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How to choose high frequency and high speed PCB material?

                                                    How to choose high frequency and high speed PCB material?

    As microprocessors and signal conversion transmitters increase in speed, digital circuits run at 100 Gbps. The use of conventional FR4 PCB materials will not achieve high-speed signal requirements, and the choice of PCB material will determine the performance of the product.

    Choosing PCB materials must strike a balance between meeting design requirements, mass production and cost. To put it simply, design requirements include both electrical and structural reliability. It is often important to choose the right material when designing a high speed PCB. For example, the commonly used FR-4 material, the dielectric loss (Df) at a few GHz frequency will be large, so may not be applicable.

    For example, a 10Gb/S high-speed digital signal is a square wave, which can be thought of as a superposition of sinusoidal signals of different frequencies. Therefore 10Gb/S contains many different frequency signals such as 5Ghz fundamental signals, 3rd order 15GHz, 5th order 25GHz, 7th order 35GHz signals and so on. Keep the integrity of the digital signal and the steepness of the upper and lower edges as well as the low-loss, low-distortion transmission of RF microwaves.

    The choice of high-frequency materials seems to be simple but there are still many factors to consider In practice,. As a PCB design engineer or a high-speed project leader, you need to have a certain understanding of the electrical properties, thermal performance, reliability, etc. of the material. In this way, We can design a PCB product with high reliability and good processing performance.

The main considerations for choosing the right material are as follows:

1. Manufacturability:

For example, the performance of material lamination, temperature performance, CAF/heat resistance and mechanical toughness (good reliability), fire rating;

2. Various properties matching the product (electricity, performance stability, etc.):

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

3. Material procurement cycle:

In addition to the commonly used high-frequency material RO4350B, many high-frequency materials have a very long procurement cycle, even 2~3 months. Therefore, if your PCBs with high-frequency materials need to be fabricated, try to communicate with PCB manufacturers as early as possible in order to prepare materials in advance.

4. Cost factors:

Look at the price sensitivity of products, whether it is consumer products, or applications in communications, medical, industrial, military;

5. Applicability of laws and regulations:

The material should be comply with environmental regulations in different countries, such as meeting RoHS and halogen-free requirements.

Among the above factors, the running speed of high-speed digital circuits is the main factor in the PCB materials selection. The higher the speed of the circuit, the smaller the Df value of PCB material.

Analysis from PCB material Df values:

The Df value is between 0.01 and 0.005, and the upper limit is 10Gb/S digital circuit;

The Df value is between 0.005 and 0.003, and the upper limit is 25Gb/S digital circuit;

The Df value ≤ 0.0015 are suitable for 50 Gb/S or even higher speed digital circuits.

Commonly used high-frequency or high-speed materials are:

1), Rogers Rogers: RO4003, RO3003, RO4350, RO5880, etc.

2), TUC : Tu862, TU872SLK, TU883, TU933, etc.

3), Panasonic: Megtron4, Megtron6, etc.

4), Isola: FR408HR, IS620, IS680, etc.

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

6), Arlon, Taconic

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