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At present, most circuit boards are made by subtracting the excess copper foil on the raw material copper clad plate to form a conductive pattern.

Manufacturing process of PCB multilayer circuit board

The reduction method is mostly chemical corrosion, which is economical and efficient. Only the chemical corrosion has no differential attack, so it is necessary to protect the required conductive pattern. A layer of anti-corrosion agent should be coated on the conductive pattern, and then the unprotected copper foil corrosion should be subtracted. In the early days, the anti-corrosive ink was printed in line by screen printing, so it was called "printed circuit board". However, as electronic products become more and more sophisticated, the image resolution of printed circuits cannot meet the needs of products, and then photoresists are used as image analysis materials. The photoresist is a kind of photosensitive material, which is sensitive to the light source of a certain wavelength and forms a photochemical reaction with it to form a polymer. After the pattern is selectively exposed with a pattern negative, the unpolymerized photoresist is stripped through the developer (e.g. 1% sodium carbonate solution) to form a pattern protective layer.

In addition, the interlayer conduction function is realized through metallized holes, so the PCB manufacturing process also needs to carry out drilling operations, and metallized electroplating operations for holes to finally achieve interlayer conduction.

Taking a conventional six layer PCB as an example, the manufacturing process is as follows: 

1、 First make two holeless double-sided boards

Blanking (raw material double-sided copper clad laminate) - inner layer pattern making (forming pattern anti-corrosive layer) - inner layer etching (minus excess copper foil)

2、 The two prepared inner core plates are bonded and pressed with epoxy resin glass fiber semi curing sheet, and the two inner core plates are riveted with the semi curing sheet. Then, a copper foil is laid on both sides of the outer layer and pressed with a press under high temperature and high pressure, so that they are bonded. The key material is the semi curing sheet, which has the same composition as the raw material. It is also epoxy resin glass fiber, but it is not fully cured. It will liquefy at 7-80 ℃. With curing agent added, it will cross link with the resin and cure at 150 ℃, and then it will no longer be reversible. Through such a semi-solid liquid solid transformation, adhesion and bonding are completed under high pressure.

PCB multilayer boards are more complex than single and double-layer boards in terms of design and manufacturing. If you are not careful, you will encounter some problems. What difficulties should we avoid in PCB multilayer board proofing?

Difficulties in multi-layer impedance plate proofing 

1. Difficulties in interlayer alignment

Due to the large number of layers in multilayer circuit boards, users have higher and higher requirements for PCB layer calibration. Generally, the alignment tolerance between layers is controlled at 75 microns. Considering the large unit size of the multilayer circuit board, the high temperature and humidity in the graphic conversion workshop, the dislocation overlap caused by the inconsistency of different core boards, and the positioning mode between layers, it is more difficult to control the alignment of the multilayer circuit board.

2. Difficulties in making internal circuits

Multilayer circuit boards are made of special materials such as high TG, high speed, high frequency, thick copper and thin dielectric layers, which put 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. The width and line spacing are small, the open circuit and short circuit increase, the short circuit increases, and the qualification rate is low; There are many thin line signal layers, and the probability of AOI leakage detection in the inner layer increases; The inner core plate is thin, easy to wrinkle, poor exposure, and easy to curl when etching; High level plates are mostly system boards with large unit size and high product scrap cost.

3. Difficulties in compression manufacturing

Many inner core plates and semi solidified plates are overlapped, and defects such as sliding plate, delamination, resin void and bubble residue are easy to appear in stamping production. In the design of laminated structure, the heat resistance, voltage resistance, glue content and dielectric thickness of materials shall be fully considered, and a reasonable material pressing scheme for multilayer circuit boards shall be formulated. Due to the large number of layers, the consistency between expansion and contraction control and size coefficient compensation cannot be maintained, and the interlaminar reliability test is easy to fail due to the thin layer insulation.

4. Difficulties in drilling

The use of high TG, high speed, high frequency, thick copper special plates increases the difficulty of drilling roughness, drilling burr and drilling dirt removal. The number of layers is large, the total copper thickness and plate thickness are accumulated, and the drilling tool is easy to break; CAF failure caused by many dense BGA and narrow hole wall spacing; Inclined drilling is easy to occur due to the thickness of the plate.