PCB can be structurally categorized into single, double and multilayer boards. Among them, multilayer board refers to a printed board with more than two layers, consisting of a number of connecting wires on an insulated substrate and pads for assembling and soldering electronic components. It not only has the function of conducting the circuit of each layer, but also has the function of insulating each other. High-speed PCBs generally use multilayer board design. Common multilayer boards are generally 4-layer boards or 6-layer boards, and complex multilayer pcb fabrication can reach dozens of layers.

PCB multilayer board features

PCB multilayer board with a single panel, double-sided board is the biggest difference between the issue is that we have increased the internal control of the enterprise power supply layer (to maintain the internal power layer) and grounding layer, power and ground for the network is mainly in the work of the power supply layer wiring. However, the multilayer board wiring system is still mainly based on the country's top and bottom layer technology, supplemented by the intermediate wiring layer.

Multilayer printed circuit boards consist of a signal layer, an internal plane, a mechanical layer, a mask, a screen and a system layer.

Multilayer PCB has many advantages, such as high assembly density, small size; shorten the connection between electronic components, fast signal transmission speed, convenient wiring; good shielding effect and so on.

Design of PCB multilayer board

In the design of multilayer boards, each layer should be kept symmetrical, preferably an even number of copper layers, if not symmetrical, easy to deform. Multilayer board wiring is based on the function of the circuit, when wired outside, more wiring in the welding surface, less wiring components, which is conducive to the maintenance of the printed circuit board and troubleshooting. In the wiring process, in order to reduce the interference between power, ground and signal, it is necessary to separate the power, ground and signal layers. Two adjacent PCB layers should be wired perpendicular to each other or diagonal or curved, not parallel lines, to reduce coupling and interference between substrate layers.

01 Determination of plate shape, size and number of layers

The shape and size of the printed circuit board shall be based on the whole structure of the product. However,PCB factory from the perspective of enterprise production technology process research can be considered, should try to use simple, generally for the aspect ratio is not too disparate rectangular, in order to facilitate the assembly to improve the efficiency of social production and management, reduce the cost of labor time.

The number of layers must be based on the performance requirements of the circuit, the size of the board and circuit density. For multilayer printed circuit boards, four-layer boards, six-layer boards are the most widely used.

Each layer of a multilayer board should be symmetrical, preferably with a uniform copper layer, i.e., four, six, eight, etc. Due to the asymmetry of the lamination, the board surface is prone to warpage, especially for surface mount multilayer boards.

02 Location and direction of components

The location and placement direction of components should first consider the circuit principle and cater to the direction of the circuit. Whether the placement is reasonable will have a direct impact on the performance of the printed circuit board, especially high-frequency analog circuits, the location of the device and the placement of the device obviously have more stringent requirements.

Therefore, engineers in the beginning of the layout of the printed circuit board, to determine the overall development of the layout of the enterprise, the circuit should be working principle can be a systematic and detailed analysis, the first to determine the location of a special component (such as large-scale IC, high-power tubes, signal sources, etc.), and then arranged as well as other components, as far as possible, to avoid students may lead to produce an impact on the interference factors.

On the other hand, the overall structure of the PCB should be considered to avoid the elements are arranged sparsely uneven and chaotic. This not only affects the aesthetics of the printed circuit board, but also brings a lot of inconvenience to the assembly and maintenance.

03 Conductor layout and wiring area requirements

Generally speaking, multi-layer PCB wiring is based on the function of the circuit. In the outer layer wiring, more wiring is required on the soldering surface and less on the component surface, which is conducive to the maintenance and troubleshooting of the printed circuit board.

Thin and dense wires and signal lines that are easily interfered with are usually arranged in the inner layer. Large areas of copper foil should be evenly distributed in the inner and outer layers, which will help reduce board warpage and obtain a more uniform surface coating during plating.

In order to prevent the product shape processing injuries and printing of various wires and machining easily caused by interlayer short circuit, the inner and outer layers for wiring area of the conductive network graphic distance from the board edge should be greater than 50 mil.

04 wire to the requirements

Multilayer board wiring should be the power layer, ground layer, signal layer to separate, reduce the power, ground, signal interference between.

Adjacent two layers of PCB wiring should be perpendicular to each other or diagonal lines, curves, not parallel lines, in order to reduce the coupling and interference between substrate layers. Conductors should be as short as possible, especially for small-signal circuits, the shorter the conductor, the lower the resistance, the less interference.

05 Requirements for safety spacing

The safety spacing should be set to meet the requirements of electrical safety. In general, the minimum spacing of the outer conductor should not be less than 4 mils, and the minimum spacing of the inner conductor should not be less than 4 mils. Where wiring can be arranged, the spacing should be as large as possible to improve the yield and reduce the potential for failure of the finished board.

06 to improve the whole board anti-interference technology capability requirements

Multi-layer printed circuit board design, but also must pay attention to the whole board at the same time anti-interference technology capabilities, general management methods are:

In the IC power supply, ground plus filter capacitors, the capacity is generally 473 or 104.

For sensitive signals on the printed circuit board, should be added separately to the accompanying shielding line, the signal source near the wiring as little as possible.

Select a reasonable grounding point.