How to design PCB multilayer board?

PCB multilayer board refers to a printed circuit board with a double-sided inner layer, two single-sided outer layers or two double-sided inner layers and two single-sided outer layers, which are alternately connected together by a positioning system and insulating adhesive materials, and the conductive patterns are connected with each other according to the design requirements, also known as multilayer printed circuit board. A PCB multilayer board with better performance can not be separated from careful design.

Design of PCB multilayer board

1. Determine the shape, size and number of layers of the board.

1) Any printed board has assembly problems with other structural parts. Therefore, the shape and size of PCB 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 rectangular with small length and width, so as 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, circuit board size and line density. For multilayer printed boards, four-layer boards and six-layer boards are the most widely used. Take a four-layer board as an example, there are two conductor layers (component surface and welding surface), a power layer and a grounding layer.

3) Each layer of the multilayer board should be symmetrical, preferably with uniform copper layers, that is, four, six and eight layers. Due to the asymmetry of lamination, the board surface is prone to warping, especially for surface-mounted multilayer boards, which should be paid more attention to.

2. Location and placement direction of components

1) the position and placement direction of components should first consider the circuit principle and cater to the trend of the circuit. Reasonable placement will directly affect the performance of printed circuit boards, especially high-frequency analog circuits, which have stricter requirements on the location and placement of devices.

2) Reasonable component layout has predicted the success of PCB design in a sense. Therefore, when arranging the PCB layout and determining the overall layout, it is necessary to analyze the circuit principle in detail, and first determine the position of special components (such as large-scale integrated circuits, high-power tubes, signal sources, etc.). ), and then arrange other components to avoid possible interference factors.

3) On 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 beauty of printed boards, but also brings a lot of inconvenience to assembly and maintenance.

3. Requirements for conductor cloth layer and wiring area

Generally speaking, the wiring of multilayer printed boards is carried out according to the circuit function. When wiring the outer layer, it is required to wire more on the welding surface and less on the component surface, which is beneficial to the maintenance and troubleshooting of printed boards. Thin and dense lines and easily disturbed signal lines are usually arranged in the inner layer.

A large area of copper foil should be evenly distributed in the inner and outer layers, which will help to reduce the warping of the board and obtain a more uniform surface coating during electroplating. In order to prevent damage to the printed circuit in shape processing and interlayer short circuit caused by mechanical processing, the distance between the conductive patterns in the inner and outer wiring areas and the edge of the board should be greater than 50 mils.

4. Requirements for conductor direction and line width

Multi-layer board wiring should separate the power layer, stratum and signal layer to reduce the interference between power, ground and signal. The lines of two adjacent layers of printed boards should be perpendicular to each other or take diagonal lines and curves as far as possible, instead of parallel lines, so as to reduce the coupling and interference between the substrates of each layer.

Moreover, the wires should be as short as possible, especially for small signal circuits. The shorter the wires, the smaller the resistance and the smaller the interference. When changing the direction of signal lines on the same floor, sharp corners should be avoided. The width of the conductor 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 a general digital board, the line width of the power input line can be 50 ~ 80 mil, and the line width of the signal line can be 6 ~ 10 mil.

Conductor width: 0.5, 1, 0, 1.5, 2.0.

Allowable current: 0.8, 2.0, 2.5, 1.9.

Conductor resistance: 0.7, 0.4 1, 0.3 1, 0.25;

When wiring, we should also pay attention to the uniform width of the wire to avoid the sudden thickening and thinning of the wire, which is conducive to impedance matching.

5. Requirements of drilling size and pad

1) The drilling size of the component on the multilayer board is related to the pin size of the selected component. Too small drilling size will affect the assembly and welding of components; The drilling hole is too large, and the solder joint is not full enough when welding.

2) The aperture of the component hole = the diameter (or diagonal) of the component pin +( 10 ~ 30 mil)

3) Element pad diameter ≥ element hole diameter+18mil.

4) As for the diameter of the via hole, it is mainly determined by the thickness of the finished board. For high-density multi-layer board, it should be generally controlled within the thickness range: aperture ≤ 5: 1.

5) via pad diameter ≥ via diameter+12mil.

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

For multilayer printed circuit boards, there is at least one power layer and one ground layer. Because all the voltages on the printed board are connected to the same power plane, the power plane must be partitioned and isolated. Generally, the size of the dividing line is 20 ~ 80 mils, and the higher the voltage, the thicker the dividing line.

In order to improve the reliability of the connection between the welding hole and the dynamic layer and stratum, and reduce the virtual welding caused by the heat absorption of a large area of metal in the welding process, the general connecting plate should be designed in the shape of a flower hole. Isolation pad aperture ≥ drilling aperture +20 mils.

7. Requirements for safe spacing

The setting of safety spacing should meet the requirements of electrical safety. Generally speaking, the minimum spacing of outer conductors should not be less than 4 mils, and the minimum spacing of inner conductors should not be less than 4 mils. When the wiring can be arranged, the spacing should be as large as possible to improve the yield and reduce the failure hidden danger of the finished board.

8. Requirements for improving the anti-interference ability of the whole board

In the design of multilayer printed board, we must also pay attention to the anti-interference ability of the whole board. The general method is as follows:

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

B. For sensitive signals on the printed board, paired shielding wires should be added respectively, and wiring near the signal source should be minimized.

C. choose a reasonable grounding point.

Do you understand many of the above PCB multilayer design skills? Nowadays, the design of PCB multilayer board is developing in the direction of high performance, high speed, high density and thinness. High-speed signal PCB circuit board design has become the focus and difficulty of electronic hardware development, paying more attention to efficiency and rigor. G34