Once you have mastered the various requirements and standards, you need to choose materials for your PCB. The choice of materials is one of the most crucial parts of the design process, as it covers everything from high frequencies to thickness and ratio of the various elements.
There are countless pitfalls here that you can fall into, and if you choose the wrong material, it can significantly delay the process – which you want to avoid.
Once again, the choice of materials depends on the type of design you are making. Consider whether it is something for a computer, and thereby high-speed digital, or a PCB for a smartphone or other communication devices, that are operating at other frequencies. The choice of material will highly depend on the frequencies.
Frequencies and isolation materials
When choosing the right material, look at the different frequencies and how fast the board signals are. It is crucial to select the best material for the signal frequency.
Especially concerning high-speed signals, the PCB material must be dimensioned correctly for the purpose in question. If, for example, there is a requirement for high frequency signals, you must take this as your starting point. When it comes to high frequencies, you also need to be aware of how much loss there is in the material you choose.
Copper, for example, is very smooth. In most cases, it is better to have a rough surface on the copper and isolation layer, so they stick together more securely. However, in terms of signal, the copper should be as smooth as possible because the electrons move on the outside of the copper. If the surface is too rough, it becomes too diffused and unspecified. You must therefore take these factors into account when choosing a material.
Remember that you must include the epsilon relative. It is necessary because it affects the isolation material. Once again, it will be an advantage if you keep in contact with the PCB manufacturer.
The epsilon relative is frequency dependent, so if, for example, you have a requirement which states the need for high frequency signals, you must have a certain impedance. You achieve the specific impedance by adjusting the width of the tracks you make, to the isolation material placed in between.
Select the correct thickness of the material
The many different electrical layers all have different thicknesses. If you choose the wrong thicknesses from the beginning, there is a risk that you will be unable to make the desired connections between the layers – and that is a problem.
Taking the construction of the various electrical layers as a starting point, it is the vias that connect them individually. If you get too great a distance between them, and it gets too thick, you will be unable to make the via connections – and you may risk that production is not possible.
There is a condition stating that the hole size must be correct in relation to the thickness of the PCB. You cannot make the hole infinitely thin, as it must have a minimum diameter to the thickness of the board. You may end up making a PCB so thick you cannot attach the component you want. Therefore, it is essential to get this ratio right.
The IPC standard also specifies the ratio of the thickness of the PCB, to the hole. It explains how large the annular rings should be in relation to the desired standard.
The higher the requirements, the larger the annular ring must be. That way you make sure it does not break when you solder it all together. Therefore, thoroughly research the ratio that applies to your class and standard before continuing the design process.
Different thicknesses of the PCB:
Left: 4-layer PCB stackup. Right: 6-layer PCB stackup
Remember to consider the material’s thermal coefficient expansion. Materials can change their size at certain temperatures, and this can have a major impact on the drilling and assembly process of the PCB. So be sure to consider the temperature of the final product when choosing the material.