Introduction to multilayer circuit boards
The double-sided board is a medium layer in the middle, and both sides are wiring layers. A multi-layer board is a multi-layer wiring layer. There is a dielectric layer between every two layers. The dielectric layer can be made very thin. A multilayer circuit board has at least three conductive layers, two of which are on the outer surface, while the remaining layer is synthesized inside the insulating board. The electrical connections between them are usually made through plated through holes in the cross-section of the board.
Advantages and disadvantages of multilayer circuit boards
High assembly density, small size, and light weight Due to the high assembly density, the connections between components (including components) are reduced, thereby improving reliability; the number of wiring layers can be increased, thereby increasing design flexibility; A circuit with a certain impedance; it can form a high-speed transmission circuit; it can be equipped with a circuit, magnetic circuit shielding layer, and a metal core heat dissipation layer to meet the needs of special functions such as shielding and heat dissipation; simple installation and high reliability.
High cost; long cycle; high reliability detection methods are required. Multilayer printed circuit is the product of the development of electronic technology in the direction of high speed, multi-function, large capacity and small volume. With the continuous development of electronic technology, especially the extensive and in-depth application of large-scale and ultra-large-scale integrated circuits, multi-layer printed circuits are rapidly developing towards high-density, high-precision, and high-level digitalization, with fine lines, small apertures through, Blind and buried vias, high thickness-to-aperture ratio and other technologies to meet the needs of the market.
The difference between multilayer PCB circuit board and double panel
A multilayer PCB circuit board is a printed circuit board formed by laminating and bonding alternating conductive pattern layers and insulating materials. The number of layers of the conductive pattern is more than three layers, and the electrical interconnection between layers is realized through metallized holes. If one double-sided board is used as the inner layer, two single-sided boards are used as the outer layer, or two double-sided boards are used as the inner layer, and two single-sided boards are used as the outer layer, the positioning system and insulating bonding materials are laminated together, and the conductive pattern is pressed The design requires interconnection, which becomes a four-layer or six-layer printed circuit board, also known as a multilayer PCB circuit board.
Compared with the general multi-layer board and double-sided board production process, the main difference is that the multi-layer board adds several unique process steps: inner layer imaging and blackening, lamination, etch back and desmear. In most of the same process, some process parameters, equipment precision and complexity are also different. For example, the metallized connection of the inner layer of the multilayer board is the decisive factor for the reliability of the multilayer board, and the quality requirements for the hole wall are stricter than those of the double-layer board, so the requirements for drilling are even higher. In addition, the number of stacked boards per drilling of multi-layer boards, the speed and feed rate of drill bits during drilling are different from those of double-sided boards. The inspection of finished and semi-finished multi-layer boards is also more stringent and complicated than that of double-sided boards. Due to the complex structure of multi-layer boards, glycerin hot-melt technology with uniform temperature should be used instead of infrared hot-melt technology that may cause excessive local temperature rise.
Process flow of multilayer circuit board PCB
- The purpose of blackening and browning
①Remove surface oil, impurities and other pollutants;
② Increase the specific surface of the copper foil, thereby increasing the contact area with the resin, which is conducive to the full diffusion of the resin and the formation of a greater bonding force;
③ Make the non-polar copper surface become the surface with polar CuO and Cu2O, and increase the polar bonding between copper foil and resin;
④The oxidized surface is not affected by moisture at high temperature, reducing the chance of delamination between copper foil and resin.
⑤The boards with the inner circuit must be blackened or browned before they can be laminated. It is to oxidize the copper surface of the inner board. Generally, the generated Cu2O is red and CuO is black, so Cu2O in the oxide layer is mainly called browning, and CuO is mainly called blackening.
- Lamination is the process of bonding each layer of circuits into a whole with the help of B-stage prepreg. This bonding is achieved through interdiffusion and penetration of macromolecules on the interface, and then interweaving. The process of bonding the various layers of circuits into a whole with a prepreg. This bonding is achieved through interdiffusion and penetration of macromolecules on the interface, and then interweaving.
- Purpose: To press discrete multi-layer boards together with bonding sheets to form multi-layer boards with the required number of layers and thickness.
①Typesetting Copper foil, bonding sheet (prepreg), inner layer board, stainless steel, isolation board, kraft paper, outer layer steel plate and other materials are laminated according to the process requirements. If the board has more than six layers, pre-typesetting is required. Copper foil, adhesive sheet (prepreg), inner layer board, stainless steel, isolation board, kraft paper, outer layer steel plate and other materials are laminated according to the process requirements. If the board has more than six layers, pre-typesetting is required.
② Lamination process Send the stacked circuit board into the vacuum heat press. Using the heat energy provided by the machine, the resin in the resin sheet is melted, so as to bond the substrate and fill the gap.
③ Lamination For designers, the first thing to consider in lamination is symmetry. Because the board will be affected by pressure and temperature during the lamination process, there will still be stress in the board after the lamination is completed. Therefore, if the two sides of the laminated board are uneven, the stress on the two sides will be different, causing the board to bend to one side, which greatly affects the performance of the PCB. Keyou Circuit specializes in the production of high-precision multi-layer circuit boards. The products are widely used in: LCD liquid crystal modules, communication equipment, instruments and meters, industrial power supplies, digital, medical electronics, industrial control equipment, led modules/modules, electric energy, transportation, Science and education research and development, automobile, aerospace and other high-tech fields. Consulting hotline: Miss Zhong
In addition, even on the same plane, if the distribution of copper is uneven, the flow rate of the resin at each point will be different, so that the thickness of the place with less copper will be slightly thinner, and the thickness of the place with more copper will be slightly thicker. Some. In order to avoid these problems, factors such as the uniformity of copper distribution, the symmetry of the stack, the design and layout of blind buried holes, etc. must be carefully considered during the design.
- Decontamination and copper sinking
Purpose: To metallize through-holes.
①The base material of the circuit board is composed of copper foil, glass fiber and epoxy resin. In the production process, the section of the hole wall after the substrate is drilled is composed of the above three parts of the material.
② Hole metallization is to solve the problem of covering the section with a uniform layer of metal copper that is resistant to thermal shock. Hole metallization is to solve the problem of covering the section with a uniform layer of metal copper that is resistant to thermal shock.
③The process is divided into three parts: 1. Decontamination process, 2. Electroless copper deposition process, 3. Thickened copper process (full board electroplating copper).
- Immersion copper and thickened copper
Hole metallization involves a concept of capability, thickness-to-diameter ratio. The aspect ratio refers to the ratio of the plate thickness to the hole diameter. , thickness-to-diameter ratio. The aspect ratio refers to the ratio of the plate thickness to the hole diameter. When the board keeps getting thicker and the hole diameter keeps decreasing, it becomes more and more difficult for the chemical liquid to enter the depth of the hole. Although the electroplating equipment uses vibration, pressure, etc. to allow the liquid to enter the center of the hole, the center caused by the concentration difference Thin coatings are still unavoidable. At this time, there will be a slight open circuit phenomenon in the drilling layer. When the voltage increases and the board is impacted under various harsh conditions, the defects are completely exposed, causing the circuit of the board to be disconnected, and the specified work cannot be completed.
Therefore, designers need to know the process capabilities of board manufacturers in a timely manner, otherwise the designed PCB will be difficult to realize in production. It should be noted that the parameter of thickness-to-diameter ratio must be considered not only in the design of through holes, but also in the design of blind buried holes.
- Outer layer dry film and graphic plating
The principle of the outer layer graphics transfer is similar to that of the inner layer graphics transfer. They both use photosensitive dry film and photographing to print the circuit graphics on the board. The difference between the outer dry film and the inner dry film is:
① If the subtraction method is used, the outer dry film is the same as the inner dry film, and the negative film is used as the board. The part of the cured dry film on the board is the circuit. The uncured film is removed, and the film is retreated after acid etching, and the circuit pattern is left on the board because of the protection of the film.