High-frequency Microwave PCB Fabrication Difficulty

The processing difficulty of high-frequency microwave board is based on the physical and chemical properties of PTFE board, which makes its processing technology different from the traditional FR4 process. If it is processed under the same conditions as the conventional epoxy resin glass fiber copper clad laminate, scrappage will occur.

Process Requirements

(1) Drilling: The base material is soft, and the number of stacked plates for drilling should be small. Usually 0.8mm plate thickness is suitable for two stacks; the speed should be slower; new drills should be used, and the top angle and thread angle of the drill have their special characteristics Requirements.

(2) Printed solder mask: After the board is etched, the board cannot be polished with a roller brush before the solder mask is printed, so as not to damage 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 the plate should be avoided as much as possible. Before spraying tin, it should be preheated at 150°C for about 30 minutes, and then spray tin immediately. The temperature of the tin tank should not exceed 245℃, otherwise the adhesion of the isolated pad will be affected.
(4) Milling shape: Fluorine resin is soft, ordinary 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 processes: 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) Etching: Strictly control side erosion, sawtooth, and notch, and strictly control the line width tolerance of ±0.2mm. Check with a 100x magnifying glass.

(7) Electroless copper deposition: The pretreatment of electroless copper deposition is a difficult and critical step in the manufacture of τefon boards. There are many methods for the pretreatment of copper precipitation, but in summary, it can stabilize the quality and is suitable for mass production. There are no more than two methods.

Method 1: Chemical method: add a solution such as tetrahydrofuran to form a sodium complex , So that the PTFE surface layer atoms in the hole are eroded to achieve the purpose of wetting the hole. 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.

Method 2: Plasma (plasma) method: Imported equipment is required, and carbon tetrafluoride (CF4) or argon (Ar2, nitrogen (N2), oxygen ( O2) Gas, the printed board is placed between the two electricity sources, and plasma is formed in the cavity to remove the drilling dirt and dirt in the hole. This method can obtain a satisfactory and 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 American Plasma equipment companies: APS March In recent years, some domestic documents have also introduced other methods, but the classic and effective methods are the above two kind.

For the ε3.38 and Rogers RO4003 high-frequency substrates, it has the similar high-frequency performance of the PTFE glass fiber substrate and the easy processing characteristics similar to the FR4 substrate. This uses glass fiber and ceramics as the filling material. High heat-resistant material with melting temperature τg>2δσ°℃. This kind of substrate drilling consumes a lot of drill bits, and requires special machine parameters. The milling profile requires frequent change of milling cutters; but other processing techniques are similar, and no special hole treatment is required, so it has been recognized by many PCB manufacturers and customers. , But RO4003 does not contain flame retardant, the board reaches 371℃, the board can cause burning.