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Introduction to Multilayer Circuit Boards
Dual-layer boards have one dielectric layer in the center and two alignment layers on both sides. Multilayer boards have multiple alignment layers, with a dielectric layer between each two layers, and the dielectric layer can be made very thin. Multilayer boards have at least three conductive layers, two of which are on the outer surface, while the remaining layer is composited within the insulation board. The electrical connection between them is usually realized by plating through holes in the cross section of the board.
Multilayer Circuit Boards
Advantages of Multilayer PCBs
Multilayer boards have high assembly density, small size, and light weight. Due to the high assembly density, the connection between each component (including components) is reduced, which improves the reliability of commemorative products. The number of wiring layers can be added, thus increasing the design flexibility. A circuit with a certain impedance can be formed. High-speed transmission circuits can be formed. Masking layers for circuits and magnetic circuits can be set, and metal core heat dissipation layers can be set to meet the needs of special functions such as masking and heat dissipation. Simple installation and high reliability.
Disadvantages of multilayer boards
Multilayer circuit boards are expensive, have a long production cycle, and require highly reliable testing methods.Multilayer printed circuit is a product of the development of electronic technology in the direction of high speed, multi-function, large capacity, small volume. With the continuous development of electronic technology,especially large-scale and ultra-large-scale integrated circuits, multi-layer printed circuits are rapidly to high-density, high-precision, high-layer digitization of the direction of the development of the emergence of a fine line,small aperture through the blind holes buried holes, high board thickness aperture ratio of the science and technology to meet market needs.
Difference between Multilayer and Duplex Boards
Multilayer boards are printed circuit boards made of alternating layers of conductive patterns and insulating materials laminated together. The number of conductive pattern layers is more than three, and the electrical interconnection between the layers is realized through metallized holes. If a double-layer circuit board as the inner layer, two single-sided circuit boards as the outer layer or two double-layer circuit boards as the inner layer, two single-sided circuit boards as the outer layer, through the positioning system and insulating adhesive materials laminated together, and the conductive pattern according to the design requirements for interconnection, it becomes a four-layer, six-layer printed circuit boards, also known as multilayer circuit boards.
Compared to the production processes of general multilayer and dual-layer boards, the main difference is that multilayer circuit boards have several new process steps unique to multilayer boards: internal imaging and blackening, lamination, etching, and de-drilling. While most of the processes are the same, there are also differences in certain process parameters, equipment accuracy and complexity. For example, the metallized connection of the inner layers of a multilayer board is a decisive factor for the reliability of multilayer boards. The quality requirements for the hole walls are more stringent than for double-layer boards, and the requirements for the drilling of commemorative products are even higher. In addition, the number of stacked boards per drilling, the speed of the drill and the amount of feed during drilling are all different for multilayer boards than for double-layer boards. The inspection of finished and semi-finished products of multilayer boards is also more stringent and complicated than that of double-layer boards. Due to the complex structure of multilayer boards, it is necessary to use the glycerin fusion process, which has a uniform temperature, rather than the infrared fusion process, which may lead to excessive localized temperature rise.
Multilayer Circuit Boards
Process Flow of Multilayer Circuit Boards
1.Blackening and browning
1.1Remove oil, impurities and other pollutants on the surface.
1.2Increase the specific surface of copper foil, so as to increase the contact area with resin, which is conducive to the full diffusion of resin and the formation of a larger bonding force.
1.3Make the non-polar copper surface become the surface with polar CuO and Cu2O, and increase the polar bonding between copper foil and resin.
1.4The oxidized surface is not affected by moisture at high temperature, which reduces the chance of delamination between copper foil and resin.
1.5The inner circuit board must be blackened or browned before lamination. It is to oxidize the copper surface of the wiring of the inner layer circuit board. Generally, Cu2O is red and CuO is black, so the oxidized layer with Cu2O as the main part is called browning and CuO as the main part is called blackening.
Lamination is the process of bonding the layers into a whole with the help of a B-stage semi-cured sheet. This bonding is realized by mutual diffusion and penetration of macromolecules at the interface, which leads to interweaving. Semi-cured sheets are used to bond the various layers into an integrated process. This bonding is achieved through the mutual diffusion and penetration of macromolecules at the interface, resulting in interweaving.
2.Multilayer circuit board lamination
Layers of discrete multilayer circuit boards are pressed together with bonding sheets to form multilayer circuit boards of the required number of layers and thickness.
2.1.Layout: Copper foils, bonding sheets (semi-cured sheets), inner laminates, stainless steel, separators, kraft paper, and outer steel laminates are laminated according to the process requirements. Pre-layout is also required for boards with more than six layers. Layer copper foil, bonding sheet (semi-cured sheet), inner sheet, stainless steel, isolation sheet, kraft paper, and outer sheet according to the process requirements. In case of more than six layers, pre-lamination is also required.
2.2.The lamination process feeds the laminated boards into the vacuum heat press. Using the heat provided by the machine,the resin inside the resin sheet is melted to bond the substrate and fill the gaps.
2.3.Lamination For designers, the first thing to consider in lamination is symmetry.This is because the circuit boards are subjected to pressure and temperature during the lamination process, and there are also stresses in the circuit boards after the lamination process is completed. If the two sides of the laminated board are not uniform, the stress on both sides will be different, causing the board to bend to one side, which will greatly affect the performance of the board.
In addition, even in the same plane,if the copper is not evenly distributed,it will cause the resin to flow at different speeds at each point, so that the thickness of the area with less copper will be thinner, while the thickness of the area with more copper will be thicker.In order to avoid these problems, in the design of the uniformity of the copper cloth, the symmetry of the laminated layer, blind buried hole design layout and other aspects of the factors must be carried out in detail rate.
3.De-drilling and copper sinking
Purpose: Metallize the through holes.
3.1.The substrate of circuit board is composed of copper foil, glass fiber and epoxy resin. In the manufacturing process, the hole wall cross-section after drilling of the substrate is composed of the above three parts.
3.2.Hole metallization is the solution to cover the cross-section with a uniform layer of copper metal that is resistant to thermal shock.Hole metallization is the solution to cover the cross-section with a uniform layer of copper metal that is resistant to thermal shock.
3.3.The process is divided into three parts: one to remove the drill dirt process, two chemical copper immersion process,three copper thickening process (full-plate copper plating).
4.Copper Immersion and Copper Thickening
Hole metallization involves a concept of ability, thickness to diameter ratio. Thickness to diameter ratio is the ratio of plate thickness to hole diameter. Thickness to diameter ratio. Thickness to diameter ratio refers to the ratio of board thickness to aperture diameter. When the circuit board board continues to become thicker, and the aperture diameter continues to decrease,the chemical solution is more and more difficult to enter the depth of the drilled holes, although the electroplating equipment using vibration, pressure and so on to allow the solution to enter the drilled holes in the center,but the concentration of the difference in the center of the plating layer caused by the thin still can not be avoided. The phenomenon of micro-open circuit in the drilled hole layer occurs,and when the voltage is increased and the circuit board is impacted under various adverse conditions, the defects are completely exposed, resulting in circuit board circuit breakage and inability to complete the specified work.
Therefore, designers need to understand the board manufacturer's process capabilities in a timely manner,otherwise it is difficult to design the PCB to be realized in production. It should be noted that the thickness to diameter ratio of this parameter must be considered not only in the through-hole design, blind buried hole design also need to be considered.
5.The outer dry film and graphic plating
The outer layer of pattern transfer and the inner layer of the principle of pattern transfer is similar, are the use of light-sensitive dry film and photo method will be printed on the circuit board line pattern.
The difference between the outer dry film and the inner dry film is that if the reduction method is used, the outer dry film is the same as the inner dry film, and a negative sheet is used to make the board.The cured dry film portion of the board is the wiring. After removing the uncured film and stripping the film after acid etching, the circuit pattern remains on the board because it is protected by the film.