PCB Tech - Radio frequency (Rf) PCB layout design

So far, microstrip is still the most commonly used transmission line structure in radio frequency and microwave design. However, as the speed and density of digital and hybrid technology designs continue to increase, the situation is becoming less and less.

Because for the same impedance, the microstrip line is usually wider than the strip line, and because the radiation associated with the microstrip line increases, it requires both more PCB wiring space and a larger distance nearby traces. In pure RF or microwave designs, this is usually not a problem, but with the demand for smaller product sizes and the consequent increase in component density, it becomes a less easily available option.

Structure

The microstrip transmission line is composed of a conductor (usually copper) with a width of W and a thickness of t. The conductor is routed on a ground plane wider than the transmission line itself and separated by a dielectric with a thickness of H. The best practice is to ensure that the ground reference plane extends at least 3H on both sides of the surface microstrip trace.

advantage

Historically, the main advantage of the microstrip line may be the ability to use only two layers of boards, while all components are mounted on one side. This simplifies the manufacturing and assembly process and is the lowest cost RF circuit board solution. Since all connections and components are on the same surface, there is no need to use vias when making connections. In addition to cost factors, this is also ideal, because the use of vias does not increase capacitance or inductance.

As for the same impedance, the microstrip trace is usually wider than the stripline trace. Therefore, since the etching tolerance in manufacturing is an absolute value, it is easier to control the characteristic impedance of the trace more strictly. Therefore, if the PCB trace width is 20 mils, and the width is reduced by 1 mil due to over-etching, then this is a very large amount compared with over-etching 5 million strip lines and reducing the width to 4 mils. Small percentage change. For example, in FR408 material, a microstrip trace that is 20 mils higher than the ground and 11.5 mils higher, with a dielectric constant of 3.8, will produce approximately 50.8 ohms. If this trace is reduced to 19 mils, the characteristic impedance will be approximately 52.6 ohms, and the characteristic impedance will increase by 3.6%. In the same material, a 5 mil stripline with 6 mils grounded at the top and bottom will generate about 50.35 ohms, but when reducing 1 mil to 4 mils, the characteristic impedance will be about 56.1 ohms, an increase of 11.5%. When completing certain designs, the characteristic impedance of the final trace is not specified, but the final width is specified. In the same over-etching scheme, reducing the 5 million traces of 1 million mils will reduce the final trace width by 20%, and reducing the 20 mil traces of 1 million mils will reduce the width by 5%.

shortcoming

Since the microstrip transmission line is usually very wide and laid on the surface of the circuit board, this means that the surface area available for component placement will be reduced. This makes microstrip useless for high-density hybrid technology designs, which are almost always valuable for space.

Microstrip transmission lines will radiate more than other transmission line types, which will be the main contributor to the overall radiated EMI of the product.

Thirdly, since the radiation from the microstrip increases, crosstalk becomes a problem, so it is necessary to provide an increase in the spacing from other circuit elements, resulting in a decrease in the available wiring density.

Microstrip designs usually require external shielding, which increases cost and complexity. In fact, this has become one of the most important issues in the design of portable devices such as mobile phones. The driving force of many products is getting smaller and smaller, and therefore thinner and thinner. This means that the shielding layer will be closer to the surface of the circuit board, which will increase the capacitance per unit length of the transmission line, thereby changing its impedance. When choosing to use microstrip transmission lines and deriving impedance models, please consider carefully. If the trace needs to pass through an external shielding wall, it may be necessary to modify the transmission line width by a small distance, usually through a "tunnel", which is usually closer to the surface of the board than the top of the shield.

The characteristic impedance of the microstrip will be affected by solder resist or other surface coatings. From one manufacturer to another, or even from one board to another board of the same PCB supplier, the application of these coatings may be very inconsistent. Therefore, the impact of these coatings on the impedance of surface microstrip traces is very unknown.