transmission speed, which also requires materials with low εr. Different materials have different εr that depends on the resin materials used. For example, the εr of FR-4 substrate is 3.9-4.5 and PTFE, 2.2-3.9. To control the Z0 value, the selection of respective insulating materials should be considered as a key factor. 

Dielectric thickness(H)

Dielectric thickness is the thickness of the insulating material between traces. It is another major factor affecting Z0. The thicker the dielectric thickness, the greater its impedance value. As the trace width and the dielectric constant of the material are determined before PCB manufacturing, controlling the laminate thickness (dielectric thickness)during the production is the main way to control the Z0, especially for multi-layer boards. Technically for high-frequency PCBs with strict impedance control, the dielectric thickness of the insulating materials has to maintain a strict tolerance that should not exceed 10%.

Impedance control is particularly proposed for the requirements of high-speed signals and is affected by many factors on the PCB. In the design process, the impedance type, trace width, trace thickness, laminate material, substrate thickness, dielectric, and other factors should be taken into account with the support of professional impedance calculation software. If the Z0 exceeds the regulated range, the signal transmitted will encounter problems such as Reflection, Dissipation, Attenuation, or Delay. This requires the manufacturer to strictly select the base material and control the manufacturing process to achieve impedance matching required by the customers.

As a professional PCB prototype and PCB assembly service provider, PCBgogo is capable of manufacturing FR-4 PCB, flex PCB, and Rigid-flex PCB with impedance tolerance controlled in the range of ±8% - ±10%. They are now giving out a 20% discount on impedance PCBs that can save up to $100. Don’t miss it out!