Electronic Design - Steps for successful PCB multi-layer board design

The PCB multilayer board is a special kind of printed board, and its existence "place" is generally special. For example, there will be a PCB multilayer board in the circuit board. This kind of multilayer board can help the machine to conduct. Various lines. Not only that, it can also have an insulating effect, and it will not allow electricity to collide with each other, which is absolutely safe. If you want to use a PCB multilayer board with better performance, you must design it carefully. Next, I will explain how to design a PCB multilayer board.

PCB multi-layer board design:

1. Board shape, size, and number of layers: 1. Any printed board has the problem of cooperating with other structural parts. Therefore, the shape and printed board size must be based on the structure of the product. However, from the perspective of the production process, it should be as simple as possible, generally a rectangle with a not too wide aspect ratio to facilitate assembly, improve production efficiency, and reduce labor costs.

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

3. The layers of the multilayer board should be symmetrical, and it is best to have an even number of copper layers, that is, four, six, eight, etc. Because of the asymmetrical lamination, the board surface is prone to warping, especially for surface-mounted multilayer boards, which should be paid more attention.

Second, the location and placement direction of components 1. The location and placement direction of components should first be considered from the 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 the high-frequency analog circuit, which makes the location and placement requirements of the device more stringent.

2. Reasonable placement of components, in a sense, has predicted the success of the printed board design. Therefore, when starting to lay out the layout of the printed circuit 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 Arrange other components and try to avoid factors that may cause interference.

3. On the other hand, it should be considered from the overall structure of the printed board to avoid uneven and disordered arrangement of components. This not only affects the beauty of the printed board, but also brings a lot of inconvenience to assembly and maintenance work.

3. Requirements for wire layout and wiring area. Under normal circumstances, multilayer printed board wiring is carried out according to circuit functions. 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 beneficial to the printed board. Repair and troubleshoot. Thin, dense wires and signal wires that are susceptible to interference are usually arranged in the inner layer. A large area of copper foil should be more evenly distributed in the inner and outer layers, which will help reduce the warpage of the board and also make the surface more uniform during electroplating. In order to prevent the shape processing from damaging the printed wires and causing inter-layer short circuits during mechanical processing, the distance between the conductive pattern of the inner and outer layer wiring areas should be greater than 50 mils from the edge of the board.

Fourth, the wire direction and line width requirements multi-layer board wiring should separate the power layer, ground layer and signal layer to reduce interference between power, ground, and signals. The lines of the two adjacent layers of printed boards should be as perpendicular to each other as possible, or follow diagonal lines or curves, and not parallel lines, so as to reduce the coupling and interference between the substrate layers. 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, avoid sharp corners 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 can be relatively small. For general digital boards, the power input line width can be 50 to 80 mils, and the signal line width can be 6 to 10 mils.

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 line width to be as consistent as possible to avoid sudden wires Thickening and sudden thinning are good for impedance matching.

V. Drilling size and requirements for pads 1. The drilling size of the component on the multilayer board is related to the size of the selected component pin. If the hole is too small, it will affect the assembly and tinning of the device; If it is too large, the solder joints are not full enough during welding. Generally speaking, the calculation method of component hole diameter and pad size is:

2. The aperture of the component hole = the diameter of the component pin (or diagonal) + (10 ~ 30mil) 3. The diameter of the component pad ≥ the diameter of the component hole + 18mil 4. As for the via hole diameter, it is mainly determined by the thickness of the finished board. High-density multilayer boards should generally be controlled within the range of board thickness: aperture ≤ 5:1. The calculation method of the via pad is:

5. Via hole pad (VIAPAD) diameter ≥ via hole diameter + 12mil.

6. Requirements for power layer, stratum partition and flower holes For multilayer printed boards, there is at least one power layer and one stratum. Since all 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 20-80 mil line width. The voltage is super high, and the partition line is thicker.

In order to increase the reliability of the connection between the welding hole and the power layer and the ground layer, in order to reduce the large-area metal heat absorption during the welding process, the joint plate should be designed into a flower hole shape.

The aperture of the isolation pad is greater than or equal to the drilling aperture + 20mil. Seven, the requirements of the safety spacing The setting of the safety spacing should meet the requirements of electrical safety. Generally speaking, the minimum spacing of the outer conductors shall not be less than 4mil, and the minimum spacing of the inner conductors shall 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 during board manufacturing and reduce the hidden danger of failure of the finished board.

8. Requirements to improve the anti-interference ability of the entire board. In the design of multilayer printed boards, attention must also be paid to the anti-interference ability of the entire board. The general methods are:

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

b. For sensitive signals on the printed board, the accompanying shielding wires should be added separately, and there should be as little wiring as possible near the signal source.

c. Choose a reasonable grounding point.

The PCB design method of themultilayer board must be known to everyone, but they don't know what the parameters of this multilayer board are. The smallest aperture of the PCB multilayer board is generally 0.4mm. This is a necessary design. When we design the PCB multilayer board, we must adjust its thickness and size to the range suitable for electrical appliances. It is too large. Not good, too small is not good either. When performing surface treatment, be sure to choose the method of electroplating gold, otherwise the insulating properties may disappear.