PCB Technical - Teach you how to easily design a multi-layer PCB board

PCB multi-layer board is a special kind of printed board, its existence "place" is generally special, for example: PCB multi-layer board will exist in the circuit board. This kind of multilayer board can help the machine conduct various circuits. Not only that, but it also has the effect of insulation and will not allow electricity to collide with each other, which is very safe. Want to get a better performance PCB multi-layer board, you need to design carefully, and then I will explain how to design a PCB multi-layer board.

PCB multi-layer design:

1. Determination of board shape, size and number of layers

Any printed circuit board has the problem of cooperating with other structural parts. Therefore, the shape and size of the printed board must be based on the product structure. 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.

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

The layers of the multilayer board should be symmetrical, and preferably 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.

2. The location and orientation of the components

The location and placement direction of the components should first be considered from the circuit principle to 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. Obviously, the location and placement requirements of the device are more stringent.

Reasonable placement of components, in a sense, has heralded 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.

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 conducive to the maintenance and troubleshooting of the printed 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 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.

4. Requirements for wire orientation and line width

Multilayer 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 thickening and sudden thinning of the wire, which is conducive to impedance matching.

5. Drilling size and pad requirements

The hole 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 soldering of the device; if the hole is too large, the solder joints are not full enough during soldering. Generally speaking, the calculation method of PCB component hole diameter and pad size is:

a. Aperture of component hole = component pin diameter (or diagonal) + (10～30mil)

b. Component pad diameter ≥ component hole diameter + 18mil

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 ≤ 5:1.

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 a power layer and a ground layer. Since all voltages on the printed 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

7. Requirements for safety clearance

The setting of the safety distance should meet the requirements of electrical safety. Generally speaking, the minimum spacing of the outer conductors should not be less than 4mil, and the minimum spacing of the inner 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 during board manufacturing and reduce the hidden danger of failure of the finished board.

8. Requirement to improve the anti-interference ability of the whole board

In the design of multi-layer 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 design method of PCB multi-layer board must be known to everyone, but they don't know what the parameters of this kind of multi-layer board are. The smallest aperture of the PCB multilayer board is generally 0.4mm. When we design the PCB board, we must adjust its thickness and size to the range suitable for electrical appliances. Too big is not good, too small is not good. When performing surface treatment, be sure to choose the method of electroplating gold, otherwise the insulating properties may disappear.