In PCB, the shield is the plane, the insulator is material, and the core wire is a trace. Impedance is measured in Ohms, but should not be confused with resistance, also measured in ohms. The resistance is for direct current (DC), while the impedance is an alternating current (AC) that becomes important as the signal frequency increases, becoming critical for printed circuit traces with signal components equal to or greater than 200 MHz. The function of a wire or trace is to transfer the signal strength from one device to another. The maximum signal power is transferred when the impedances are matched.

A TV antenna has a “natural” impedance . For Radio Frequencies (FR), the transfer of the maximum signal power from the antenna to the cables requires that the impedance of the cable matches the impedance of the antenna. In addition, the impedance of the TV must match the impedance of the cable.

Although we have to focus on wire interconnections, the same considerations apply to signal transfer through traces on a PCB. As recently as 1997, only the then high-speed exotic devices required PCBs with controlled impedance . These amounted to perhaps 20% of the manufactured PCBs.

In 2000, about 80% of all multilayer PCBs were manufactured with traces of controlled impedance . These included boards for all types of technologies including :
– Telecommunications
– Video signal processing
– High-speed digital processing
– Real graphic processing
– Process control

Most homes today have a great number of low-cost applications of these technologies, for example:
– Modem, phones, satellite TV
– Radar
– Video games
– Low-cost PC
– Auto engine control modules

What kind of impedance design can we meet on PCB?

Embedded Microstrip

An embedded microstrip transmission line, similar to microstrip, is made up of a conductor, usually copper, of width W and thickness t routed over a ground plane that is wider than the transmission line itself and separated by a dielectric substrate of thickness H1.

Offset Stripline

In the offset Stripline configuration, the signal trace is sandwiched between two planes and may or may not be equally spaced between the two planes. This construction is often referred to as Dual Stripline.

Edge-coupled Coated Microstrip

The edge-coated microstrip is a differential configuration in which there are two traces of controlled impedance on the surface, coated with a resist and a plane on the other side of the laminate.

Edge-coupled Offest Stripline

Edge coupled offset Stripline is a differential configuration with two controlled-impedance paths sandwiched between two planes. The traces are staggered, but they could be halfway between the planes (2H1 + T = H)

Edge-coupled Stripline

This differential configuration has two traces separated by a laminate and sandwiched between two planes. Although the diagram shows the offset of the traces, the manufacturing objective is to have the traces without offset, i.e, one must be directly above the other

Coated Coplanar Strips

In this Coated Colplanar Strips configuration, there is a single trace of controlled impedance with two ground traces of a specified width (W2/W3) on each side. All the traces are coated with resist.

Coated Coplanar Waveguide with Ground

The coplanar waveguide has a single trace of controlled impedance with planes on each side (or very wide ground traces), a continuous plane on one side and a laminate only on the other.

Offset Coplanar Waveguide

The Coplanar Waveguide is similar to the above configuration, with the exception that there are planes on both sides of the laminate and a plane on the same layer as the controlled impedance trace.

How to Control Impedance?

During PCB manufacturing, a test coupon must be added to the working panel. This coupon is the best way to be sure that the build-up and the copper image match the requested impedance . Due to the fact that the PCB impedance needs to be measured, the PCB manufacturer has to add a test coupon in the working panel representing exactly the configuration of the impedance print in the PCB board. Those test coupons must be designed as a model described by the test measurement system. Using the same material, same production process, and same parameters, the test coupon is exactly the replica of the impedance of the PCB.

What is a test coupon?

The typical test coupon is a PCB approximately 200 x 30 mm with exactly the same trace construction as the main PCB. It has traces that are designed to be the same width and on the same layer as the controlled traces on the main circuit. This is the best way to assure a good result. The test coupon avoids any additional pads or any changes which can influence the PCB impedance. In cases where the laminate thickness is specified, the manufacturer will adjust the trace width to achieve the value of impedance . This coupon will be tested and checked with appropriate testing equipment. For each test coupon, a report can be assigned showing the value of the impedance measured.

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