PCB multilayer board is a special printed circuit board, and its "features" are generally special. For example, there will be PCB multilayer boards in six-layer circuit boards. This kind of multi-layer board can help the machine to conduct all kinds of different circuits, not only that, but also has the effect of insulation, which will not let electricity and electricity collide with each other, which is completely safe. If you want to use a PCB multi-layer board with better performance, you must concentrate on the preset. Next, I will explain how to preset the PCB multi-layer board.
1. Confirm the shape, size and number of layers of the board
a. No matter what kind of printed board, there is a problem of proper assembly with other structural parts. Therefore, the shape and size of the printed board must be based on the overall structure of the product. However, when thinking about the problem from the perspective of production technology, it should be as simple as possible, usually a rectangle with a length-to-width ratio that is not too far apart, so as to facilitate the assembly to increase the production rate and reduce labor costs.
b. In terms of the number of layers, it must be determined according to the requirements of circuit performance, the size of the board and the density of the circuit. For multi-layer printed boards, four-layer boards and six-layer boards are the most widely used. Taking four-layer boards as an example, there are two conductor layers (component surface and welding surface), a power supply layer and a ground layer.
c. The layers of the multi-layer board should be kept symmetrical, and preferably double-numbered copper layers, that is, four, six, eight layers, etc. Due to the wrong lamination, the appearance of the board is prone to warping, especially for surface-mounted multilayer boards, which should be paid attention to.
2. The position and placement direction of the components
a. The position and placement direction of the components should first be considered from the perspective of the circuit principle and cater to the trend 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 has stricter requirements on the location and placement of components.
b. Reasonable placement of components, in a sense, has shown the success of the PCB preset. Therefore, when starting to arrange the layout of the printed board and the layout of the voting group, a thorough analysis of the circuit principle should be carried out, and the location of special components (such as large-scale ICs, high-power transistors, signal sources, etc.) should be confirmed first, and then Then place other components and try to prevent factors that may cause interference.
c. On the other hand, the problem should be considered from the group structure of the printed board to prevent the arrangement of components from being uneven and upside down. This not only affects the appearance of the printed board, but also brings a lot of inconvenience to the assembly and maintenance office.
3. Requirements for wire layout and wiring area
Under normal working conditions, the multi-layer printed board wiring is implemented according to the circuit function. When wiring the outer layer, it is required to have more wiring on the welding surface and less wiring on the component surface, which is helpful for the maintenance and troubleshooting of the printed board. Thin and dense conductors are mild and disturbed signal lines are generally placed on the inner layer. The copper foil of the size of the large plane or the surface of the object should be distributed evenly in the inner and outer layers, which will help to detract from the warpage of the board and achieve a more even coating on the surface during electroplating. In order to avoid damage to the printed wires and short circuits between layers during machining, the distance between the conductive patterns in the inner and outer layer wiring areas from the edge of the board should be greater than 50 mils.
4. Requirements for wire direction and line width
The multi-layer board traces should separate the power layer, the ground layer and the signal layer to reduce the interference between the power, ground and signal. The lines of two adjacent layers of printed boards should try their best to be perpendicular to each other or follow oblique lines and curves, and two lines that do not intersect should not be taken to reduce the interlayer coupling and interference of the substrate. And the wires should be as short as possible, especially for small signal circuits, the shorter the wire, the smaller the resistance and the less interference. Signal lines on the same layer should be prevented from turning sharply when changing directions. The width of the wire should be confirmed according to the current and impedance requirements of the circuit. The power input wire should be larger, and the signal wire should be relatively smaller. For ordinary digital boards, the power input line width can be considered appropriate and use 50-80 mil, and the signal line width can be considered appropriate and use 6-10 mil.
Wire width: 0.5, 1, 0, 1.5, 2.0;
Permitted current: 0.8, 2.0, 2.5, 1.9;
Wire resistance: 0.7, 0.41, 0.31, 0.25;
When wiring, it should also be noted that the width of the lines should be as exactly the same as possible, to prevent the wires from suddenly becoming thicker and thinner, and helping to match the impedance.
5. Requirements for drilling volume and pad
a. The drilling volume of the components on the multi-layer board is related to the size of the selected component pins. If the drilling is too small, it will affect the assembly and tinning of the components; if the drilling is too large, the solder joints are not full enough during welding. . Generally speaking, the calculation method of component hole diameter and pad volume is:
b. Diameter of component hole = component pin diameter (or diagonal) + (10 ~ 30mil)
c. Component pad diameter ≥ component hole diameter + 18mil
d. As for the via hole diameter, it is mainly determined by the thickness of the finished board. For multi-layer boards with high density and density, it should generally be controlled within the range of board thickness: diameter ≤ 5:1. The calculation method of the via pad is:
e. Via pad (VIAPAD) diameter ≥ via diameter + 12mil.
6. Requirements for power supply layer, stratum partition and flower hole
For multi-layer printed boards, there is a minimum power plane and a ground plane. Because all the voltages on the printed board are connected to the same power supply layer, it is necessary to implement partition isolation on the power supply layer. The volume of the partition line is generally considered appropriate, and the line width of 20-80 mil is appropriate. The voltage is super high, and the partition line thicker.
The welding hole is connected with the power supply layer and the stratum. In order to increase its reliability and reduce the heat absorption of the large and small metals on the large plane or the surface of the object during the welding process, the common connection plate should be preset as a pattern of holes. The hole diameter of the isolation pad is greater than or equal to the hole diameter of the drill hole + 20mil.
7. Requirements for safe distance
The setting of the safety distance shall meet the requirements of electrical safety. Generally speaking, the minimum spacing of outer conductors cannot be less than 4 mils, and the minimum spacing of inner conductors cannot be less than 4 mils. In the case where the wiring can be arranged, the spacing should be as large as possible to increase the yield of the board and reduce the hidden danger of failure of the finished board.
8. Experienced requirements for increasing the whole board anti-interference
For the preset of multi-layer printed boards, it is also necessary to pay attention to the anti-interference of the whole board. The common methods are:
a. Add filter capacitors near the power supply and ground of each IC, and the volume is generally 473 or 104.
b. For the sensitive signal on the printed board, the accompanying shielded wire should be added, and the wiring should be minimized near the signal source.
c. Select a reasonable grounding point.
You must have understood the preset method of PCB multilayer boards, but you don't know what the parameters of this multilayer board are. The minimum aperture of the PCB multilayer board is generally 0.4mm, which is a must preset. When we preset the PCB multilayer board, we must adjust its thickness and size to the range suitable for the use of electrical appliances. Too big is very bad, too small is very bad. When performing surface treatment, the form of electroplating must be selected, otherwise the special properties of insulation may be lost.
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