5G mm-Wave era brings significant changes to many areas of our life. New technology requires not only completely novel approaches in concepting and design but also in the manufacturing of PCBs. There is a big amount of possible complexities when it comes to high frequencies, you should consider manufacturing limits as early as possible in your device creation, ideally – on the simulation phase.

Dielectric material tolerances

The most significant part of every PCB is its dielectric substrate. We all are getting used to FR-4 materials as they are cheap and reliable, but it only works up to 6GHz, best case scenario up to 10GHz. What is happening when you go to the mm-Wave range, which is up to 40GHz?
Well, there are several things that start bringing a lot of problems. First of all – dielectric losses. Normal FR-4 material has loss tangent in a range of 0.03 at 1GHz and it gets even worse and less predictable when frequency increases. This loss level might be not so important for cellular bands, but the higher frequency the bigger losses per signal line’s length, so you might face real problems with standard FR-4 material at mm-Wave bands.

The second thing to consider is the tolerance of dielectric constant and material thickness. The fluctuation of those factors is quite normal for nowadays manufacturing process and its effects getting worse as frequency increases. It is really important to take it into account in the early stages of simulation to have a good idea of what to expect. Figure 1 shows how to return loss changes with dielectric constant and thickness fluctuation in the example design of a wideband 5G antenna array.

Effect of dielectric fluctuation mmWave

Figure 1. Fluctuation of dielectric constant and thickness in 5G antenna array design.

The physical limitation of minimum copper element sizes

5G mm-Wave technology might be very appealing due to really small sizes of antenna topologies or feed network structures, but the same advantage brings new manufacturing challenges. When it comes to sub 6GHz frequencies usually these things are not vital as you rarely come close to physical limits, but in mm-Wave designs it is crucial. Minimum signal line width, the gap between copper elements, VIA diameters: all of these things need to be considered in the simulation. In figure 2 you can see Vivaldi antenna topology designed for 30GHz frequency. The marked gap size is too small for manufacturing, thus topology needs to be modified to meet requirements.

vivaldi antenna model with a gap too small for manufacturing

Figure 2. 5G mm-Wave Vivaldi antenna component.

Components’ and connectors’ behavior

Sometimes you might need to place SMD components into your PCB, for example for power divider topology or antenna’s matching network. Usually, all components work well up to 6GHz but after that properties start shifting. On figure 3 a plot from a high-frequency resistor datasheet is shown. You can clearly see, that 100 Ohm resistance becomes 60 Ohm after 10GHz.

Resistor impedance can vary greatly from nominal value at mmWave frequencies

Figure 3. Vishay high frequency resistor’s characteristics.

When it comes to connectors, there are even more issues. High-frequency connectors are pretty expensive and very tricky to use. You should take them into account in the simulation process or you might face rather different results in real life. Figure 4 shows a high-frequency connector included in the mm-Wave feed network simulation. As you can see in the picture, the connector is attached to PCB with a pair of screws. At 40GHz these screws might easily start resonating and bring losses to the system.

Even the screws of RF connector cans tart to resonate

Figure 4. Simulation model of a connector.

Listed challenges are only few and most obvious, but they might give a hint about what to expect when it comes to mm-Wave topologies manufacturing. The best way to predict outcome and to be sure that manufacturing tolerances will not ruin your plans is to take every possible fluctuation into account in electromagnetic simulations. Revealing design’s weakest spots at the earliest phases of the product development brings competitive advantages and reduces overall expenses.

Disclaimer: The views and opinions expressed in this article are those of the author. It is intended only as a sharing of antenna design knowledge for educational purposes.

Katerina GalitskayaAntenna and RF Design Engineer