Radio Frequency (RF) PCB Layout Design – Microstrip Transmission Lines

To date, microstrip remains the most commonly used transmission line structure in RF and microwave designs. However, this is becoming less and less the case as the speed and density of digital and hybrid technology designs continue to increase.

Because microstrip lines are typically wider than ribbon lines for the same impedance, and because of the increased radiation associated with microstrip lines, it requires both more wiring space and a greater distance around the alignment. In a pure RF or microwave design, this is usually not an issue, but with the need for smaller product sizes and the consequent increase in component density, it becomes a less readily available option.

Structured Body

A microstrip transmission line consists of a conductor (usually copper) of width W and thickness t that is routed on a ground plane wider than the transmission line itself and separated by a dielectric of thickness H (see figure below). Best practice is to ensure that the ground reference plane extends at least 3H on either side of the surface microstrip line.

Advantages:

Historically, the main advantage of microstrip lines may have been the ability to use only two layers of boards with all components mounted on one side. This simplifies the manufacturing and assembly process and is the lowest cost RF 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 the cost factor, this is ideal because the use of via holes does not increase capacitance or inductance.

Since a microstrip will typically be wider than a strip line alignment for the same impedance, it is easier to have tighter control over the characteristic impedance of the alignment since the etch tolerance in manufacturing is absolute. So, if you have an alignment width of 20 mils and over-etch and reduce the width by 1 mil, this is a very small percentage change compared to over-etching a 5 mil strip line and reducing it to a 4 mil width. For example, in FR408 material with a trace 20 mils higher than the ground and 11.5 mils higher than the tape line with a dielectric constant of 3.8 would yield about 50.8 ohms. if this trace were reduced to 19 mils, the characteristic impedance would be about 52.6 ohms, an increase in characteristic impedance of 3.6%. Whereas a 5 mil ribbon trace in the same material with a 6 mil top and bottom ground will yield approximately 50.35 ohms, but when reduced by 1 mil to 4 mils, the characteristic impedance will be approximately 56.1 ohm, an increase of 11.5%. In completing some designs, the characteristic impedance of the final alignment was not specified, but rather the final width. In the same over-etch scenario, a 5 million trace reduction of 1 million mils would reduce the final alignment width by 20%, while a 20 mil trace reduction of 1 million mils would reduce the width by 5%.

Disadvantages:

Since microstrip transmission lines are typically very wide and are laid on the surface of the board, this means that the surface area available for component placement is reduced. This makes microstrip less useful for high-density hybrid technology designs where space is almost always at a premium.

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

Once again, crosstalk becomes an issue due to the increased radiation from the microstrip, and thus the need to provide increased spacing from other circuit elements, resulting in a reduction in the available wiring density.

Microstrip designs often require external shielding, which adds cost and complexity. In fact, this has become one of the most important issues in the design of portable devices such as cell phones. Many products are getting smaller and therefore thinner drivers. This means that the shield will be closer to the surface of the board, which will increase the capacitance per unit length of the transmission line and thus change its impedance. Consider carefully when choosing to use microstrip transmission lines and deriving impedance models. If the alignment needs to pass through an external shield wall, it may be necessary to modify the width of the transmission line by a small distance, usually through a “tunnel” that is usually closer to the board surface than the top of the shield.

The characteristic impedance of the microstrip will be affected by solder resist or other surface coatings. The application of these coatings can be very inconsistent from one manufacturer to another, and even from one board to another from the same supplier, so the effect of these coatings on the surface microstrip line impedance is very unknown.