PCB Tech - PCB multilayer circuit board design suggestions

PCB multilayer circuit board design suggestions

The PCB multilayer circuit board is a special kind of printed circuit board, and its existence "location" is generally special. For example, there will be a PCB multilayer board in the circuit board. This kind of multi-layer board can help the machine to conduct various circuits, not only that, but also has an insulating effect, will not let electricity and electricity collide with each other, 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 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 multilayer printed boards, four-layer boards and six-layer boards are the most widely used. Taking the four-layer board as an example, there are two conductor layers (component surface and soldering surface), a power layer and a ground layer.

2. The layers of the PCB multi-layer circuit board should be symmetrical, and it is best to have an even-numbered copper layer, that is, a four-layer circuit board, a six-layer PCB, an eight-layer circuit board, etc. Because of the asymmetrical lamination, the surface of the PCB circuit board is prone to warpage, especially for surface-mounted PCB multilayer circuit boards, which should be paid more attention.

3. Any printed circuit board has the problem of cooperating with other structural parts. Therefore, the shape and size of the printed circuit board 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 location and orientation of the components

1. On the other hand, it should be considered from the overall structure of the printed circuit 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.

2. The location and placement direction of the 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.

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

3. Requirements for wire layout and wiring area

In general, the wiring of multi-layer printed circuit boards is carried out according to circuit functions. In the outer layer wiring, more wiring is required 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 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 interlayer short circuits during mechanical processing, the distance between the conductive patterns in 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

Multilayer circuit 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; also pay attention to the line width when wiring to avoid sudden wire Thickening and sudden thinning are good for impedance matching.

Five, the size of the hole and the requirements of the pad

1. The drilling size of the components on the PCB multi-layer circuit board is related to the selected component pin size. If the drilling is too small, it will affect the assembly and tinning of the device; the drilling is too large, and the solder joints are not enough during soldering. full. Generally speaking, the calculation method of component hole diameter and pad size is:

2. Aperture of component hole = component pin diameter (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: the diameter of the via pad (VIAPAD) ≥ the diameter of the via + 12mil.

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

For multilayer printed boards, there is at least one power layer and one ground layer. 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 ≥ drilling aperture + 20mil

Seven, the requirements of the safety gap The setting of the safety gap 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:

1. Choose a reasonable grounding point.

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

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