2024-01-20
What is a high-frequency PCB board? What are the high-frequency boards? How to choose?
When processing and testing high-frequency PCB boards and high-frequency antennas, beginners often choose the incorrect high-frequency PCB, resulting in unsatisfactory product results. What are the PCB high frequency sheets? how to choose?
Definition of PCB high frequency board
High-frequency circuit boards refer to special circuit boards with higher electromagnetic frequencies. They are PCBs 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). They are microwave substrates. CCL is a circuit board produced by using part of the ordinary rigid circuit board manufacturing method 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 are designed for applications in the microwave frequency band (>1GHZ) or even in the millimeter wave field (77GHZ) (such as the now very popular vehicle-mounted 77GHz millimeter wave antenna), which also means As the frequency becomes higher and higher, the requirements for the base material of circuit boards are also getting higher and higher. For example, the substrate material needs to have excellent electrical properties and good chemical stability. As the frequency of the power signal increases, the loss on the substrate is required to be very small, so the importance of high-frequency boards has emerged.
Classification of PCB high frequency circuit boards
A. Divided by material:
a. Organic materials: phenolic resin, glass fiber/epoxy resin, Polyimide, BT/Epoxy, etc. all belong to this category.
b. Inorganic materials: aluminum, Copper-invar-copper, ceramic, etc. all belong to this category. Mainly choose its heat dissipation function
B. Distinguish the finished product by softness and hardness
a. Rigid PCB,
b. Flexible PCB,
c. Rigid-Flex PCB
Taiyao TUC: Tuc862, 872SLK, 883, 933, etc.
C. Divided by structure
a.Single panel,
b.Double-sided panel,
c.Multilayer board
D. According to purpose
Communications/consumable electronics/military/computers/semiconductors/electrical test boards…
Commonly used high-speed plates (manufacturers)
National boards are cost-effective and their performance is not inferior to imported products. The representative ones are: Dongguan Shengyi, Shanghai Nanya New Materials, Taizhou Wangling, Taixing Microwave, Changzhou Zhongying, Gongli Ceramic Board, TUC: Tuc862, 872SLK, 883, 933, etc.
Foreign ones include:
1) Rogers: Rogers: RO4003, RO3003, RO4350, RO5880, etc. With the development of 5G millimeter waves, Rogers has also launched a variety of low-loss circuit boards suitable for millimeter waves.
RO3000 series: Based on ceramic-filled PTFE circuit materials, models include: RO3003, RO3006, RO3010, RO3035 high-frequency laminates.
RT6000 series: Based on ceramic-filled PTFE circuit materials, 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 board materials that are not listed one by one. Among them, Arlon (acquired by Rogers and also an old brand RF microwave board manufacturer).
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 changing characteristics of the material DK at different frequencies. For requirements that focus on high-speed signal transmission or characteristic impedance control, the focus is on examining DF and its performance under conditions such as frequency, temperature and humidity.
General substrate materials show large changes in DK and DF values under the condition of changing frequency. Especially in the frequency range from 1 MHz to 1 GHz, the changes in their DK and DF values are more obvious. For example, the DK value of a general epoxy resin-glass fiber cloth-based substrate material (general FR-4) at a frequency of 1MHz is 4.7, while the DK value at a frequency of 1GHz changes to 4.19. Above 1GHz, the change trend of its DK value is gentle. Its changing trend is that as the frequency increases, it becomes smaller (but the change is not large). For example, at 10GHz, the DK value of FR-4 is generally 4.15. Substrate materials with high-speed and high-frequency characteristics change in frequency. In this case, the change in DK value is small. At the changing frequency from 1MHz to 1GHz, DK mostly maintains a change in the range of 0.02. Its DK value has a slight downward trend under different frequency conditions from low to high.
The dielectric loss factor (DF) of general substrate materials is affected by frequency changes (especially changes in the high-frequency range), which causes the DF value to change larger than DK. The law of change is that it tends to increase. Therefore, when evaluating the high-frequency characteristics of a substrate material, the focus of investigation is the change of its DF value. Substrate materials with high-speed and high-frequency characteristics have two obviously different types of general substrate materials in terms of changing characteristics at high frequencies: one is that its (DF) value changes very little as the frequency changes. . There is another type that is similar to general substrate materials in terms of change amplitude, but has a lower (DF) value.
How to choose high-speed circuit board materials for high-frequency circuit boards
The choice of PCB board material must strike a balance between meeting design requirements, mass production, and cost. Simply put, design requirements include both electrical and structural reliability components. Usually this board material issue will be more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the dielectric loss Df (Dielectric loss) of the commonly used FR-4 material at frequencies of several GHz will be very large and may not be suitable.
For example, the 10Gb/S high-speed digital signal is a square wave, which can be regarded as a superposition of sine wave signals of different frequencies. Therefore, 10Gb/S contains many different frequency signals: 5Ghz fundamental wave 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 are the same as the low-loss and low-distortion transmission of radio frequency microwaves (the high-frequency harmonic part of the digital signal reaches the microwave band). Therefore, in many aspects, the requirements for PCB material selection for high-speed digital circuits are similar to those for radio frequency and 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 rational use of lamination is used to design a product with high reliability and good processability, and various factors are considered to be optimized.
The following will introduce the main factors to consider when choosing the right plate:
1. Manufacturability:
For example, the performance of multiple laminations, temperature performance, etc., CAF/heat resistance, mechanical toughness (stickiness) (good reliability), and fire protection level;
2. Various properties matching the product (electrical, performance stability, etc.):
Low loss, stable Dk/Df parameters, low dispersion, small coefficient of variation with frequency and environment, small tolerances on 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. "Xingsen Technology-Agilent (High Speed/RF) Joint Laboratory" has solved the performance problem of inconsistent simulation results and testing. It has done a large number of closed-loop verifications of simulation and actual testing, and can achieve consistency between simulation and actual measurement through a unique method.
3. Timely availability of materials:
The procurement cycle for many high-frequency boards is very long, even 2-3 months; except for the conventional high-frequency boards RO4350, which is in stock, many high-frequency boards need to be provided by customers. Therefore, high-frequency plates need to be communicated with the manufacturer in advance and materials prepared as early as possible;
4. Cost factors:
Depend on the price sensitivity of the product, whether it is a consumer product or a communication, medical, industrial, or military application;
5. Applicability of laws and regulations, etc.:
It must be integrated with the environmental protection regulations of different countries and meet RoHS and halogen-free requirements.
Among the above factors, the running speed of high-speed digital circuits is the main factor to consider in PCB selection. The higher the speed of the circuit, the smaller the selected PCBDf value should be. Circuit boards with medium and low losses will be suitable for 10Gb/S digital circuits; boards with lower losses will be suitable for 25Gb/s digital circuits; boards with ultra-low losses will be suitable for faster high-speed digital circuits, and their rates can be 50Gb /s or higher.
From the material Df:
Df ranges from 0.01 to 0.005. Circuit board materials are suitable for 10Gb/S digital circuits;
Df ranges from 0.005 to 0.003. The suitable upper limit of circuit board materials is 25Gb/S digital circuits;
Circuit boards with Df not exceeding 0.0015 are suitable for 50Gb/S or higher high-speed digital circuits.
processing method:
1. Cutting materials: The protective film must be retained when cutting materials to prevent scratches and indentations.
2. Drilling:
2.1 Use a new drill tip (standard 130), one piece at a time is best, and the presser foot pressure is 40psi
2.2 Use the aluminum sheet as the cover plate, then use 1mm melamine backing plate to tighten the PTFE plate
2.3 After drilling, use an air gun to blow out the dust in the hole.
2.4 Use the most stable drilling rig and drilling parameters (basically, the smaller the hole, the faster the drilling speed, the smaller the chip load, the smaller the return speed)
3. Hole treatment
Plasma treatment or sodium naphthalene activation treatment is beneficial to via metallization
4.PTH copper immersion
4.1 After micro-etching (the micro-etching rate has been controlled to 20 micro inches), start feeding the plate from the oil removal cylinder in the PTH
4.2 If necessary, pass the second PTH and just start from the estimated? The cylinder begins to feed the plate
5. Solder mask
5.1 Pretreatment: Use acidic plate washing, do not use mechanical grinding.
5.2 Bake the plate after pre-treatment (90℃, 30min), brush with green oil and solidify
5.3 Bake the plate in three stages: one at 80°C, 100°C, and 150°C, each for 30 minutes (if oil is found on the surface of the base material, you can rework: wash off the green oil and reactivate it)
6. Gong board
Lay the white paper on the circuit surface of the PTFE board, and clamp it up and down with a 1.0MM thick FR-4 base material plate or phenolic base plate that has been etched to remove copper.
The above summarizes how to select high-speed plates and design considerations. Actual applications must be analyzed based on specific cases.