There are two factors that you need to take into account when choosing the right materials for a printed circuit board with high frequency for PCB assembly. The first one is how well the materials meet the needs of an end-use application. The second one is how much effort is needed to fabricate the desired circuit. These two things should work together.
Actually, there are no proven guidelines or step-by-step procedures in choosing the right materials. However, you can simply decide by evaluating the materials in terms of their relevance to meeting the needs of an end-use application and to circuit fabrication.
Taking into consideration the end-use applications
There are some factors involved in choosing materials based on the end-use applications. These factors are thermal conductivity, dielectric constant (Dk), moisture absorption and conductor losses.
The first one is thermal conductivity which refers to the capability to conduct heat. In high-power applications this parameter is very important to take into account because high temperature should be dissipated. The substrate for high-power applications should have high thermal conductivity to be effective. Basically, most printed circuit board materials have 0.25 W/m/K thermal conductivity. The best way to improve the thermal conductivity of a circuit material is by adding a ceramic filler. Moreover, incorporating a material with ceramic filler can help solve some issues on thermal management.
Next, dialectric constant (Dk) is the value needed to determine as a process control for making the substrate. One of the most common methods today to get the Dk value is through software simulation tools. However, it yields erroneous results. When the process Dk values are not suitable for printed circuit board design, technicians determine a second set of Dk values via micro strip transmission lines circuits.
Another important factor to consider during PCB assembly is the moisture absorption. For example, a very humid environment will cause increases in Dk which can negatively affect the PCB performance. In this type of environment, it is ideal to use circuit materials with high moisture absorption to avoid degraded performances. Most printed circuit board materials have 1% moisture absorption.
In addition, conductor losses should also be taken into account, especially for thin circuits. Some of the factors that affect conductor losses are roughness of the copper, circuit configuration, circuit design and thickness of conductive metals. Materials with high amount of copper surface roughness have higher conductor losses compared to materials that have smoother ones.
Considering the fabrication issues
There are different mechanical processes involved in the high frequency PCB fabrication. These include multilayer lamination, PCB assembly, plated-through-hole (PTH) preparation and drilling.
The main concern with multilayer lamination is the fact that dissimilar materials are bonded together. These dissimilar materials can complicate the PTH preparation and drilling processes. Moreover, a mismatch between material properties can cause reliability problems. Generally, materials with low lamination temperatures are used to avoid these problems.
When it comes to PCB assembly the main issue is the effects of thermal stress in the soldering process. Circuit materials with a lower coefficient of thermal expansion (CTE) are preferred because these materials can handle the thermal stress better. This is why different circuit materials are used in multilayer PCB construction. The combination of materials with lower CTE and good electrical properties makes it possible to assemble and design a robust printed circuit board.
PTH preparation process requires special processing. The use of ceramic-filled PTFE-based materials gives more options. In contrast, the use of non-ceramic-filled PTFE materials needs a specific process and provides limited choices.
Lastly, there are different issues during the drilling process. Some of these are fracturing of materials and smearing. The process of fracturing is not appropriate for nonwoven glass hydrocarbon materials. Smearing can be fatal for PTFE-based materials.