PCB multilayer circuit board is a special printed circuit board, and its "location" is generally special. For example, there will be PCB multilayer boards in circuit boards. This kind of multi-layer board can help the machine to conduct various lines. Not only that, but also can play an insulating effect, so that it will not let electricity collide with each other, which is absolutely safe. If you want to use a pcb multilayer board with better performance, you must carefully design it. Next, I will explain how to design a pcb multilayer circuit board.
一 Determination of the shape, size and number of layers of the circuit board
1. In terms of the number of layers, it must be determined according to the requirements of circuit performance, board size and density of circuits. For multi-layer printed boards, four-layer boards and six-layer boards are the most widely used. Taking four-layer boards as an example, there are two wire layers (component surface and soldering surface), one power layer and one ground layer.
2. The layers of PCB multilayer circuit boards should be kept symmetrical, and it is better to have an even number of copper layers, that is, four-layer circuit boards, six-layer PCBs, eight-layer circuit boards, etc. Because of the asymmetric lamination, the PCB circuit board surface is prone to warping, especially for surface-mounted PCB multilayer circuit boards, more attention should be paid.
3. Any printed circuit board has the problem of being assembled with other structural parts. Therefore, the shape and size of the printed circuit board must be based on the structure of the whole product. However, from the perspective of production technology, it should be as simple as possible. Generally, it is a rectangle with a relatively small aspect ratio, so as to facilitate assembly, improve production efficiency and reduce labor costs.
二.the location and placement of components
1. On the other hand, the overall structure of the printed circuit board should be considered to avoid uneven and disorderly arrangement of components. This not only affects the appearance of the printed board, but also brings a lot of inconvenience to the assembly and maintenance work.
2. The position and placement direction of components should first be considered from the perspective of circuit principle and cater to the direction 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, the requirements for the location and placement of components are more stringent.
3. Reasonable placement of components, in a sense, has indicated the success of the printed board design. Therefore, when starting to arrange the layout of the printed board and determine the overall layout, a detailed analysis of the circuit principle should be carried out, first determine the location of special components (such as large-scale ICs, high-power tubes, signal sources, etc.), and then Arrange other components to avoid factors that may cause interference as much as possible.
三 Requirements for wire layout and wiring area
In general, multilayer printed circuit board wiring is carried out according to the circuit function. When wiring on the outer layer, it is required to have more wiring on the soldering 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 in the inner layer. The large-area copper foil should be evenly distributed 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 the shape processing from damaging the printed wires and causing interlayer short circuit during mechanical processing, the distance between the conductive pattern in the inner and outer layer wiring area and the edge of the board should be greater than 50mil.
四. Requirements for wire direction and line width
The wiring of multi-layer circuit boards should separate the power layer, ground layer and signal layer to reduce the interference between power, ground and signal. The lines of two adjacent layers of printed boards should try to be perpendicular to each other or follow oblique lines or curves instead of parallel lines, so as to reduce the interlayer coupling and interference of the substrate. And the wires should be as short as possible, especially for small signal circuits, the shorter the wire, the smaller the resistance and the smaller the 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 line should be larger, and the signal line can be relatively smaller. For general digital boards, the line width of the power input line can be 50~80mil, and the line width of the signal line can be 6~10mil.
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, you should also pay attention to the width of the lines to be as consistent as possible to avoid wires Sudden thickening and sudden thinning are conducive to impedance matching.
五. Drilling hole size and pad requirements
1. The size of the component drilling holes on the PCB multilayer circuit board is related to the pin size of the selected components. If the drilling holes are too small, it will affect the installation and tinning of the components; if the drilling holes are too large, the solder joints will not be enough during welding. full. Generally speaking, the calculation method of component hole diameter and pad size is:
2. The diameter of the component hole = the diameter of the component pin (or diagonal) + (10~ 30mil)
3. Component pad diameter ≥ component hole diameter + 18mil 4. As for the via hole 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: aperture ≤ 5:1.
4. The calculation method of the via pad is: via pad (VIAPAD) diameter ≥ via diameter + 12mil.
六. Requirements for power supply layer, stratum partition and flower hole
For multilayer printed boards, there are at least one power layer and one ground layer. Since all the voltages on the printed board are connected to the same power layer, the power layer must be partitioned and isolated. The size of the partition line is generally 20~80mil in width. The voltage is super high, and the thicker the partition line is.
In order to increase the reliability of the connection between the welding hole and the power layer and the ground layer, and reduce the virtual welding caused by large-area metal heat absorption during the welding process, the general connection plate should be designed in the shape of a flower hole. Aperture diameter of isolation pad ≥ drilling aperture + 20mil
七. Requirements for safety distance The setting of safety distance should meet the requirements of electrical safety
Generally speaking, the minimum spacing between the outer layer conductors should not be less than 4mil, and the minimum spacing between the inner layer 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 the failure of the finished board.
八. The requirements for improving the anti-interference ability of the whole board The design of multi-layer printed boards must also pay attention 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, accompanying shielding lines should be added respectively, and the wiring near the signal source should be as little as possible.
