What is impedance control?
Electrical impedance is the measure of the opposition a circuit presents to a current when a sinusoidal AC voltage is applied. This cable is a coaxial cable. A common example of controlled impedance that most of us are familiar with, consisting of a round inner conductor, separated from the outer cylindrical conductor (the shield) by an insulator. Instead of a coaxial cable, an antenna can be connected to the device by a cable formed of two round wires spaced apart from a flat plastic strip. As with the coaxial cable, the dimensions and materials of this wire are carefully controlled to give it the correct electrical impedance.
These two cables are examples of different configurations of a controlled impedance but there are many others. In PCB, the shield is the plane, the insulator is the material and the core wire is a trace. Impedance is measured in Ohms, but should not be confused with a 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 of 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 :
- 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
- Video games
- Low cost PC
- Auto engine control modules
What kind of impedance design can we meet on PCB?
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.
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)
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. In general, this configuration is difficult to manufacture.
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 the PCB manufacturing, a test coupon must be added in the working panel. This coupon is the best way to be sure that the build up and the copper image matches 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, same parameters, the test coupon is exactly the replica of the impedance of the PCB.
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 which 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 addition pads, any changes which can influence the PCB impedance. In case 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 a appropriate testing equipment. For each test coupon, a report can be assigned showing the value of impedance measured. In 2019, ICAPE Group invested in a new Polar machine to control impedance with the latest technology available on the market.