PCB News - Basic essentials of multi-layer PCB design

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

Any printed board has the problem of matching with other structural parts. Therefore, the shape and size of the printed 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.

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.

The layers of the multi-layer board should be symmetrical, and it is best to have an even number of copper layers, that is, four, six, eight, and so on. 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 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 the high-frequency analog circuit, which makes the location and placement requirements of the device 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 wiring layer and wiring area

In general, the wiring of the multilayer printed board is carried out according to the circuit function. 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 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. Wire direction and line width requirements

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.

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. Drill size and pad requirements

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

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

The diameter of the component pad ≥ the diameter of the component hole + 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 (VIA PAD) is greater than or equal to the diameter of the via + 12 mil.

6. Requirements for power layer, stratum partition 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 is greater than or equal to the aperture of the drilled hole + 20mil.

7. Safety clearance requirements

The setting of the safety distance 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 for improving 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 as follows:

a. Add filter capacitors near the power and ground of each IC, and 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 above is an introduction to the basic essentials of multi-layer PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.