High Frequency Circuit Materials
Specialty high frequency circuit materials have been used in the PCB industry for decades and for many different reasons. There are several attributes of these materials that are very unique when compared to the more traditional PCB materials. When these attributes are well understood, PCB fabricators and OEMs can benefit greatly from improved electrical performance. In addition there are many other non-electrical improvements that can be achieved, which the PCB industry audience may not be aware. In order to realize the full potential of the benefits these materials offer, PCB fabrication issues must be well understood. Most high frequency applications will have multiple demands for PCB materials and not just one specific item of interest. For a specific application, many times one concern is paramount while others are secondary. Understanding these needs and the attributes of high frequency materials allows the user to choose the optimum material for the specific application at hand. A couple of application examples will demonstrate the various attributes to consider.
Why Do Materials Matter?
The right PCB materials will keep your circuits from sending mixed signals. Things like losses in poor quality dielectrics or sub-optimal copper foil can have more of an effect than you might think.
How exactly do dielectrics introduce loss into your system? All dielectrics are made up of polarized molecules. These molecules vibrate in the magnetic fields generated by signals. The higher the frequency, the more vibration, which results in energy lost as heat. This energy loss can be mitigated by using low loss dielectrics, but more on that later.
Another significant loss is introduced by the copper conductors themselves. You may recall learning something about skin depth in college. Remember that electrons do not always flow through the centers of conductors. As frequency increases, currents are limited to a maximum “skin depth.” Well, if your copper is finished with nickel, that can mean most of the current is flowing through that nickel layer. Boom, loss. Even if your entire conductor is made of copper, the micro-profile of the copper can make a difference. If the copper has micro ridges, currents will travel up and down those ridges, increasing resistance and loss.
