Choose the best high frequency circuit materials
The correct choice of high frequency circuit materials will make your circuit avoid sending mixed signals. The effect of poor quality dielectric constant or loss of suboptimal copper foil may be greater than you think. How can dielectric accurately introduce losses into your system? All dielectric materials are composed of polarized molecules. These molecules vibrate in a magnetic field generated by the signal. The higher the frequency, the greater the vibration, resulting in the loss of energy as heat. This energy loss can be reduced by using a low loss dielectric, but it will be more in the future.
Copper conductor itself brings another big loss. You may remember what you learned about skin depth in college. Remember, electrons don’t always flow through the center of the conductor. As the frequency increases, the current is limited to the maximum skin depth. Well, if your copper is made of nickel, it may mean that most of the current flows through the nickel layer. Prosperity, loss. Even if your entire conductor is made of copper, the micro profile of copper will be different. If copper has micro ridges, the current will move up and down on these ridges, increasing resistance and loss. The relative permittivity and conductor on a printed circuit board can cause significant differences in signal integrity.
Material selection based on circuit manufacturing problems
As a part of high frequency PCB manufacturing, many different mechanical processes are needed. In general, the most critical ones are drilling, electroplated through-hole (PTH) preparation, multilayer lamination and assembly. The drilling process is usually associated with the creation of a cleaning hole, which is then metallized to form a through hole for electrical connection from one conductive layer to another.
Some of the problems during drilling include coating, burring and cracking of materials. Since stains cannot be removed, they can be fatal to PCB manufacturing using PTFE based materials. Fracturing can be fatal for some nonwoven glass hydrocarbon materials. However, most woven glass hydrocarbon materials do not have this problem.
For most non PTFE materials, the definition of the preparation process of PTH is relatively clear and simple, although special treatment is needed to form PTH of PTFE based materials. Ceramic filled PTFE based materials provide more tolerant PTH preparation options. However, non ceramic filled PTFE materials require special processes, which will limit the yield of the final circuit.
Manufacturing multilayer PCB presents many challenges. It is a fact that heterogeneous materials are usually bonded together and may have properties that complicate the drilling and PTH preparation process. Moreover, mismatches between some material properties, such as the coefficient of thermal expansion (CTE), can lead to reliability problems when thermal stress is applied to the circuit during assembly. The goal of the material selection process is to find a good combination of circuit materials for Multilayer PCB to realize the actual manufacturing process and meet the requirements of end use.
Designers and manufacturers can choose a variety of materials to bond copper clad laminates together to form multilayer PCB. As shown in Table 2, the materials differ in dielectric constant, dissipation factor and processing temperature. Generally, a lower lamination temperature is preferred. However, if the PCB must be soldered or some other form of heat exposure, it is necessary to use an adhesive material with a high reflow (remelting) temperature, which has strong thermal stability and does not reflow at elevated processing temperatures.
When selecting high frequency circuit materials based on manufacturing problems, the most preferred choice is ceramic filled hydrocarbon materials with woven glass. These materials have a low dissipation factor typically on the order of 0.003 and are robust in most circuit manufacturing processes. If better electrical properties are needed, PTFE woven glass filled with ceramic glass can be selected. The dissipation factors of these materials are usually in the range of 0.002 and are generally process friendly.
The main problems in PCB fabrication with these materials are related to drilling and PTH preparation. In order to obtain the best electrical performance, microfiber glass PTFE should be selected, although this material may be difficult to make more complex circuit structures. This material is almost pure PTFE and is usually used in simple high frequency circuits such as microstrip filters and couplers. In addition, it is used in the manufacture of other multi-layer PCB materials, which are more suitable for the manufacturing of multi-layer circuits.