PCB multilayer board is a special kind of printed circuit board, its “location” is generally special, for example, PCB multilayer board will exist in the circuit board.
This kind of multi-layer board can help the machine conduct all kinds of different lines. Not only that, it can also have the effect of insulation. It will not let electricity collide with each other, so it is absolutely safe.
If you want to use a better performance PCB multilayer board, you must design it carefully. Next, I will explain how to design PCB multilayer board.
PCB multilayer design
1. Determination of plate shape, size and number of layers
1) Any printed circuit board has the problem of matching with other structural parts. Therefore, the shape and size of printed circuit board must be based on the overall structure of the product. However, from the point of view of production technology, it should be as simple as possible. Generally, it is a rectangle with not too large length width 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 the circuit performance, the size of the board and the density of the circuit. For multi-layer printed circuit board, four layer board and six layer board are most widely used. Taking four layer board as an example, there are two conductor layers (component surface and welding surface), one power layer and one layer.
3) The layers of the multilayer board should be symmetrical, and it is better to have even number of copper layers, i.e. four, six, eight, etc. Because of asymmetric lamination, the board surface is prone to warpage, especially for surface mounted multi-layer boards.
2. Position and direction of components
1) The position and placement direction of components should be considered from the circuit principle to cater to the trend of the circuit. Whether the placement is reasonable or not will directly affect the performance of the printed circuit board, especially for the high-frequency analog circuit, the requirements for the location and placement of devices are more strict.
2) Reasonable placement of components, in a sense, has predicted the success of the PCB design. Therefore, when we start to make the layout of the board and decide the overall layout, we should make a detailed analysis of the circuit principle, first determine the location of special components (such as large-scale IC, high-power tube, signal source, etc.), and then arrange other components to avoid possible interference factors.
3) On the other hand, we should consider the overall structure of the printed circuit board to avoid the uneven arrangement of components. This not only affects the appearance of the printed circuit board, but also brings a lot of inconvenience to the assembly and maintenance work.
3. Requirements for wire layer and wiring area
In general, the wiring of multilayer printed circuit board is carried out according to the circuit function. When wiring in the outer layer, it requires more wiring on the welding surface and less wiring on the component surface, which is conducive to the maintenance and troubleshooting of printed circuit board. Thin and dense wires and easily disturbed signal wires are usually arranged in the inner layer.
The large area of copper foil should be evenly distributed in the inner and outer layers, which will help to reduce the warpage of the plate and obtain a more uniform coating on the surface during electroplating. In order to prevent the appearance processing from damaging the printed wires and causing inter layer short circuit during mechanical processing, the distance between the conductive pattern of the inner and outer layer wiring area and the board edge should be greater than 50 Mil.
4. Requirements for conductor direction and line width
Multilayer board wiring should separate the power layer, stratum and signal layer to reduce the interference among power, ground and signal. The lines of two adjacent layers of PCB should be perpendicular to each other or follow oblique lines and curves, 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 circuit, the shorter the wire is, the smaller the resistance is and the smaller the interference is. The signal line on the same layer should avoid sharp corner when changing direction. 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 can be smaller.
For general digital board, the width of power input line can be 50-80 mil, and the width of signal line 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;
Conductor resistance: 0.7, 0.41, 0.31, 0.25;
When wiring, the width of lines should be consistent as far as possible to avoid sudden thickening and thinning of wires, which is conducive to impedance matching.
5. Requirements of drilling size and pad
1) The size of the hole on the multilayer board is related to the pin size of the selected components. If the hole is too small, it will affect the device assembly and tinning; if the hole is too large, the solder joint is not full enough. Generally speaking, the calculation method of element hole diameter and pad size is as follows:
2) Aperture of element hole = diameter of element pin (or diagonal) + (10-30mil)
3) Pad diameter ≥ hole diameter + 18mil
4) As for the through-hole diameter, it is mainly determined by the thickness of the finished board. For high-density multilayer board, it should be controlled in the range of plate thickness: aperture ≤ 5:1. The calculation method of via pad is as follows:
5) Viapad diameter ≥ via diameter + 12mil.
6. Requirements for power layer, stratum division and flower hole
For Multilayer PCB, there is at least one power layer and one layer. Since all the voltages on the printed circuit 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 20-80mil. The thicker the partition line is, the higher the voltage is.
In order to increase the reliability of the joint between the welding hole and the power layer and stratum, and reduce the false welding caused by large area metal heat absorption in the welding process, the general connection plate should be designed into the shape of flower hole. The hole diameter of isolation pad is greater than or equal to drilling hole diameter + 20MIL
7. Requirements for safety distance
The setting of safety distance should meet the requirements of electrical safety. Generally speaking, the minimum spacing of outer conductor shall not be less than 4mil, and that of inner conductor shall not be less than 4mil. In the case that the wiring can be arranged properly, the spacing should be as large as possible, so as to improve the yield and reduce the hidden trouble of finished board failure.
8. Requirements for improving the anti-interference ability of the whole board
In the design of multilayer printed circuit board, the anti-interference ability of the whole board must also be paid attention to
a. Filter capacitors are added near the power supply and ground of each IC, and the capacity is generally 473 or 104.
b. For the sensitive signals on the printed circuit board, the accompanying shielding wires should be added respectively, and the wiring near the signal source should be minimized.
c. Choose a reasonable grounding point.
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