High frequency board concept 2
Article source：Zhengjiaxiang Technology / Author：Zhengjiaxiang Technology / Publication time：2021-11-02
1. Application field of high frequency board
At present, high frequency and induction heating technology has the highest heating efficiency, the fastest speed, low consumption and environmental protection. It has been widely used in hot processing, heat treatment, hot assembly, welding, smelting and other processes of metal materials in all walks of life. It can not only heat the workpiece as a whole, but also heat the workpiece locally; It can realize the deep heat penetration of the workpiece, or only the surface and surface can be heated centrally; It can not only directly heat metal materials, but also indirectly heat non-metallic materials. wait. Therefore, induction heating technology will be more and more widely used in all walks of life. It has very high requirements for physical performance, precision and technical parameters. It is commonly used in automobile anti-collision system, satellite system, radio system and other fields. High frequency of electronic equipment is the development trend.
2. High frequency board material selection
It is made of materials with high dielectric constant and low high frequency loss. At present, the most commonly used high-frequency plate substrate is fluoro dielectric substrate, such as polytetrafluoroethylene (PTFE), usually called Teflon, which is usually used at 5GHz above. FR-4 is also useful Or PPO Base material, can be used for 1GHz ~ 10GHz The physical properties of the three high-frequency substrates are compared as follows.
Epoxy resin and PPO used at this stage Among the three types of high-frequency substrate materials, epoxy resin and fluorine resin, epoxy resin is the cheapest, while fluorine resin is the most expensive; Considering the dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorine resin is the best and epoxy resin is poor. When the frequency of product application is higher than 10GHz Only fluorine resin printed boards can be used. Obviously, the performance of fluorine resin high frequency substrate is much higher than that of other substrates, but its shortcomings are poor rigidity and large coefficient of thermal expansion in addition to high cost. For polytetrafluoroethylene (PTFE), in order to improve the performance, a large number of inorganic substances (such as silica, SiO2) or glass cloth are used as reinforcing filling materials to improve the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertia of PTFE resin itself, it is not easy to combine with copper foil, so it needs special surface treatment on the joint surface with copper foil. The treatment methods include chemical etching or plasma etching on the surface of polytetrafluoroethylene, increasing the surface roughness, or adding an adhesive film between copper foil and polytetrafluoroethylene resin to improve the adhesion, but it may affect the dielectric performance. The development of the whole fluorine series high-frequency circuit substrate needs raw material suppliers, research units, equipment suppliers and PCBs Manufacturers cooperate with communication product manufacturers to keep up with the needs of the rapid development of high-frequency circuit boards in this field.
3. Basic characteristics and requirements of high frequency substrate materials
(1) The dielectric constant (DK) must be small and stable. Usually, the smaller the better. The transmission rate of the signal is inversely proportional to the square root of the dielectric constant of the material. High dielectric constant is easy to cause signal transmission delay.
(2) The 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 coefficient of thermal expansion shall be consistent with that of copper foil as much as possible, because inconsistency will cause copper foil separation in cold and heat changes.
(4) Low water absorption and high water absorption will affect the dielectric constant and dielectric loss when affected by moisture.
(5) Other heat resistance, chemical resistance, impact strength and peel strength must also be good.
4. How to distinguish FR4 circuit board from ordinary board or high-frequency board?
Ordinary board Cotton fiber paper
High frequency board Alkali free glass cloth
The high-frequency board is usually pressed by FR4 fiberglass board, and the whole piece of epoxy resin glass cloth. In terms of color, the whole board is relatively uniform and bright. The density is larger than that of low-frequency board, which is the focus of weight.
Many low-frequency boards are made of low-end materials, such as paper board, composite board, epoxy board (also known as 3240 epoxy board and phenolic board) and FR-4 fiberglass board (splicing board). The overall density of paper board and composite board should be low and the back color should be the same, but it is easy to see that there is basically no fiberglass cloth pattern inside the board. The difference between epoxy board and FR4 glass fiber splicing board is that the color of the back substrate is different. When the epoxy board is scraped by hand or other tools at the fracture, it is easy to see white powder, and the color is off white. FR4 splicing board is easier to see because it is pressed with the leftover materials of FR4 glass fiber cloth, and large stripes can be seen on the back of the whole board.
5. What kind of failure will occur when the performance of the high frequency board is poor
The poor performance of high-frequency board is mainly due to the low dielectric constant, which affects the working stability of high-frequency circuit. The high frequency has skin effect, and the low dielectric constant of the circuit board causes leakage of high frequency components, resulting in signal weakening, frequency offset drift and serious vibration stop. The overall electrical performance index is reduced.
6. Characteristics of high frequency board
For the high-frequency circuit board provided by the utility model, the edges of the upper opening and the lower opening of the hollow groove of the core board are provided with a retaining edge that can block the glue flow, so that when the core board is bonded with the copper clad laminate placed on its upper and lower surfaces, the glue will not enter the hollow groove, that is, the bonding operation can be completed by one-time pressing. Compared with the high-frequency circuit board in the prior art, which can be completed by two-time pressing, The high-frequency circuit board in the utility model has the advantages of simple structure, low cost and easy manufacture.
7. Manufacturing requirements of high frequency board
High frequency board is one of the most difficult boards, so it must meet the manufacturing requirements as much as possible.
1. The drilling feed speed should be slow to 180 / S a new drill nozzle shall be used, and aluminum sheets shall be padded up and down. It is best to drill with a single PNL, and water shall not be encountered in the hole
2. Pore forming agent PTH hole The sample can use concentrated sulfuric acid (preferably not used) for 30min
3, Grinding plate Copper precipitation The circuit is made as normal double-sided
4. Special attention: the high-frequency board does not need to remove the glue residue.
1. If the high-frequency plate needs green oil backing, it is not allowed to grind the plate before resistance welding, and a red seal shall be stamped in MI.
2. If the high-frequency board needs to be printed with green oil on the substrate, it shall be printed with green oil twice (to prevent green oil foaming on the substrate). The Board shall not be ground before etching and tin withdrawal, but can only be air dried 。 For the first priming, use 43T screen plate for normal printing and sectional baking plate: 50 degrees 50Min 75 ° 50min 95 degrees 50Min 120 degrees 50Min 135 ° 50min one hundred and fifty 50 min degrees ， Exposure with line film, grinding the plate after development, and normal production for the second time. Note in MI: line film alignment is used for the first priming.
3. If the high-frequency board needs to be printed with green oil on some substrates and not on some substrates, it needs to produce a "backing film". The backing film only retains the green oil on the substrate, and the second normal production can be carried out after the backing baking board. The following picture shows the need of 018212, especially the "backing film".
Special attention should be paid to printing green oil only once if it is not printed on the substrate like 018092 (see the figure below, the blue part is the green oil window), so as to prevent the green oil on the substrate from developing after the first green oil priming.
Bake before tin spraying 150 degrees 30Min Before spraying tin
The line width tolerance without requirements shall be ± 0.05mm If required, it shall be made according to the customer's requirements.
To use the specified sheet See requirements. Because the plate price is expensive, only 1pnl can be opened.