What is a high-frequency PCB board? What are the high-frequency boards? How to choose?
High-frequency circuit board refers to a special circuit board with a higher electromagnetic frequency. It is used in the fields of 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).
It is a circuit board produced by using some processes of ordinary rigid circuit board manufacturing methods or special processing methods on microwave substrate copper clad boards. 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 millimeter wave antenna on vehicles), 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.
Classification of PCB high-frequency circuit boards
A. Classification by material:
a. Organic materials: phenolic resin, glass fiber/epoxy resin, Polyimide, BT/Epoxy, etc.
b. Inorganic materials: aluminum, copper-invar-copper, ceramic, etc. Mainly for its heat dissipation function
B. Classification by the hardness and softness of the finished product
a. Rigid PCB,
b. Flexible PCB,
c. Rigid-Flex PCB
Taiyao TUC: Tuc862, 872SLK, 883, 933, etc.
C. Classification by structure
a. Single-sided board,
b. Double-sided board,
c. Multilayer board
D. According to the purpose
Communication/consumable electronics/military/computer/semiconductor/electrical test board…
Commonly used high-speed boards (manufacturers)
National boards have high cost performance and performance is not inferior to imported products. Representative ones are: Dongguan Shengyi, Shanghai Nanya New Materials, Taizhou Wangling, Taixing Microwave, Changzhou Zhongying, Gongli Ceramic Board, Taiyao TUC: Tuc862, 872SLK, 883, 933, etc.
Foreign ones are:
1), Rogers: Rogers: RO4003, RO3003, RO4350, RO5880, etc. With the development of 5G millimeter wave, Rossjie has also launched a number of low-loss circuit boards suitable for millimeter waves.
RO3000 series: PTFE circuit materials based on ceramic filling, models are: RO3003, RO3006, RO3010, RO3035 high-frequency laminates.
RT6000 series: PTFE circuit materials based on ceramic filling, designed for electronic circuits and microwave circuits that require high dielectric constants, models include: RT6006 dielectric constant 6.15, RT6010 dielectric constant 10.2.
TMM series: composite materials based on ceramics, hydrocarbons, and thermosetting polymers, models: TMM3, TMM4, TMM6, TMM10, TMM10i, TMM13i. Etc.
2), Taconic: TLX series, TLY series, etc.
3), Panasonic: Megtron4, Megtron6, etc.
4), Isola: FR408HR, IS620, IS680, etc.
5), Nelco: N4000-13, N4000-13EPSI, etc.
Of course, there are many other high-frequency circuit boards that are not listed one by one. Among them, Arlon (which has been acquired by Rogers and is also an old brand RF microwave board factory)
What are the important indicators for selecting high-frequency and high-speed PCB materials?
When selecting substrates for PCBs used in 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 requirements, the focus should be on DF and its performance under conditions such as frequency, temperature and humidity.
General substrate materials show a large change in DK and DF values under frequency changes.
Especially within 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 material (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 tend to be gentle. Its changing trend is that it becomes smaller as the frequency increases (but the change is not large).
For example, at 10GHz, the DK value of general FR-4 is 4.15.
The DK value of substrate materials with high-speed and high-frequency characteristics changes less when the frequency changes. At the frequency change from 1MHz to 1GHz, DK mostly maintains a change 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 changing 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 different types of general substrate materials in terms of changing characteristics at high frequencies: one is that its (DF) value changes very little with the change of frequency. Another type is similar to general substrate materials in terms of change amplitude, but its own (DF) value is lower.
How to choose high-frequency circuit board high-speed board
The selection of PCB board must strike a balance between meeting design requirements, mass production and cost. Simply put, the design requirements include electrical and structural reliability. Usually, this board issue 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, and may not be applicable.
For example, the 10Gb/S high-speed digital signal is a square wave, which can be regarded as the superposition of sinusoidal 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 the digital signal 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 will have a certain understanding of the characteristics and selection of boards. Understand the electrical properties, thermal properties, reliability, etc. of the board. And use the stacking reasonably to design a product with high reliability and good processability, and optimize the consideration of various factors.
The following will introduce the main considerations for selecting suitable boards:
- Manufacturability:
For example, how are the multiple pressing performances, temperature performance, etc., CAF resistance/heat resistance and mechanical toughness (adhesion) (good reliability), fire rating;
- 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 routing 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.
- Timely availability of materials:
Many high-frequency boards have a very long procurement cycle, 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 prepared as early as possible;
- Cost factor Cost:
It depends on the price sensitivity of the product, whether it is a consumer product, or a communication, medical, industrial, or military application;
- 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 circuit rate, 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, with a rate of 50Gb/s or higher.
From the material Df point of view:
Circuit boards with Df between 0.01 and 0.005 are suitable for digital circuits with an upper limit of 10Gb/S;
Circuit boards with Df between 0.005 and 0.003 are suitable for digital circuits with an upper limit of 25Gb/S;
Circuit boards with Df not exceeding 0.0015 are suitable for 50Gb/S or even higher speed digital circuits.
Processing method:
- Cutting: The protective film must be retained to prevent scratches and indentations
- Drilling:
2.1 Use a new drill bit (standard 130), one piece per stack is the best, and the pressure foot pressure is 40psi
2.2 Use the aluminum sheet as the cover plate, and then use a 1mm melamine pad to tighten the PTFE plate
2.3 Use an air gun to blow out the dust in the hole after drilling
2.4 Use the most stable drill 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)
- Hole treatment
Plasma treatment or sodium naphthalene activation treatment is conducive to through-hole metallization
- PTH copper deposition
4.1 After micro-etching (the micro-etching rate has been controlled at 20 micro-inches), start the PTH pull from the oil removal cylinder to enter the board
4.2 If necessary, go through the second PTH, just from the expected? The cylinder starts to enter the board
- Solder mask
5.1 Pretreatment: Use acid to wash the board, do not use mechanical grinding
5.2 Bake the board after pretreatment (90℃, 30min), brush green oil to cure
5.3 Bake the board in three stages: 80℃, 100℃, and 150℃, each for 30min (if the substrate surface is found to be oily, it can be reworked: wash off the green oil and reactivate it)
- Gong board
Put white paper on the circuit surface of the PTFE board, and clamp it with a 1.0MM thick FR-4 substrate board or phenolic base board with copper etched off
The above summarizes how to choose high-speed boards and design precautions. In practice, the application still needs to be analyzed according to specific cases.
