PCB multilayer board is a special printed board, and its "location" is generally special. For example, there will be PCB multilayer boards in circuit boards.
This kind of PCB multi-layer circuit board can help the machine to conduct various lines. Not only that, but also can play an insulating effect to prevent electricity from colliding 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 board.
1. Determination of PCB circuit board shape, size and number of layers
① Any printed circuit board has the problem of being assembled with other structural parts, so 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 a rectangle with a relatively small aspect ratio, so as to facilitate assembly, improve production efficiency and reduce labor costs.
② 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 multilayer printed boards, four-layer boards and six-layer boards of PCB multilayer circuit boards are the most widely used. Taking four-layer boards as an example, there are two wire layers (component surface and welding surface), a power supply layers and a stratum.
③ Each layer of the PCB circuit board should be symmetrical, and it is best to have an even number of copper layers, that is, four, six, eight layers, etc. Because of asymmetrical lamination, the board surface is prone to warping, especially for surface-mounted multilayer boards, more attention should be paid.
2. The position and placement direction of components
① 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 devices are more stringent.
② Reasonable placement of components has predicted the success of the printed board design in a sense. 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, and the location of special components (such as large-scale ICs, high-power tubes, signal sources, etc.) should be determined first, and then arranged. Other components try to avoid factors that may cause interference.
③On the other hand, the overall structure of the printed 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.
3. 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, more wiring is required on the soldering surface, and less wiring on the component surface 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 circuits during mechanical processing, the distance between the conductive patterns in the inner and outer layer wiring areas and the edge of the board should be greater than 50mil.
4. Wire direction and line width requirements
PCB multilayer board wiring should separate the power layer, ground layer and signal layer; 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 wire 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, it should also be noted that the width of the lines should be as consistent as possible to avoid sudden thickening and thinning of the wires, which is conducive to impedance matching.
5. PCB circuit board drilling size and pad requirements
The size of the component drilling on the @PCB multilayer board is related to the pin size of the selected components. 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 will not be full enough during soldering.
②Generally speaking, the calculation method of component hole aperture and pad size is: component hole aperture = component pin diameter (or diagonal) + (10~30mil) ③ The calculation method of via pad is: via hole Pad (VIAPAD) diameter ≥ via diameter + 12mil. 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 boards, it should generally be controlled within the range of board thickness: aperture s5:1.
6. Requirements for power supply layer, stratum division and flower hole
For PCB multilayer printed circuit boards, there are at least one power layer and one ground layer. Since all the voltages on the printed circuit 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 large-area metal heat absorption during the welding process to cause virtual welding, the general connection plate should be designed in the shape of a flower hole. Aperture diameter of isolation pad ≥ drilling aperture + 20mil
7. Requirements for safety distance
The setting of the safety distance shall 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.
8. Improve 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 Add a filter capacitor near the power supply and ground of each IC, and the capacity is generally 473 or 104.
② For the sensitive signals on the printed board, accompanying shielding lines should be added respectively, and the wiring near the signal source should be as little as possible.
③Choose a reasonable grounding point.
