How do I select the appropriate RF PCB material?

Question :Is the material the most important factor for RF PCB?

Material is one factor, though standard FR4 is adequate for all except the highest frequencies. Once you get to frequencies where you might use a waveguide, PCB substrates become rather specialised, looking rather like cardboard.As important is the physical dimensions of tracking and good component layout; components and tracking can talk to each other in undesirable ways, and transmission lines must be the right width and spaced over the ground plane by the right thickness of insulator.

 

Below are the suggestions for how to choose the appropriate RF PCB material.

By sampling different types of line laminate materials, it is possible to summarize some of the differences in material selection. For example, whether the material is added with glass cloth, whether special ceramic filler is added. The addition of special ceramic fillers to PTFE materials helps to improve the performance of both CTE and TCDk parameters; the addition of ceramic fillers also allows the Dk values of PTFE line laminates to reach a wide range (approximately between 3 and 10). Without ceramic fillers, PTFE achieves better loss characteristics (low Df), but has poorer CTE and TCDk properties than ceramic filled materials. In general, PTFE line laminates with higher Dk values will exhibit higher Df values and greater anisotropy as Dk increases.Ceramic-filled hydrocarbon line laminates reinforced with glass cloth generally have better CTE, TCDk and thermal conductivity than PTFE materials, but have a higher Df (meaning greater loss) than PTFE materials. The line material of PPE and PPO generally has a higher Df value than the PTFE line, and is roughly the same as the Df value of the hydrocarbon material at a frequency of 10 GHz or less. PPE and PPO board materials have some special properties, such as excellent long-term aging characteristics, but they have higher water absorption than other types of high-frequency line laminates.

 

What kind of material is commonly used for an RF PCB?

RF PCB usually needs to select a high-frequency board to carry the circuit. The main considerations in selecting the appropriate HIGH-FREQUENCY PCB material are: 

1.Manufacturability

For example, how many times the lamination performance, temperature performance, etc., CAF/heat resistance and mechanical toughness (adhesion) (good reliability), fire rating;

2. Various performances (electrical, performance stability, etc.) that match the product

Low loss, stable Dk / Df parameters, low dispersion, small coefficient of variation with frequency and environment, small material thickness and glue content tolerance (good impedance control), if the wiring is long, consider low-roughness copper foil. Also, the high-speed circuit design needs simulation in the early stage, and the simulation result is the reference standard for the design.

3. Timely availability of materials

The purchase cycle of many high-frequency boards is very long, even 2-3 months. Except for the conventional high-frequency boards RO4350, many high-frequency boards need to be provided by customers. Therefore, the high-frequency sheet needs to communicate well with the manufacturer in advance and prepare materials as soon as possible;

4. Cost

It depends on the price sensitivity of the product, whether it is a consumer product or communication, medical, industrial, military application;

5. Applicability of laws and regulations

It must be integrated with environmental protection regulations of different countries to meet RoHS and halogen-free requirements.

Among the above factors, the speed of high-speed digital circuits is the main factor for PCB selection. The higher the circuit speed, the smaller the PCB Df value should be. Medium and low-loss circuit boards will be suitable for 10Gb / S digital circuits; plates with lower losses will be suitable for 25Gb / s digital circuits; ultra-low-loss boards will be adapted for faster high-speed digital circuits, and its speed can be 50Gb / s or higher.

From the material Df:

Df is between 0.01 ～ 0.005 PCB sheet is suitable for digital circuits with a limit of 10Gb / S

Df is between 0.005 ～ 0.003 PCB board is suitable for digital circuits with an upper limit of 25Gb / S;

Df does not exceed 0.0015 PCB sheet is suitable for 50Gb / S and even higher speed digital circuits.

Common high-speed plates are:

Rogers: RO4003, RO3003, RO4350B, RO5880, etc.

Tai Yao TUC: Tuc862, 872SLK, 883, 933, etc.

Panasonic: Megtron4, Megtron6, etc.

Isola: FR408HR, IS620, IS680, etc.

Nelco: N4000-13, N4000-13EPSI, etc.

Dongguan Shengyi, Taizhou Wangling, Taixing Microwave, etc.

Of course, there are many other high-frequency materials, such as Arlon (acquired by Rogers) and Taconic, which are all old-fashioned RF microwave plate factories with guaranteed performance.

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