We mostly use high frequency laminates in high frequency PCB. However, these are often difficult to fabricate. Because they must maintain the thermal heat transfer of the application due to the sensitivity of the transmitted signal. So, we need special materials for high frequency PCB manufacturing.
When you are selecting a material for High frequency PCB, you must keep the following in mind.
It is the ability of a material to store energy when we apply an electric field. However, it is a directional property which means that it will change with the axis of the material. So, the material you intend to use should have a small dielectric constant. Therefore, it will deliver stable input and there will be no delay in the transmission signal.
Your material should also have a small dissipation factor. Because a high dissipation factor can affect the quality of the transmitted signal. However, a small dissipation factor will allow for less signal wastage.
It depends on the molecular structure of the material and it can affect the RF material going through high frequencies.
It is important in terms of skin effect and cross talk. Crosstalk takes place when the PCB starts interacting with itself and we observe undesired coupling between the components. So, we need to ensure the minimum distance between the plane and trace in order to avoid crosstalk. The skin effect is related to the resistance of the trace. However, skin effect becomes prominent as the resistance increases. So, this can lead to warming the board. Therefore, the trace length and width must be such that it can’t affect the PCB at high frequencies.
The diameter of the VIA
VIAs with smaller diameters have low conductance hence, they are more suited when we are dealing with high frequencies.
Coefficient of thermal expansion
It determines the impact of temperature on the size of the material. So, this becomes important during the assembly and drilling processes. Because even a slight change in the temperature can significantly alter the size of the material. So, you must ensure that the thermal expansion of the foil must be the same as that of the substrate. Otherwise, the foil might dissociate when we subject it to high temperatures.
So, based on these considerations, we recommend the following materials for high frequency PCB,
- Taconic RF-35 Ceramic
- Rogers RO3001
- Taconic TLX
- Rogers RO3003
- ISOLA IS620 E-fiber glass
- Rogers 4350B HF
- ARLON 85N