When processing and testing high-frequency PCB boards and high-frequency antennas, beginners often choose the incorrect high-frequency PCB, resulting in manufacturing unsatisfactory products. Let’s briefly what are high-frequency PCB boards and how to choose high-frequency PCB substrates.
Definition of high-frequency PCB 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. This circuit board is 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 is 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 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. 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 choose its heat dissipation performance.
B. Distinguish the finished product by softness and hardness
C. by structure
a.Single panel, b.Double panel, c.Multilayer board
D. By purpose
electrical test boards…
Commonly Chinese high-speed plate manufacturers
China PCB boards are cost-effective and their performance is not inferior to imported products. The representative ones are: Taizhou Wangling, Changzhou Zhongying, Taiyao TUC: Tuc862, 872SLK, 883, 933, etc.
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, and 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.
Taconic: TLX series, TLY series, etc.
Panasonic: Megtron4, Megtron6, etc.
Isola: FR408HR, IS620, IS680, etc.
Nelco: N4000-13, N4000-13EPSI, etc.
There are many other high-frequency PCB board materials that are not listed above.
What are the important indicators for selecting high-frequency and high-speed PCB materials?
When selecting PCB substrates 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), resulting in changes in the DF value that are 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 is the change in its DF value.
There are 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.
another 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
When you choose PCB board material, you must keep a balance between meeting design requirements, mass production, and cost. 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), such as 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 PCB circuits are similar to those for radio frequency and microwave circuits.
In actual engineering operations, Choosing high-frequency PCB boards seems simple, but there are still many factors that need to be considered, such as:
For example, the performance of multiple laminations, temperature performance, etc., CAF/heat resistance, mechanical toughness (stickiness) (good reliability), and fire protection level;
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.
Simulation is required in the early stage of high-speed circuit design, and the simulation results are the reference standard for design.
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, many high-frequency PCB boards need to be provided by customers. Therefore, high-frequency plates need to be communicated with the PCB manufacturer in advance and materials prepared as early as possible;
Cost factors Cost:
Depends on 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 the environmental protection regulations of different countries and meet RoHS and halogen-free requirements.
Among the above factors, high-speed digital circuits are the main factor to consider in PCB selection. The higher the speed of the circuit, the smaller the selected PCB Df value should be.
Medium and low losses PCB will be suitable for 10Gb/S digital circuits;
Lower losses PCB will be suitable for 25Gb/s digital circuits;
ultra-low losses PCB 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;
Df not exceeding 0.0015 are suitable for 50Gb/S or higher high-speed digital circuits.
The above guide introduces how to select high-speed plates and design considerations. We must analyze specific cases in actual applications.