From the onset, you must have realized that RF Printed Circuit Boards are quite different from the other types of PCBs.
They rely on special materials and special considerations, as outlined below.
1)Scaling
As the PCBs are cured during heat lamination, the internal layers will lose quite some mass. Because of this anticipated loss, it is crucial to scale up the circuitry by a certain percentage.
This ensures that when the layers lose weight, they come to their desired dimensions.
The high-frequency materials used in RF PCBs usually behave differently. This means scaling is very difficult because you will need to understand the specific material.
Calculating the scale factor for the particular material that you are using is a challenge that you have to take.
Failure to do this, you will end up with boards with bad registration from drill to pad and layer to layer. Such boards will not perform correctly.
Over time, however, you should be able to determine consistent scale factors for the materials, making it easier.
2)Surface Preparation
PTFE types are very sensitive when it comes to multilayer surface preparation.
A very aggressive preparation can deform the material because it is relatively softer. Significant deformation will render the PCB scrap.
During deburring, the substrate can end up getting polished, which badly affects adhesion. It requires that you always anticipate the need for special handling.
3)Hole Preparation
Because of the different nature of the materials used in RF PCB, you will need to use different methods in hole preparation.
Therefore, you must adjust drill machine parameters to ensure there is no surface smearing.
Again, different gases than the usual ones are used in treating the holes after drilling. You must, therefore, plan all the necessary steps to ensure that the holes will come out clean.
4)Thermal Expansion Rates
If you are designing a hybrid multilayer PCB by combining high-frequency materials with FR4, you must properly match the materials.
This is because if the material characteristics fail to match, the expansion rates of the layers will be different.
You will also need to match the hole fill materials you use to plug the vias with the other materials in the stack.
If you are an expert designer and fabricator, this should not be a big problem. You have most likely had to analyze these material characteristics before.
5)Machining
The behavior of RF materials when subjected to different machines usually differ from FR4 laminates.
During drilling, for example, ceramic impregnated materials can prove very hard on drill bits.
In such cases, the RPM and spindle infeed should be customized based on the specific characteristics of the material. This will also help you to avoid fibers that usually remain inside the hole walls
To avoid poor quality of the edge, use special bits that are developed for RF laminates. If you use a wrong type of router, the tool is likely to pill against the board’s fibers, giving it a furry edge.
V-scoring may also damage the RF material, and is not recommended unless very necessary. The blade of the v-scoring machine is likely going to pull off the copper from the surface.
6)PCB Material
As we already saw in the earlier sections of this guide, RF PCBs are very “choosy” when it comes to base materials.
Materials such as FR4 are of very low quality when it comes to manufacturing RF PCBs.
The danger with choosing the wrong material is that the problems are mostly realized after production, leading to extreme losses.
If you are hiring a PCB manufacturer, you may have to inspect samples if you doubt the choice of materials. Otherwise, you risk getting a whole order of scrap.
7) Choice of Transmission Line
Choosing the right transmission line is critical for the performance of your RF PCB.
There are two main options when choosing transmission lines for RF PCB: Grounded coplanar waveguides (GCPW) and Microstrip.
If your design is more compact, a GCPW will give better performance because:
- They are characterized with unbroken ground plane
- The grounded layers enable greater isolation for transmission lines. This minimizes losses and reflections.
Despite these strengths, most designers prefer microstrip because most programs do not support the GCPWs.
8)Dimensions of the Transmission Lines
The dimensions of the transmission lines usually affect PPCB performance. All transmission lines and components should be at an impedance of 50 O or very close thereabout.
Again, using very thin transmission lines are likely to lead to lousier performance.
9)Via Spacing and Placement
Via spacing must always be appropriate because the vias are essential for RF PCB performance. It is the vias that help in reducing ground inductance in the RF PCB.
They also help in solving the problems that come with unwanted coupling.
10)Routing on Bias and Ground Layers
In RF PCB design, the system bias layers’ return current path should be well checked.
Signal layers between bias and ground layers lead to larger return paths. This causes noise coupling on signal layers.
For a good layout, there should be no signal line between the ground layers and the bias.