The pcb multilayer circuit board is a special printed circuit board, and its existence "location" is generally special. For example, there will be pcb multilayer boards in the circuit board. This kind of multi-layer board can help the machine to conduct various different circuits. Not only that, but also has the effect of insulation, which will not let electricity and electricity collide with each other, which is absolutely safe. If you want to use a pcb multi-layer board with better performance, you must design it carefully. Next, I will explain how to design a pcb multi-layer circuit board.

　　1. Determination of the shape, size and number of layers of the circuit board

　　1. The number of layers must be determined according to the requirements of circuit performance, board size and circuit density. For multi-layer printed boards, four-layer boards and six-layer boards are the most widely used. Taking a four-layer board as an example, there are two conductor layers (component surface and welding surface), a power supply layer and a ground layer.

　　2. The layers of the PCB multilayer circuit board should be kept symmetrical, and preferably even-numbered copper layers, that is, four-layer circuit boards, six-layer PCBs, eight-layer circuit boards, etc. Because of the asymmetric lamination, the surface of the PCB circuit board is prone to warpage, especially for the surface-mounted PCB multilayer circuit board, which should be paid more attention.

　　3. Any printed circuit board has the problem of coordinating with other structural parts. Therefore, the shape and size of the printed circuit board must be based on the overall structure of the product. However, from the perspective of production technology, it should be as simple as possible, generally a rectangle with a less disparate aspect ratio, so as to facilitate assembly, improve production efficiency and reduce labor costs.

　　2. The position and orientation of components

　　1. On the other hand, the overall structure of the printed circuit board should be considered to avoid uneven arrangement of components and disorder. This not only affects the appearance of the printed board, but also brings a lot of inconvenience to assembly and maintenance work.

　　2. The position and orientation of components should first be considered from the perspective of circuit principles and cater to the trend of the circuit. Whether the placement is reasonable or not will directly affect the performance of the printed board, especially for high-frequency analog circuits, which have stricter requirements on the location and placement of components.

　　3. Reasonable placement of components, in a sense, has heralded the success of the printed board design. Therefore, when starting to arrange the layout of the printed board and decide the overall layout, the circuit principle should be analyzed in detail, and the position of special components (such as large-scale ICs, high-power transistors, signal sources, etc.) should be determined first, and then Arrange other components to avoid possible interference factors.

　　3. Requirements for wire layout and wiring area

　　Under normal circumstances, the wiring of multi-layer printed circuit boards is carried out according to the circuit function. When wiring the outer layer, it is required to have more wiring on the welding surface and less wiring on the component surface, which is conducive to the maintenance and troubleshooting of the printed circuit board. Thin, dense wires and signal lines that are susceptible to interference are usually arranged on the inner layer. A large area of copper foil should be distributed evenly in the inner and outer layers, which will help reduce the warpage of the board and also enable a more uniform coating on the surface during electroplating. In order to prevent damage to the printed wires and short circuits between layers during machining, the distance between the conductive patterns in the inner and outer wiring areas and the edge of the board should be greater than 50 mils.

　　4. Requirements for wire direction and line width

　　The multi-layer circuit board traces should separate the power supply layer, the ground layer and the signal layer to reduce the interference between the power supply, ground and signal. The lines of the two adjacent layers of printed boards should be perpendicular to each other as much as possible or follow oblique lines and curves, but not parallel lines, so as to reduce the interlayer coupling and interference of the substrate. And the wire should be as short as possible, especially for small signal circuits, the shorter the wire, the smaller the resistance and the less interference. For signal lines on the same layer, sharp corners should be avoided when changing directions. The width of the wire should be determined according to the current and impedance requirements of the circuit. The power input wire should be larger, and the signal wire should be relatively smaller. For general digital boards, the power input line width can be 50-80 mil, and the signal line width can be 6-10 mil.

　　Wire width: 0.5, 1, 0, 1.5, 2.0; allowable current: 0.8, 2.0, 2.5, 1.9; wire resistance: 0.7, 0.41, 0.31, 0.25; when wiring, pay attention to the width of the lines as much as possible to avoid sudden Thickening and sudden thinning are beneficial to impedance matching.

　　5. Requirements for drilling size and pad

　　1. The size of the component drilling on the PCB multilayer circuit board is related to the size of the selected component pins. If the drilling is too small, it will affect the installation and tinning of the device; if the drilling is too large, the solder joints are not enough during welding. full. Generally speaking, the calculation method of component hole diameter and pad size is:

　　2. The diameter of the component hole = component pin diameter (or diagonal) + (10 ~ 30mil)

　　3. The diameter of the component pad ≥ the diameter of the component hole + 18mil4. As for the via diameter, it is mainly determined by the thickness of the finished board. For high-density multilayer circuit boards, it should generally be controlled within the range of board thickness: diameter ≤ 5:1.

　　4. The calculation method of the via pad is: the diameter of the via pad (VIAPAD) ≥ the diameter of the via hole + 12mil.

　　6. Requirements for power supply layer, stratum partition and flower hole

　　For multi-layer printed boards, there is at least one power layer and one ground layer. Since all the voltages on the printed board are connected to the same power supply layer, the power supply layer must be partitioned and isolated. The size of the partition line is generally 20-80 mil line width. The voltage is too high, and the thicker the partition line.

　　In order to increase the reliability of the connection between the welding hole and the power supply layer and the ground layer, in order to increase its reliability and reduce the virtual welding caused by the heat absorption of a large area of metal during the welding process, the general connection plate should be designed in the shape of a flower hole.

　　Diameter of isolation pad ≥ drilling hole diameter + 20mil

　　7. Requirements for safe distance The setting of safe distance should meet the requirements of electrical safety

　　Generally speaking, the minimum spacing of outer conductors should not be less than 4mil, and the minimum spacing of inner conductors should not be less than 4mil. In the case that the wiring can be arranged, the spacing should be as large as possible to improve the yield of the board and reduce the hidden danger of failure of the finished board.

　　8. Requirements for improving the anti-interference ability of the whole board In the design of multi-layer printed boards, attention must also be paid to the anti-interference ability of the whole board. The general methods are:

　　1. Choose a reasonable grounding point.

　　2. Add filter capacitors near the power supply and ground of each IC, and the capacity is generally 473 or 104.

　　3. For the sensitive signals on the printed circuit board, the accompanying shielded wires should be added separately, and the wiring should be as little as possible near the signal source.

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