At present, most of the circuit boards are made by the subtraction method, that is, the excess copper foil on the raw material copper clad laminate is subtracted to form a conductive pattern.
The manufacturing process of PCB multilayer circuit board
The reduction method is mostly chemical corrosion, which is economical and efficient. It is only chemical corrosion indistinguishable attack, so it is necessary to protect the conductive pattern, apply a layer of resist on the conductive pattern, and then subtract the unprotected copper foil corrosion. The early resist was to print the anti-corrosion ink in the form of circuit by screen printing, so it was called "printed circuit". However, with the increasing precision of electronic products, the image resolution of the printed circuit can not meet the needs of the product, so photoresist is used as the image analysis material. Photoresist is a kind of photosensitive material, which is sensitive to a certain wavelength of light source and forms a photochemical reaction with it to form a polymer. the pattern is selectively exposed by a graphic negative, and then the unpolymerized photoresist is stripped off by a developer (for example, 1% sodium carbonate solution) to form a pattern protective layer.
In addition, the interlayer conduction function is realized through the metallized hole, so it is necessary to drill the hole and realize the metallization electroplating operation in the PCB fabrication process, and finally realize the interlayer conduction.
Take a regular six-layer PCB as an example, the production process is as follows:
First, make two double panels without holes.
Open material (raw material double-sided copper clad laminate)-inner layer pattern fabrication (forming pattern resist layer)-inner layer etching (minus excess copper foil)
Second, two pieces of inner core board are bonded and pressed with epoxy resin glass fiber semi-solidified sheet , two inner core plates and semi-cured sheet are riveted, and then a piece of copper foil on each side of the outer layer is pressed by a press under high temperature and high pressure to make it adhesive. The key material is semi-curing sheet, which has the same composition as the raw material, and is also epoxy resin glass fiber, but it is not completely solidified and will be liquefied at 7-80 degrees Celsius, in which curing agent is added, which will react and cure with the resin at 150 degrees Celsius, and then it is no longer reversible. Through such a semi-solid-liquid-solid conversion, the adhesion is completed under high pressure.
PCB multilayer board is more complex than single and double layer board in terms of design and manufacture. If we are not careful, we will encounter some problems. What difficulties should we avoid in PCB multilayer circuit board proofing?
Difficulties in proofing multilayer impedance plates
1. Difficulties in interlayer alignment.
Due to the large number of middle layers of multi-layer circuit boards, users have higher and higher requirements for the calibration of PCB layer. In general, the alignment tolerance between layers is controlled at 75 microns. Considering the large size of the multi-layer circuit board unit, the high temperature and humidity in the pattern conversion workshop, the dislocation overlap caused by the inconsistency of different core boards, the interlayer positioning mode and so on, the alignment control of the multi-layer circuit board is more difficult.
2. Difficulties in the manufacture of internal circuits.
The multilayer circuit board adopts special materials such as high TG, high speed, high frequency, thick copper, thin dielectric layer and so on, which puts forward high requirements for internal circuit fabrication and graphic size control. For example, the integrity of impedance signal transmission increases the difficulty of internal circuit manufacturing. Width and line spacing is small, open circuit and short circuit increase, short circuit increases, the qualified rate is low; thin line signal layer, inner layer AOI leak detection probability increase; inner core plate thin, easy to wrinkle, poor exposure, easy to curl when etching machine; high level plate is mostly system board, unit size is larger, and the product scrapping cost is high.
3. Difficulties in compression manufacturing
Many inner core panels and semi-cured plates are superimposed, and there are easily some defects in stamping production, such as skateboard, delamination, resin void and bubble residue. In the design of laminated structures, the heat resistance, pressure resistance, rubber content and dielectric thickness of materials should be fully considered, and a reasonable material pressing scheme for multilayer circuit boards should be made. Due to the large number of layers, the expansion and contraction control and size coefficient compensation can not be consistent, and the thin interlayer insulation layer can easily lead to the failure of interlayer reliability test.
4. Difficulties in drilling.
The use of high-TG, high-speed, high-frequency, thick copper special plate increases the difficulty of drilling roughness, drilling burr and drilling dirt. Many layers, cumulative total copper thickness and plate thickness, drilling tool is easy to break; dense BGA, narrow hole wall spacing caused by CAF failure; plate thickness is easy to lead to oblique drilling problem.
