When processing and testing high-frequency PCB boards and high-frequency antennas, beginners often choose incorrect high-frequency PCBs, resulting in unsatisfactory product results. Let’s talk briefly today, what are the PCB high-frequency boards? How to choose?

1. Definition of PCB high-frequency boards

High-frequency circuit boards refer to special PCB circuit boards with higher electromagnetic frequencies. They are used in high-frequency (frequency greater than 300MHZ or wavelength less than 1 meter) and microwave (frequency greater than 3GHZ or wavelength less than 0.1 meter) fields. They are produced on microwave substrate copper-clad boards using some processes of ordinary rigid circuit board manufacturing methods or using special processing methods. Generally speaking, high-frequency boards can be defined as circuit boards with frequencies above 1GHz.

With the rapid development of science and technology, more and more equipment designs are used in the microwave frequency band (>1GHZ) or even in the millimeter wave field (77GHZ) (such as the popular 77GHz car-mounted millimeter wave antenna), which also means that the frequency is getting higher and higher, and the requirements for the substrate of the circuit board are getting higher and higher. For example, the substrate material needs to have excellent electrical properties and good chemical stability. The loss on the substrate is required to be very small as the power signal frequency increases, so the importance of high-frequency boards is highlighted.

II. Classification of PCB high-frequency circuit boards

1. Divided by material: Organic materials: phenolic resin, glass fiber/epoxy resin, Polyimide, BT/Epoxy, etc. are all included. Inorganic materials: aluminum, copper-invar-copper, ceramic, etc. are all included. Mainly for its heat dissipation function
2. Divide by the hardness and softness of the finished product: rigid board Rigid PCB, flexible board Flexible PCB, rigid-flex PCB;
3. Divided by structure: single-sided board; double-sided board; multi-layer board.
4. Divided by purpose: communication/consumable electronics/military/computer/semiconductor/electrical test board…

III. What are the important indicators for selecting high-frequency and high-speed PCB materials

When selecting the substrate used for PCBs for high-frequency circuits, special attention should be paid to the material DK and its changing characteristics at different frequencies. For requirements that focus on high-speed signal transmission or characteristic impedance control, the focus is on DF and its performance under frequency, temperature and humidity.

General substrate materials show a large change in DK and DF values ​​under frequency changes. Especially in the frequency range of 1 MHz to 1 GHz, the changes in their DK and DF values ​​are more obvious. For example, the DK value of general epoxy resin-glass fiber cloth-based substrate materials (general FR-4) at a frequency of 1 MHz is 4.7, while the DK value at a frequency of 1 GHz changes to 4.19. Above 1 GHz, its DK value changes slowly. Its change trend is that it becomes smaller as the frequency increases (but the change range is not large). For example, at 10 GHz, the DK value of general FR-4 is 4.15. Substrate materials with high-speed and high-frequency characteristics have a small change in DK value under frequency changes. At the frequency change from 1 MHz to 1 GHz, DK mostly remains in the range of 0.02. Its DK value tends to decrease slightly under different frequency conditions from low to high.

The dielectric loss factor (DF) of general substrate materials changes more than DK when affected by frequency changes (especially changes in the high-frequency range). Its change law tends to increase. Therefore, when evaluating the high-frequency characteristics of a substrate material, the focus of its investigation is the change of its DF value. For substrate materials with high-speed and high-frequency characteristics, there are two obvious differences in the change characteristics at high frequencies. One is that its (DF) value changes very little with the change of frequency. Another type is that although the change amplitude is similar to that of general substrate materials, its own (DF) value is lower.

Fourth, how to choose high-speed board materials for high-frequency circuit boards
The selection of PCB board materials must strike a balance between meeting design requirements, mass production, and cost. Simply put, design requirements include electrical and structural reliability. Usually, this board problem is more important when designing very high-speed PCB boards (frequencies greater than GHz). For example, the commonly used FR-4 material has a large dielectric loss Df (Dielectric loss) at frequencies of several GHz, which may not be applicable.

For example, 10Gb/S high-speed digital signal is a square wave, which can be regarded as the superposition of sine wave signals of different frequencies. Therefore, 10Gb/S contains many different frequency signals: 5Ghz fundamental signal, 3rd order 15GHz, 5th order 25GHz, 7th order 35GHz signal, etc. Maintaining the integrity of digital signals and the steepness of the upper and lower edges is the same as the low-loss and low-distortion transmission of RF microwaves (the high-frequency harmonic part of the digital signal reaches the microwave frequency band). Therefore, in many aspects, the selection of PCB materials for high-speed digital circuits is similar to the requirements of RF microwave circuits.

In actual engineering operations, the selection of high-frequency boards seems simple, but there are still many factors to consider. Through the introduction of this article, as a PCB design engineer or high-speed project leader, you have a certain understanding of the characteristics and selection of boards. Understand the electrical properties, thermal properties, reliability, etc. of the board. And use stacking reasonably to design a product with high reliability and good processability, and optimize the consideration of various factors.

5. The following will introduce the main considerations for selecting suitable boards:
6. Manufacturability: such as multiple lamination performance, temperature performance, CAF resistance/heat resistance and mechanical toughness (adhesion) (good reliability), fire resistance level;
7. Various performances matching the product (electrical, performance stability, etc.): low loss, stable Dk/Df parameters, low dispersion, small coefficient of variation with frequency and environment, small tolerance of material thickness and glue content (good impedance control), if the trace is long, consider low roughness copper foil. Another point is that simulation is required in the early stage of high-speed circuit design, and the simulation results are the reference standard for design. “Xinsen Technology-Agilent (High Speed/RF) Joint Laboratory” has solved the performance problem of inconsistent simulation results and tests, and has done a lot of simulation and actual test closed-loop verification. Through a unique method, it can achieve consistency between simulation and actual measurement.
8. Timely availability of materials: The procurement cycle of many high-frequency boards is very long, even 2-3 months; except for the conventional high-frequency board RO4350, which is in stock, many high-frequency boards need to be provided by customers. Therefore, high-frequency boards need to be communicated with manufacturers in advance and materials should be prepared as early as possible;
9. Cost factor Cost: Look at the price sensitivity of the product, whether it is a consumer product, or a communication, medical, industrial, or military application;
10. Applicability of laws and regulations, etc.: It must be integrated with environmental protection laws and regulations of different countries to meet RoHS and halogen-free requirements.

Among the above factors, the operating speed of high-speed digital circuits is the main factor to consider in PCB selection. The higher the rate of the circuit, the smaller the selected PCBDf value should be. Circuit boards with medium and low loss will be suitable for 10Gb/S digital circuits; boards with lower loss are suitable for 25Gb/s digital circuits; boards with ultra-low loss will adapt to faster high-speed digital circuits, and their rates can be 50Gb/s or higher.

From the material Df point of view: Df between 0.01 ~ 0.005 circuit board is suitable for 10Gb/S digital circuit; Df between 0.005 ~ 0.003 circuit board is suitable for 25Gb/S digital circuit; Df not exceeding 0.0015 circuit board is suitable for 50Gb/S or even higher speed digital circuit.

VI. Processing method:

1. Cutting: The protective film must be kept to prevent scratches and indentations.
2. Drilling: Use a new drill bit (standard 130), one piece per stack is the best, the pressure foot pressure is 40psi; aluminum sheet is the cover plate, and then use 1mm melamine pad to tighten the PTFE plate; blow out the dust in the hole with an air gun after drilling; use the most stable drilling rig and drilling parameters (basically, the smaller the hole, the faster the drilling speed, the smaller the chip load, and the smaller the return speed).
3. Hole treatment: plasma treatment or sodium naphthalene activation treatment is conducive to via metallization.
4. PTH copper deposition: After micro-etching (micro-etching rate has been controlled at 20 micro-inches), the PTH pulls the board from the oil removal cylinder; if necessary, the second PTH is passed, and the board only needs to be fed from the expected cylinder.
5. Solder mask: Pre-treatment: Use acid to wash the board, and do not use mechanical grinding; bake the board after pre-treatment (90℃, 30min), brush green oil for curing; bake the board in three stages: one stage at 80℃, 100℃, and 150℃, each for 30min (if oil is found on the substrate surface, it can be reworked: wash off the green oil and reactivate it).
6. Gong board: Lay white paper on the circuit surface of the PTFE board, and clamp it with a FR-4 substrate board or phenolic base board with a thickness of 1.0MM that has been etched to remove copper.

The above summarizes how to choose high-frequency PCB high-speed board and design precautions. In practice, the application still needs to be analyzed according to specific cases.