PCB Technical - 8 Steps in PCB Multilayer Board Design

PCB multilayer board is a special kind of printed circuit board. Its existing "characteristics" are generally special. For example, PCB multilayer board will exist in the circuit board. This kind of PCB multilayer board can help the machine to conduct various kinds of different circuits. Moreover, it can also have the effect of insulation. It will not let electricity collide with each other, so it is completely safe. If you want to use a better performance PCB multilayer board, you must pay attention to preset. Next, I will explain how to preset PCB multilayer board.

PCB multilayer boardpreset:

Number of layers and dimensions

1. No matter what kind of printed circuit board, there is the problem of proper assembly with other structural parts. Therefore, the shape and size of printed circuit board must be based on the structure of the whole product. But from the perspective of production technology, we should try our best to be simple. The rectangle with the ratio of length to width is not very different, so as to facilitate assembly, increase the production rate and reduce the labor cost.
   
   

2. The number of layers must be determined according to the requirements of circuit performance, board size and circuit density. For multilayer printed circuit board, the four-layer board and six-layer board are the most widely used. Take the four-layer board as an example, there are two wire layers (component surface and welding surface), one power layer and one layer.

   
   

3. Each layer of multilayer board should be symmetrical, and the best is even copper layer, i.e. four, six and eight layers. As a result of the misbalanced lamination, the appearance of the board is prone to warpage, especially for the multilayer board with external surface mounting, which should be paid more attention to.
   

Position and installation direction of components

1. The position of components and the direction of installation and placement should be considered from the circuit principle to meet the trend of the circuit. Whether the placement of the swing is reasonable or not will directly affect the performance of the printed circuit board, especially for the high-frequency analog circuit, the position of the components and the placement requirements of the swing are more strict.

2. Reasonable placement of components, in a sense, has shown the success of the PCB preset. Therefore, at the beginning of the layout and voting group layout of the printing board, the circuit principle should be carefully analyzed. The location of special components (such as large-scale IC, high-power transistor, signal source, etc.) should be confirmed first, and then other components should be placed to prevent possible interference factors.

3. On the other hand, we should consider the problem from the group structure of PCB to prevent the uneven arrangement of components. This not only affects the good-looking PCB, but also brings many inconveniences to assembly and maintenance office.

Requirements of wire layer and wiring area

Under normal conditions, the wiring of multilayer printed circuit board is implemented according to the circuit function. When wiring in the outer layer, more wiring is required on the welding surface and less wiring on the component surface, which is helpful for the maintenance and troubleshooting of the printed circuit board. Thin and dense wires with mild attitude and disturbed signal lines are generally placed in the inner layer. The copper foil with large plane or surface size should be evenly distributed in the inner and outer layers, which will help to reduce the warpage of the plate and to obtain a more average coating on the surface during electroplating. In order to avoid damage to printed wire and short circuit between layers caused by machining, the distance between conductive figure in inner and outer wiring area and board edge should be greater than 50mil.

Requirements of conductor direction and line width

Multi layer board wiring should separate power layer, stratum and signal layer to reduce the interference between power supply, ground and signal. The lines of two adjacent layers of printed circuit boards shall be vertical or inclined to each other as far as possible, instead of two disjoint straight lines, so as to reduce the interlayer coupling and interference of the substrate. And the conductor should try to run short wire, especially for small signal circuit, the shorter the wire, the smaller the resistance and the smaller the interference. The signal lines on the same layer shall be prevented from turning at acute angle when changing direction. The width of the wire should be confirmed according to the current and impedance requirements of the circuit. The power input line should be larger, and the signal line can be relatively small. For ordinary digital board, the line width of power input line can be considered as appropriate and 50 ~ 80mil, and the width of signal line can be considered as appropriate and 6 ~ 10mil.

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

Allowable current: 0.8, 2.0, 2.5, 1.9;

Conductor resistance: 0.7, 0.41, 0.31, 0.25;

When wiring, we should also pay attention to the same width of lines as much as possible to prevent the wires from suddenly becoming thicker and thinner, which is helpful for impedance matching.

Requirements for drilling volume and pad

1. The drilling volume of the components on the multilayer board is related to the pin size of the selected components. If the drilling hole is too small, it will affect the assembly and tin filling of the components; if the drilling hole is too large, the solder joint is not full enough. Generally speaking, the calculation method of element hole diameter and pad volume is as follows:

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

3. Component pad diameter ≥ element hole diameter + 18mil

4. As for the via hole diameter, it is mainly decided by the thickness of the finished plate. For the multilayer board with high density, it should be controlled in the range of plate thickness: aperture ≤ 5:1. The calculation method of via pad is as follows:

5. Viapad diameter ≥ via diameter + 12mil.

Requirements of power layer, stratum division and trepanning

For Multilayer PCB, there should be at least one power layer and one layer. Because all the voltages on the printed circuit board are connected to the same power layer, the power layer must be partitioned and isolated. The volume of the partition line is generally considered to be appropriate, but the line width of 20-80mil is appropriate. If the voltage is too high, the thicker the partition line is.

In order to increase the reliability and reduce the heat absorption of the metal on the large plane or the surface of the object during the welding process, the common joint signboard should be preset as the pattern of the flower hole.
The hole diameter of the isolation pad is greater than or equal to the hole diameter + 20MIL
Requirements of safety distance

The setting of safety distance should meet the requirements of electrical safety. Generally speaking, the minimum distance between the outer conductor and the inner conductor should not be less than 4mil. When the wiring can be arranged, the spacing should be as large as possible to increase the yield and reduce the hidden trouble of the finished board.
Increase the requirement of anti-interference experience of whole board

For the preset of multi-layer printed circuit board, it is also important to pay attention to the anti-interference of the whole board
a. The capacity of the IC is 473 or 104 when the filter capacitor is added near the power supply and ground of each IC.
b. For the sensitive signals on the printed board, the accompanying shielding wire should be added, and the wiring near the signal source should be minimized.
c. Choose a reasonable grounding point.

Requirements of power layer, stratum division and trepanning
For Multilayer PCB, there should be at least one power layer and one layer. Because all the voltages on the printed circuit board are connected to the same power layer, the power layer must be partitioned and isolated. The volume of the partition line is generally considered to be appropriate, but the line width of 20-80mil is appropriate. If the voltage is too high, the thicker the partition line is.

In order to increase the reliability and reduce the heat absorption of the metal on the large plane or the surface of the object during the welding process, the common joint signboard should be preset as the pattern of the flower hole.

The hole diameter of the isolation pad is greater than or equal to the hole diameter + 20MILsafety distance
The setting of safety distance should meet the requirements of electrical safety. Generally speaking, the minimum distance between the outer conductor and the inner conductor should not be less than 4mil. When the wiring can be arranged, the spacing should be as large as possible to increase the yield and reduce the hidden trouble of the finished board.

Increase the requirement of anti-interference experience of whole board
For the preset of multilayer printed circuit board, it is also important to pay attention to the anti-interference of the whole board
a. The capacity of the IC is 473 or 104 when the filter capacitor is added near the power supply and ground of each IC.
b. For the sensitive signals on the printed board, the accompanying shielding wire should be added, and the wiring near the signal source should be minimized.
c. Choose a reasonable grounding point.

We must have understood the default method of PCB multilayer board, but we don't know what the parameters are. The minimum aperture of PCB multilayer board is usually 0.4mm, which is a necessary preset. When we preset PCB multilayer board, we must adjust its thickness and size to the range suitable for electrical appliance application. Too large is not good, and too small is also very bad. In the implementation of surface treatment, we must choose the form of gold plating, otherwise the special properties of insulation may disappear.