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High frequency pcb refers to special circuit boards with high electromagnetic frequency, used in the field of high-frequency (frequency greater than 300MHZ or wavelength of less than 1 metre) and microwave (frequency greater than 3GHZ or wavelength of less than 0.1 metres) PCB, is the use of ordinary copper-clad laminates in the microwave substrate on the rigid circuit board manufacturing method of the part of the process or the use of special treatment methods and the production of circuit boards. Generally speaking, high-frequency pcb can be defined as the frequency of 1GHz or more circuit boards.
With the rapid development of science and technology, more and more equipment design is in the microwave band (> 1GHZ) or even with the millimetre wave field (30GHZ) above the application, which means that the frequency is getting higher and higher, the substrate of the circuit board requirements are also getting higher and higher. For example, the substrate material needs to have excellent electrical properties, good chemical stability, with the increase in the frequency of the power supply signal in the substrate loss requirements are very small, so the importance of high-frequency pcb came to the fore.
The basic process of high-frequency pcb production:
1. Material selection: high-frequency pcbsare usually selected from special high-frequency materials, such as PTFE (polytetrafluoroethylene), ceramic filler materials, modified polyphenylene ether resins, etc. These materials have excellent electrical properties and good chemical stability.
2. Inner layer fabrication: fabrication of the inner layer graphics, including copper foil preparation, image transfer, etching and other steps.
3. Pressing: The inner and outer copper foils are pressed together with insulating materials to form the multilayer board structure.
4. Drilling: Drill the required holes in the multilayer structure, including through holes, blind holes and buried holes.
5. Hole metallisation: the drilled holes are chemically or physically metallised to form electrical connections.
6. Outer Layer Fabrication: Image transfer, etching, etc. of the outer copper foil to form the outer circuit.
7. Pattern plating: Pattern plating on the outer layer of copper to increase the thickness of the copper layer and improve the reliability of the circuit.
8. Soldermask production: Coat the copper layer with soldermask to protect the copper layer from contamination and corrosion.
9. surface treatment: surface treatment as needed, such as tin spraying, OSP (organic protective film), ENIG (electroless nickel gold plating), etc..
10. Testing: Electrical performance testing of circuit boards, including impedance testing, conduction testing, insulation resistance testing, etc.
11. Inspection: Inspect the appearance and dimensions of the circuit boards to ensure compliance with quality standards.
12. Forming: Cut and form the circuit boards according to the shape requirements of the final product.
13. Packing for shipment: After completing all processes, the qualified circuit boards will be packed and ready for shipment.
Difficulties in the processing of high frequency pcb
High frequency pcb processing due to its unique physical and chemical properties of the polytetrafluoroethylene (PTFE) board, so that its processing process and the traditional FR4 board has a significant difference, according to the epoxy resin glass fibre copper-cladding board processing of the conventional process, it is difficult to obtain a qualified finished product.
Drilling, due to the substrate is relatively soft, the number of single drilling stacked boards should be controlled low, usually 0.8 mm thickness of the plate to two layers is appropriate; drilling speed needs to be slower, and must use a new drill bit, and the top angle of the drill bit and thread angle has strict special requirements.
In the process of printing solder resist (green oil), high-frequency pcb after etching, can not be used to roller brush mechanical grinding board to avoid damage to the substrate, recommended the use of chemical methods for surface treatment to ensure that the solder resist after the completion of the line and the copper surface uniformity and no oxidation layer, this operation is more difficult.
Hot air levelling (tin spraying) treatment, due to the material properties of fluorocarbon resin, should avoid rapid heating of the board, tin spraying before the need to preheat at 150 ℃ for about 30 minutes, and the tin tank temperature can not exceed 245 ℃, otherwise it will affect the adhesion of the isolated pad.
When milling the profile, due to the soft characteristics of fluorine resin, the conventional milling cutter will easily lead to a lot of burrs and the surface is not flat, you need to use a special special milling cutter to ensure the processing quality.
Special attention should be paid to the handling between each process, it is forbidden to put the board vertically, it should be put flat between the paper, and it is forbidden to touch the line graphic directly with the finger to prevent scratching and abrasion, and any scratches, pinholes, indentation, or dents on the line will affect the signal transmission and lead to the board being rejected.
The etching process is extremely demanding, requiring precise control of side etching, serrations and notches, with line width tolerances strictly controlled within ±0.02 mm, and inspection with at least 100x magnification.
The treatment before chemical copper immersion is the key and difficult point in the manufacture of PTFE high frequency pcb. Stable and suitable for mass production of copper immersion pre-treatment there are two main methods:
Plasma method (Plasma): in a vacuum state, between the two high-pressure electrodes injected with carbon tetrafluoride (CF4), argon (Ar2), nitrogen (N2) and oxygen (O2) and other gases, the formation of plasma, to remove dirt in the drilled holes, the effect of uniformity and is suitable for mass production, but the equipment cost is high (a single unit of more than one hundred thousand U.S. dollars), the famous manufacturers of equipment are APS and March. Chemical method: Using a specific chemical solution to remove the dirt from the borehole.
Chemical method: A specific chemical solution such as tetrahydrofuran is used to form a complex that erodes the PTFE surface layer in the hole to improve wettability. This method is traditional and stable, but suffers from high toxicity, flammability and high operational risk and requires specialised management.
For ε3.38 and Rogers Ro4003 high frequency substrates, these materials combine the excellent high-frequency properties of PTFE glass fibre substrates with the ease of processing of FR4 substrates, filled with glass fibre and ceramic materials, and have a high heat resistance with a Glass Transition Temperature (Tg) of greater than 280°C. The materials have been designed for drilling holes in a wide range of applications, including the drilling of holes in a wide range of materials.This type of substrate drilling on the drill bit consumption, the need to use special drilling parameters, milling the shape of the milling cutter needs to be changed frequently; but the other processing steps and ordinary high frequency pcb is similar and does not require special hole processing, and therefore is recognised by a large number of PCB factories and customers. It should be noted that Ro4003 does not contain flame retardant,the board may have a risk of combustion when it reaches 371 ℃.Some domestic manufacturers also use modified polyphenylene ether (PPO) type sheet such as state-owned 704 factory LGC-046, the dielectric constant of about 3.2, its processing performance close to FR4, and get some recognition and use.