Microwave Tech - How to choose high-frequency and high-speed plates

How to choose high-frequency and high-speed plates

The selection of PCB boardmust achieve a balance point in meeting the requirements of the design, mass production, and cost center. Simply put, the design requirements include electrical and layout reliability.

When designing extremely high-speed PCB boards(frequency greater than GHz), this board problem will be more important.

For example, the FR-4 material that is often used at the moment has a large dielectric loss Df (Dielectricloss) at a frequency of several GHz, which 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 includes many different frequency signals: 5Ghz fundamental wave signal, 3rd order 15GHz, 5th order 25GHz, 7th order 35GHz signal and so on.

The completeness of the digital signal and the steepness of the high and low edges are the same as the low-consumption and low-distortion transmission of radio frequency microwave (the high-frequency harmonic part of the digital signal reaches the microwave frequency band).

Therefore, in many aspects, the selection of high-speed digital circuit PCB materials is similar to the requirements of radio frequency microwave circuits.

In the actual engineering operation, the selection of high-frequency plates seems simple, but there are still many identities that need to be considered. Through the introduction of this article, as a PCB design engineer or a high-speed project contributor, the characteristics and selection of the plates are Inevitable understanding.

Understand the electrical energy, thermal performance and reliability of sheet metal. And fair use of cascading, conceived a product with high reliability and good processability, the consideration of various identities reached the best.

The above coating will be introduced online, and the selection of suitable plates should be based on the identity:

1. Manufacturability:

It is like repeated pressing function, temperature function, etc., CAF/heat resistance and machine toughness (stickiness) (good reliability), fireproof grade;

2. Various functions (electrical, functional invariance, etc.) matched with the product:

Low consumption, constant Dk/Df parameters, low dispersion, small conversion coefficient with frequency and conditions, low material thickness and glue content (good impedance control), if the trace is long, consider the copper foil with low roughness. In addition, the design of high-speed circuits requires simulation in the early stage, and the simulation results are the reference scale of the design. "Xingsen Technology-Agilent (High Speed/RF) Combination Lab" has dealt with the difficulties of simulation results and testing of inconsistent deeds, and has done a lot of simulation and reality testing closed-loop verification, and can achieve differences between simulation and actual measurement through an exclusive method .

3. Real-time availability of materials:

Many high-frequency plates have a very long procurement cycle, even 2-3 months; except for the general high-frequency plate RO4350, which is in stock, many high-frequency plates need to be supplied by customers. Therefore, the demand for high-frequency plates is the same as that of the manufacturers in advance, and the materials are prepared as soon as possible;

4. Cost of capital status:

Look at the price sensitivity level of the product, whether it is a consumable product, or it is the use of communications, medical, industry, and ordnance;

5. The suitability of laws and regulations, etc.:

Integrate with the environmental protection laws of different countries and meet the requirements of RoHS and halogen-free.

Among the above factors, the operating speed of high-speed digital circuits is the primary factor for PCB selection. The higher the circuit speed, the smaller the selected PCBDf value.

Circuit boards with medium and low consumption will be suitable for 10Gb/s digital circuits; boards with lower consumption will be combined with 25Gb/s digital circuits; boards with ultra-low consumption will be compatible with faster high-speed digital circuits, and the speed can be 50Gb /s or higher.

From the perspective of material Df: The suitable upper limit for Df is between 0.01～0.005 is 10Gb/S digital circuit; the suitable upper limit for Df is between 0.005～0.003 is 25Gb/S digital circuit; The suitable upper limit for Df is not more than 0.0015 is 50Gb/S Even higher-speed digital circuits.

The above summarizes how to select high-speed plates and the considerations of ideas, and the actual application has to be analyzed in detail based on specific cases. .

Overview of PCB high-frequency board selection and production and processing methods in PCB circuit board factory Definition of PCB high-frequency board

The high frequency board of the circuit board factory refers to the special circuit board with high electromagnetic frequency. It is used in 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) in the field of PCB.

A circuit board is manufactured on a microwave substrate copper clad laminate or a circuit board manufactured by a special processing method by using a traditional rigid circuit board manufacturing method.

Generally, a high-frequency board can be defined as a circuit board with a frequency higher than 1 GHz. With the rapid development of science and technology, more and more equipment designs are applied to the microwave frequency band (> 1GHZ) or even the millimeter wave field (30GHZ), which means that the frequency is getting higher and higher, and the base number is getting higher and higher. The requirements are getting higher and higher.

For example, the substrate material needs to have excellent electrical properties, good chemical stability, and as the frequency of the power signal increases, the loss on the substrate is very small, thus highlighting the importance of the high-frequency board.

PCB high frequency board application field mobile communication products; power amplifiers, low-noise amplifiers and other passive equipment, such as power splitters, couplers, duplexers, filters; high Frequency electronic equipment is a development trend.

Classification of high-frequency boards Powder ceramic filled thermosetting materials

A, Manufacturer: Rogers’ 4350B/4003C Aron’s 25N/25FR Taconic’s TLG series B, processing method: similar to the epoxy resin/glass woven fabric (FR4) process, the sheet is relatively fragile, easy to break, drill and The service life of the drill bit and boring tool when drilling is reduced by 20%.

PTFE (polytetrafluoroethylene) material A, manufacturer: Rogers’ RO3000 series, RT series, TMM series, Arlon’s AD/AR series, IsoClad series, CuClad series, Taconic’s RF series, TLX series, TLY series, Taixing Microwave’s F4B, F4BM, F4BK, TP-2 B, processing method:

1. Cutting: The protective film must be kept to prevent scratches and creasing.

2. Drilling 1. Use a new drill nozzle (standard 130), one piece is the best, and the presser foot pressure is 40 psi. 2. The aluminum plate is the cover plate, and then tighten the PTFE plate with a 1mm melamine pad. 3. After drilling the hole, use an air gun to blow out the dust in the hole. 4. With the most stable drilling machine, drilling parameters (basically the smaller the hole, the faster the drilling speed, the smaller the chip load, the smaller the return speed)

3. Pore treatment Plasma treatment or sodium naphthalene activation treatment is conducive to pore metallization

4. PTH sinking copper 1 After micro-etching (the micro-etching rate is controlled by 20 micro inches), PTH is pulled out of the ejector into the circuit board. 2 If necessary, pass the second PTH, starting from the expected cylinder

5. Solder mask 1 pretreatment: use pickling plates, mechanical grinding plates cannot be used. 2 pretreatment slabs (90°C, 30 minutes), drawing green oil curing 3 points and 3 sections slabs: one section 80° C, 100°C, 150°C, 30min each time (if oyster sauce is found on the surface of the substrate, you can rework: wash the green oil, reactivate the treatment) 6. Put white paper on the PTFE board wiring surface on the seesaw, and use etching The MM-4 substrate or phenolic bottom plate with a thickness of 1.0mm clamps the upper and lower sides.

The high-frequency raft stacking method needs to carefully scrape the edge of the back of the fascia board by hand to prevent damage to the substrate and copper surface, and then separate it with a considerable size of sulfur-free paper and visually inspect it.

In order to reduce burrs, the focus is on the slab process. Process NPTH PTFE sheet processing and cutting-drilling-dry film-inspection-etching-etching-welding mask-characters-spraying-molding-testing-final inspection-packaging-transportation PTH PTFE sheet processing cutting drilling processing (plasma treatment Or sodium naphthalene activation treatment)-sink board electro-dry film inspection-pattern electro-etching-etching resistance-characteristics-spray tin-forming-testing-inspection-packaging-transportation

To sum up the difficulties of dealing with high-frequency boards

1. Copper sink: the hole wall is not easy to copper

2. Image transfer, etching, line width and line gap, sand hole control

3. Green oil process: green oil adhesion, green oil foaming control

4. Strictly control the scratches on the surface of the board during each process.