PCB Tech - What Are the Basic PCB High Frequency Board Materials?

A few days ago, a graduate student took a FR4 processed "receiving antenna" to measure 77GHz index. When processing and testing high frequency PCB board and receiving antenna, there are always scholars who have just begun to study. The final result of the product is not satisfactory because of the wrong high frequency PCB. Today, I will talk about it. What are the PCB high frequency board materials? How to choose!

Definition of PCB high frequency board:

High frequency board is a kind of special circuit board with high electromagnetic frequency, which is used in the fields of high frequency (frequency greater than 300MHz or wavelength less than 1m) and microwave (frequency greater than 3GHz or wavelength less than 0.1M), Is it the local process of using the usual rigid circuit board manufacturing method on the microwave substrate copper clad laminate or the circuit board produced by using the special treatment method when it is considered appropriate. Generally speaking, a high frequency board can be defined as a circuit board with a frequency above 1GHz. With the rapid development of science and technology, more and more facilities are designed to be used in the microwave band (> 1GHz) or even in the millimeter wave field (77GHz) or above (for example, the popular 77GHz millimeter wave receiving antenna in vehicle nowadays), which also means that the frequency is higher and higher, and the requirements for the substrate of circuit board are higher and higher. For example, the substrate material needs to have good electrical properties and satisfactory chemical stability. With the increase of power signal frequency, the loss on the substrate is very small, so the tightness of high-frequency plate is highlighted.

Classification of PCB high frequency sheet:

A. By material:

a. Organic materials: phenolic natural resin, glass fiber / epoxy natural resin, polyimide, metamorphosis / epoxy, etc.

b. Inorganic materials: aluminum, copper invar copper, ceramic, etc. It mainly takes its heat dissipation function

B. By distinguish between hard and soft products:

a. Rigid PCB,

b. Flexible PCB, Rigid-Flex PCB

C. By structure:

a. Single panel

b. double panel

c. multi panel

D. According to usage:

a.Communication

b.consumer electronics

c.military

d.computer

e.semiconductor

f.electrical measuring board

Commonly used high-speed sheet metal (manufacturers) are --- warmly welcome supply more

Rogers series:RO4003, RO3003, RO4350, RO5880,etc

Ro3000 series : PTFE circuit materials based on ceramic supplement, models are: ro3003, ro3006, ro3010, ro3035 high frequency laminate

RT6006 series : Based on the PTFE circuit material supplemented by porcelain ceramics, it is preset for the electronic circuit and microwave circuit with high dielectric constant. The models are rt6006 with dielectric constant of 6.15 and rt6010 with dielectric constant of 10.2

TMM series : Based on ceramic, carbon hydride, thermosetting polymer composites, models: tmm3, tmm4, tmm6, tmm10i, tmm13i, and so on.

Taconic series : TLX series, TLY series, etc

Panasonic series : Megtron4, Megtron6, etc

Isola series:FR408HR, IS620, IS680, etc

Nelco:N4000-13, N4000-13EPSI, etc

TUC series: Tuc862, 872SLK, 883, 933,etc

Dongguan Shengyi, Taizhou Wangling, Taixing microwave, Changzhou Zhongying and so on.

When selecting the substrate for PCB used in high frequency circuit, we should pay special attention to the special properties of material DK under different frequencies. For the requirements of high-speed signal transmission or special property impedance control, DF and its performance under the conditions of frequency, temperature and humidity are mainly investigated. Under the condition of frequency variation, the regular pattern of DK and DF value variation is expressed. Especially in the frequency range of 1 MHz to 1 GHz, their DK and DF values change more superficially. For example, the DK value of ordinary FR-4 is 4.7 at lmhz and 4.19 at lghz. Above 1 GHz, the variation of DK tends to be gentle. For example, at 10 GHz, the DK value of common FR-4 is 4.15. For substrate materials with high speed and high frequency, the DK value changes little when the frequency changes. At frequencies from 1 MHz to 1 GHz, the DK value changes in the range of 0.02. The values of DK tended to decrease slightly at different frequencies from low to high. The medium defect factor (DF) of common substrate materials is affected by the frequency variation (especially in the high frequency range), and the variation of DF is larger than that of DK. The law of variation tends to increase. Because of this, when a substrate material has high frequency and special properties, the focus of investigation is its DF value variation. There are two kinds of substrate materials with special properties of high speed and high frequency, which have different surface properties at high frequency. One is that with the change of frequency, its (DF) value changes very little. The other is similar to the common substrate material in variation range, but its DF value is lower. The selection of PCB board must be in the satisfaction of preset needs, mass production, cost half to get the balance point. In short, presupposition needs to include electrical and structural reliability. Generally, when a very high-speed PCB wrench is preset (frequency greater than GHz), the problem of sheet metal will be more closely related. For example, the FR-4 material, which is commonly used nowadays, has a very large DF (dielectric loss) at a frequency of several GHz, and may not be suitable for use.

Loss of different high frequency plates

For example, 10Gb / s high-speed digital signal is a square wave, which can be regarded as the superposition of sine wave signals with different frequencies. Because this 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 microwave (the high frequency harmonics of the digital signal reach the microwave band). Because of this, in various aspects, high-speed digital circuit PCB material and RF microwave circuit needs are roughly similar.

High frequency domain signal domain characteristics

In the actual engineering operation, the selection of high-frequency sheet seems simple, but there are still too many factors to think about. After the introduction of this paper, as a PCB design engineer or high-speed project leader, I have a certain understanding of the special nature and selection of sheet. Understand electrical properties, thermal properties and reliability of sheet metal. And the reasonable use of overlap, preset a reliable, good processing products, various factors to optimize. Next, we will introduce the selection of suitable sheet metal and consider the main factors:

1. Manufacturability:

   For example, how about the multiple pressing performance, temperature performance, CAF / heat resistance and mechanical toughness (good reliability), fire rating

2. All kinds of performance (electrical, performance stability, etc.) matched with the product:

   Low damage, stable Dk / DF parameter, low dispersion chromatic light, small variation coefficient with frequency and background, small tolerance of material thickness and glue content (good impedance control). If the wiring is long, consider the problem of low finish copper foil. In addition, simulation is required in the previous stage of high-speed circuit design, and the final result of simulation is the reference standard of design“ "XINGSEN science and technology Agilent (high speed / RF) joint laboratory" solves the problem that the final result of simulation is not exactly the same as that of test. It has done a large number of closed-loop verification of simulation and actual test. Through the unique method, it can make the simulation and test exactly the same.

3. Availability of materials as early as possible:

   Many high-frequency sheet metal acquisition and purchase cycle is very long, even 2-3 months; In addition to the common high-frequency plate ro4350 has storage, many high-frequency plates are required by customers. Because of this, high frequency plate demand and manufacturers communicate well in advance, as soon as possible to prepare.

4. Cost factor:

   In terms of price sensitivity, consumer products are still used in communication, medical, industrial and military industries.

5. The applicability of laws and regulations

   
   

   
   

   

We should integrate with different national environmental protection laws and regulations, and satisfy RoHS and halogen-free requirements. Among the above factors, the speed of high-speed digital circuit is the main factor of PCB selection and thinking. The higher the efficiency of the circuit, the smaller the PCBdf value should be selected. The board with medium and low damage consumption will adapt to 10gb/s digital circuit; The board with lower damage is suitable for 25gb/s digital circuit; The ultra-low damage board will be suitable for faster high-speed digital circuits, and its efficiency can be 50gb/s or higher.

The above summarizes how to select high frequency sheet metal and preset attention items, and the actual application should be based on the specific cases.