PCB Tech - Summary of the processing difficulties of high-frequency boards

Based on the physical and chemical characteristics of PTFE, the high-frequency board processing technology is different from the traditional FR4 process. If it is processed under the same conditions as the conventional epoxy resin glass fiber copper clad laminate, it will not be qualified. The product.

1.High-frequency board processing drilling: the base material is soft, the number of drilling stacks should be small, usually 0.8mm thickness is suitable for two stacks; the speed should be slower; new drills should be used, the top angle of the drill, and the thread The angle has its special requirements.

2. Printed solder mask: After the high-frequency board is etched, it is not allowed to use a roller to polish the board before printing the solder mask to avoid damage to the substrate. It is recommended to use chemical methods for surface treatment. To achieve this: without grinding the board, after printing the solder mask, the circuit and the copper surface are uniform and there is no oxide layer, which is by no means easy.

3. Hot air leveling: Based on the inherent properties of fluororesin, rapid heating of high-frequency plates should be avoided as much as possible. Before spraying tin, do a pre-heat treatment at 150°C for about 30 minutes, and then spray tin immediately. The temperature of the tin tank should not exceed 245 degree Celsius, otherwise the adhesion of the isolated pad will be affected.

4. Milling profile: Fluorine resin is soft, the general milling cutter has a lot of burrs, and it is not flat. It needs to be milled with a suitable special milling cutter.

5. Transport between high-frequency board processing procedures: It cannot be placed vertically, only placed in the basket flat with paper, and no finger is allowed to touch the circuit pattern in the board during the whole process. The whole process prevents scratches and scratches. Line scratches, pinholes, indentations, and dents will affect the signal transmission, and the board will be rejected.

6. High-frequency board processing and etching: Strictly control side erosion, sawtooth, and notches, and strictly control the line width tolerance of ±0.02mm. Check with a 100x magnifying glass.

7. High-frequency plate processing electroless copper: The pre-treatment of electroless copper is a difficult point in the manufacture of high-frequency plates, and it is also a key step. There are many methods for the pretreatment of copper immersion, but in summary, there are two methods that can stabilize the quality and be suitable for mass production of high-frequency boards:

Method 1: Plasma (plasma) method: Imported equipment is required, and carbon tetrafluoride (CF4) or argon (Ar2) nitrogen (N2) and oxygen are injected between two high-voltage power supplies in a vacuumed environment. (O2) gas, the printed board is placed between the two electricity, plasma is formed in the cavity, so as to remove the dirt and dirt in the hole. This method can obtain a satisfactory uniform effect, and mass production is feasible. But to invest in expensive equipment (approximately more than one hundred thousand US dollars per machine), there are two well-known Plasma equipment companies in the United States: APS and March.

Method 2: Chemical method: Add a solution such as tetrahydrofuran to form a sodium complex, so that the surface layer atoms of the polytetrafluoroethylene in the pores are eroded to achieve the purpose of wetting the pores. This is a classic and successful method with good results and stable quality, but it is highly toxic, flammable, and dangerous, and requires special management.

For high-frequency board processing, ε3.38 and RogersRo4003 high-frequency substrates have high-frequency performance similar to PTFE glass fiber substrates, and have the characteristics of easy processing similar to FR4 substrates. This uses glass fiber and ceramics as fillers. High heat-resistant material with glass transition temperature Tg>280 degree Celsius. This kind of substrate drilling is very consuming drill bit, special drilling machine parameters are needed, and the milling shape needs to be changed often; but other high-frequency board processing technology is similar, no special hole treatment is required, so many PCB manufacturers and customers However, Ro4003 does not contain flame retardant, the board reaches 371 degree Celsius, and the high-frequency board can cause burning. State-owned 704 factory LGC-046 sheet is a modified polyphenylene ether (PPO) type, with a dielectric constant of 3.2 and processing performance as FR4. This product has also been approved for use in many domestic orders.