Definition of high-frequency PCB board

High frequency board refers to the special electromagnetic frequency circuit board, used in high frequency (greater than 300 MHZ frequency or wavelength is less than 1 meter) and microwave (greater than 3 GHZ frequency or wavelength is less than 0.1 meters) in the field of PCB, is on the microwave base copper clad using common rigid circuit board manufacturing method of a part of the process or the use of special processing methods and the production of circuit boards. In general, 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 design is in the microwave band (> 1GHZ) and even with the millimeter wave field (30GHZ) above the application, which also means that the frequency is higher and higher, the substrate of the circuit board requirements are also higher and higher. For example, substrate materials need to have excellent electrical properties, good chemical stability, with the increase of power signal frequency in the substrate loss requirements are very small, so the importance of high frequency plate is highlighted.

Classification of PCB high frequency plate

1, at the end of the ceramic filled thermosetting material

Processing method:

And epoxy resin/glass woven cloth (FR4) similar processing process, but the plate is more brittle, easy to break, drilling and gong plate drill nozzle and gong knife life is reduced by 20%.

2. PTFE (polytetrafluoroethylene) material

Processing method:

1. Material opening: protective film must be retained to prevent scratches and indentation

2. The drill:

2.1 use a new drill (standard 130), one piece stacked is the best, the presser foot pressure is 40psi

2.2 Aluminum sheet as cover plate, then use 1mm dense amine plate, tighten the PTFE plate

2.3 Blow the dust out of the hole with air gun after drilling

2.4 With the most stable drilling rig, drilling parameters (basically, the smaller the hole, the faster the drilling rate, the smaller Chip load, the smaller the return rate)

3. The hole processing

Plasma treatment or sodium – naphthalene activation treatment is beneficial to metallization of pores

4. PTH sink copper

4.1 After micro-etching (the micro-etching rate has been controlled by 20 micro-inches), the plate is fed from the oil removing cylinder in PTH pull

4.2 If necessary, go through the second PTH, just from the forecast? The cylinder began to enter the plate

5. The resistance welding

5.1 Pre-treatment: use acid washing plate instead of mechanical grinding plate

5.2 After pre-treatment, bake plate (90℃, 30min), brush green oil and cure

5.3 Three baking plates: one is 80℃, 100℃ and 150℃ for 30min each (if oil is found on the substrate surface, it can be reworked: wash off the green oil and reactivate it)

6. Gong board

Lay the white paper on the PTFE board circuit surface, and clamp it with fr-4 base plate or phenolic base plate with a thickness of 1.0mm and copper removal: As shown in the figure:

What are the PCB high frequency boards? PCB high frequency plate classification

High frequency and high speed sheet material

When selecting the substrate for PCB for high frequency circuits, special consideration should be given to the variation characteristics of material DK at different frequencies. For the requirements of signal high-speed transmission or characteristic impedance control, DF and its performance under the conditions of frequency, temperature and humidity are mainly investigated.

Under the condition of frequency variation, the DK and DF values of general substrate materials change greatly. Especially in the frequency range from L MHz to L GHz, their DK and DF values change more obviously. For example, the GENERAL epoxy – glass fiber substrate material (general FR-4) has a DK value of 4.7 at lMHz and a DK value of 4.19 at lGHz. Above lGHz, its DK value changes gently. For example, under l0GHz, the DK value of FR-4 is 4.15. For substrate materials with high speed and high frequency characteristics, the DK value changes slightly. From lMHz to lGHz, the DK value mostly stays within 0.02 range. The DK value tends to decrease slightly at different frequencies from low to high.

The dielectric loss factor (DF) of the general substrate material is larger than that of DK due to the influence of frequency variation (especially in the high frequency range). Therefore, when evaluating the high frequency characteristics of a substrate material, we should focus on the change of its DF value. The substrate materials with high speed and high frequency characteristics are obviously different from the general substrate materials in terms of the variation characteristics at high frequency. One is that with the change of frequency, its (DF) value changes very little. The other is similar to the general substrate material in the range of variation, but its own (DF) value is lower.

How to choose high frequency high speed plate

PCB board selection must meet the design requirements, mass production and cost of the balance between. In short, the design requirements consist of two components: electrical and structural reliability. This is usually important when designing very high speed PCB boards (frequencies greater than GHz). For example, the fr-4 material commonly used today may not be applicable due to its large Df (Dielectricloss) at several GHz frequencies.

What are the PCB high frequency boards? PCB high frequency plate classification

For example, a 10Gb/S high-speed digital signal is a square wave, which can be regarded as a superposition of sinusoidal signals of different frequencies. Therefore, 10Gb/S contains many different frequency signals: 5Ghz fundamental signal, 3 order 15GHz, 5 order 25GHz, 7 order 35GHz signal, etc. 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 rf microwave (the high frequency harmonic part of the digital signal reaches the microwave band). Therefore, in many respects, the PCB material selection of high-speed digital circuits is similar to the requirements of RF microwave circuits.

What are the PCB high frequency boards? PCB high frequency plate classification

In practical engineering operations, the selection of high-frequency plates seems simple, but there are still many factors to be considered. Through the introduction of this paper, AS a PCB design engineer or a high-speed project leader, I have a certain understanding of the characteristics and selection of plates. Understand electrical properties, thermal properties, reliability, etc. And rational use of stacking, design a piece of high reliability, good processing products, various factors to consider the best.

The following will introduce the main factors to consider in selecting the appropriate plate:

1, manufacturability:

Such as multiple pressing performance, temperature performance, CAF/ heat resistance and mechanical toughness (viscosity) (good reliability), fire rating;

2, with the product matching performance (electrical, performance stability, etc.) :

Low loss, stable Dk/Df parameters, low dispersion, small change coefficient with frequency and environment, small tolerance of material thickness and rubber content (good impedance control), if the wire is long, consider low roughness copper foil. In addition, simulation is needed in the early stage of high-speed circuit design, and simulation results are the reference standard for design. “Xingsen Technology – Agilent (high speed/RADIO frequency) Joint laboratory” solved the performance problem of inconsistent simulation results and tests, and did a large number of simulation and actual test closed-loop verification, through a unique method to achieve the consistency of simulation and measurement.

What are the PCB high frequency boards? PCB high frequency plate classification

3. Timely availability of materials:

Many high-frequency plate procurement cycle is very long, even 2-3 months; In addition to the conventional high frequency plate RO4350 has inventory, many high frequency plates need to be provided by customers. Therefore, high frequency plate and manufacturers need to communicate well in advance, as soon as possible;

4. Cost factors:

Depending on the price sensitivity of the product, whether it is a consumer product, or a telecommunications, medical, industrial, military application;

5. Applicability of laws and regulations, etc.

To be compatible with environmental regulations of different countries and meet the requirements of RoHS and halogen-free.

Among the above factors, the running speed of high-speed digital circuit is the main factor to consider in PCB selection. The higher the circuit speed, the smaller the selected PCBDf value should be. The circuit plate with medium and low loss will be suitable for 10Gb/S digital circuit; The plate with lower loss is suitable for 25Gb/s digital circuit; Panels with ultra-low loss will accommodate faster, high-speed digital circuits at rates of 50Gb/s or higher.

From the material Df:

Df between 0.01 ~ 0.005 circuit board suitable for the upper limit of 10Gb/S digital circuit;

Df between 0.005 ~ 0.003 circuit board suitable for the upper limit of 25Gb/S digital circuit;

Circuit boards with Df not more than 0.0015 are suitable for 50Gb/S or higher speed digital circuits.

Commonly used high-speed plates are:

1), Rogers: RO4003, RO3003, RO4350, RO5880, etc

2), Taiyao TUC:Tuc862, 872SLK, 883, 933, etc

3), Panasonic: Megtron4, Megtron6, etc

4), Isola: FR408HR, IS620, IS680, etc

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

6), Dongguan Shengyi, Taizhou Wangling, Taixing Microwave, etc