1.  High frequency board application field

High frequency and induction heating technology currently have the highest heating efficiency and speed for metal materials, and are low-cost and environmentally friendly. It has been widely used in various industries in the heat processing, heat treatment, hot assembly, welding, smelting and other processes of metal materials. It can not only heat the workpiece as a whole, but also heat the workpiece locally; it can achieve deep heat penetration of the workpiece, or only heat its surface and surface layer; it can not only directly heat metal materials, but also indirectly heat non-metallic materials. And so on. Therefore, induction heating technology will be more and more widely used in various industries. Its various physical properties, precision, and technical parameters are very high, and it is often used in automotive anti-collision systems, satellite systems, radio systems and other fields. High frequency electronic equipment is a development trend.

 2.  Material selection for high frequency board

Made of materials with high dielectric constant and low high-frequency loss. The most commonly used high-frequency board substrate is fluorine-based dielectric substrate, such as polytetrafluoroethylene (PTFE), commonly known as Teflon, which is usually used above 5GHz. In addition, there are FR-4 or PPO substrates, which can be used for products between 1GHz and 10GHz. The physical properties of these three high-frequency substrates are compared as follows.

At present, the three major types of high-frequency substrate materials used are epoxy resin, PPO resin and fluorine resin. Epoxy resin is the cheapest, while fluorine resin is the most expensive. In terms of dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorine resin is the best and epoxy resin is worse. When the frequency of the product application is higher than 10GHz, only fluorine resin printed boards can be used. Obviously, the performance of fluorine resin high-frequency substrates is much higher than other substrates, but its shortcomings are poor rigidity and large thermal expansion coefficient in addition to high cost. For polytetrafluoroethylene (PTFE), in order to improve the performance, a large amount of inorganic substances (such as silicon dioxide SiO2) or glass cloth is used as reinforcing filling materials to improve the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertness of polytetrafluoroethylene resin itself, it is not easy to bond with copper foil, so special surface treatment of the bonding surface with copper foil is required. The treatment methods include chemical etching or plasma etching on the surface of PTFE to increase the surface roughness or adding an adhesive film layer between the copper foil and the PTFE resin to improve the bonding strength, but it may affect the dielectric performance. The development of the entire fluorine-based high-frequency circuit substrate requires cooperation from raw material suppliers, research institutions, equipment suppliers, PCB manufacturers, and communication product manufacturers to keep up with the rapid development of the high-frequency circuit board field.

 3  Basic property requirements of high frequency substrate materials

(1) The dielectric constant (Dk) must be small and very stable. Generally, the smaller the better. The signal transmission rate is inversely proportional to the square root of the dielectric constant of the material. A high dielectric constant can easily cause signal transmission delays.

(2) Dielectric loss (Df) must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss.

(3) The thermal expansion coefficient should be as consistent as possible with that of the copper foil, as inconsistency will cause the copper foil to separate during hot and cold changes.

(4) The water absorption should be low. High water absorption will affect the dielectric constant and dielectric loss when exposed to moisture.

(5) Other properties such as heat resistance, chemical resistance, impact strength, peel strength, etc. must also be good.

 4  FR4 circuit board material, how to distinguish between ordinary board and high-frequency board?

Ordinary board Cotton fiber paper

High frequency board Alkali-free glass cloth

High frequency boards are usually made of FR4 fiberglass boards, and are made of a whole sheet of epoxy resin glass cloth. The color of the board is relatively uniform and bright. The density is greater than that of low frequency boards, so the weight is the focus.

Many low-frequency boards are made of low-end materials, such as paper substrates, composite substrates, epoxy boards (also called 3240 epoxy boards, phenolic boards), and FR-4 fiberglass boards (patchwork boards). Paper substrates and composite substrates have low overall density and the back color is consistent, but if you look carefully, you can easily see that there is basically no fiberglass cloth texture inside the substrate. The difference between epoxy boards and FR4 fiberglass patchwork boards is that the back substrate color varies. At the fracture of epoxy boards, you can easily see white powder by scraping with your hands or other tools. The color is off-white. FR4 patchwork boards are easier to see because they are pressed with the scraps of FR4 fiberglass cloth. Large stripes can be seen on the back of the entire board.

 5.  What kind of fault phenomenon will occur when the high-frequency board has poor performance ?

The poor performance of high-frequency boards is mainly due to the low dielectric constant value, which affects the working stability of high-frequency circuits. High frequencies have skin effect characteristics, and the low dielectric constant of the circuit board causes leakage of high-frequency components, which results in signal weakening, frequency deviation drift, and even oscillation stop in severe cases. The overall electrical performance indicators are reduced

 6High  frequency board characteristics

The high-frequency circuit board provided by the utility model is provided with retaining edges capable of blocking the flow of glue at the edges of the upper opening and the lower opening of the hollow groove of the core plate, so that when the core plate is bonded to the copper-clad plates placed on the upper and lower surfaces thereof, the flow of glue will not enter the hollow groove, that is, the bonding operation can be completed by one pressing. Compared with the high-frequency circuit board in the prior art which requires two pressings to complete, the high-frequency circuit board in the utility model has a simple structure, low cost and is easy to manufacture.

 7.  High frequency board production requirements

High-frequency boards are one of the most difficult boards, so they must meet the production requirements as much as possible.

drilling

1. The drilling feed speed should be slow, 180/S. Use a new drill bit, put aluminum sheets on the top and bottom, and it is best to drill with a single PNL. Do not allow water to enter the hole.

2. The PTH hole sample can be treated with concentrated sulfuric acid (preferably not) for 30 minutes.

3. The copper circuit of the grinding plate is made in the same way as the normal double-sided

4. Special note: There is no need to remove glue residue from high-frequency boards.

Solder Mask

1. If the high-frequency board needs green oil primer, grinding is not allowed before solder mask, and a red seal is stamped in MI.

2. If the high-frequency board needs to be printed with green oil on the substrate, it needs to be printed twice (to prevent the green oil on the substrate from bubbling). The board cannot be ground from the time of etching and before de-tinning, and can only be air-dried. For the first primer, use a 43T screen to print and bake the board in sections: 50 degrees for 50 minutes, 75 degrees for 50 minutes, 95 degrees for 50 minutes, 120 degrees for 50 minutes, 135 degrees for 50 minutes, and 150 degrees for 50 minutes. Use circuit film to expose, and the board can be ground after development. The second production can be done normally. It is necessary to note in MI: the first primer is aligned with circuit film.

3. If the high-frequency board needs to be printed with green oil on part of the substrate and not on part of the substrate, a “priming film” is required. The primer film only retains the green oil on the substrate. After the primer is baked, the second normal production can be carried out. The following picture shows that 018212 needs to be specially produced with a “priming film”.

Pay special attention that if green oil is not printed on the substrate like 018092, green oil can only be printed once to prevent the green oil on the substrate from being unable to be developed after the first green oil primer.

Spray tin

Before spraying tin, bake at 150 degrees for 30 minutes before spraying tin

Line tolerance

The line width tolerance is ±0.05mm for non-required ones and can be made according to customer requirements for required ones.

Plate

Please refer to the requirements for the specified plate. Because the plate is expensive, only 1PNL is used if possible.

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