Basic Design Essentials of Multilayer Printed Board)

I. Overview
Next, we will focus on the electrical properties of printed boards. Based on the process requirements, we will talk about the basic essentials of Multilayer Board design from the perspective of the stability and reliability of printed boards.

Ii. Necessary work before Design of printed boards
1. carefully check the schematic diagram: the design of any printed board is inseparable from the schematic diagram. The accuracy of the schematic diagram is the precondition for whether the Printed Board is correct or not. Therefore, prior to the design of the printed board, the signal integrity of the schematic diagram must be carefully and repeatedly checked to ensure the correct connection between devices.
2. device Selection

III. Basic Requirements of Multilayer Printed Board Design
1. determine the shape, size, and layers of the Board
For multilayer printed boards, the four and six layers are the most widely used. Taking the four layers as an example, there are two Wire Layers (component surface and welding surface), one power layer and one formation. The layers of the multi-layer laminate should be symmetric, and it is best to have an even copper layer, that is, four, six, and eight layers. Due to asymmetric laminated, the board surface is prone to warping, especially for the surface-mounted multi-board, more attention should be paid.
2. component Position and placement direction
3. Requirements for Wire Layout and wiring Areas
Multi-wiring of welding surface and less wiring of component surface are conducive to the maintenance and troubleshooting of printed boards. Fine, dense wires and easily disturbed signal lines are usually arranged in the inner layer. A large area of copper foil should be uniformly distributed in the interior and outer layers, which will help reduce the warpage of the plate and make the Plating Surface more even. In order to prevent the appearance of industrial injury and printed wires and mechanical processing, resulting in short-circuit between layers, the distance between the conductive drawing of the outer and outer layers of the wiring area should be greater than 50mil.
4. Wire Direction and line width requirements
The power supply layer, ground layer, and signal layer must be separated to reduce interference between power supply, ground, and signals. The lines of two adjacent printed boards should be vertical or diagonal lines and curves as much as possible. Parallel lines cannot be used to reduce the layer coupling and interference of the substrate. The shorter the wire is, the smaller the resistance and the smaller the interference. The signal line on the same layer should avoid sharp turns when changing the direction. The width and width of the wire should be determined according to the current and impedance requirements of the circuit. The power input line should be larger, and the signal line may be smaller. For General Digital boards, the power input line width can be 50 ~ 80mil, signal line width can be 6 ~ 10mil. When wiring, you should also pay attention to the line width should be consistent as much as possible, to avoid sudden or sudden thickening of the wire, is conducive to the matching of impedance.
5. Drilling size and pad requirements
The drill size of the component on the multi-layer board is related to the Pin Size of the selected component. If the drill size is too small, the insertion and tin loading of the component will be affected. If the drill size is too large, the solder joints will be insufficient during welding. Generally, the calculation method of component hole aperture and pad size is as follows:
Component hole aperture = component Pin Diameter (or diagonal) + (10 ~ 30mil );
Component pad diameter ≥ component hole diameter + 18mil.
As for the pore size, it is mainly determined by the thickness of the finished sheet. for high-density multi-board, it should be controlled in
Thickness: the aperture is less than or equal to 5: 1.
The calculation method is as follows: via pad diameter ≥ diameter + 12mil
6. Requirements for power supply layer, Formation Partition and flower Holes
For multilayer printed boards, there is at least one power supply layer and one formation layer. Because all the voltage on the printed board is connected to the same power supply layer, the power supply layer must be partitioned and isolated. The size of the partition line is generally 20 ~ The line width of 80 mil is recommended. The voltage is too high and the partitioning line is thicker.
7. Security spacing requirements
Safety spacing should be set to meet electrical safety requirements. In general, the minimum spacing of the outer wire shall not be less than 4 mil, and the minimum spacing of the inner wire shall not be less than 4 mil. In the case that wiring can be arranged, the padding should be used as much as possible to improve the yield rate and reduce the hidden risks of failure.
8. Improve the anti-interference capability of the entire board
The multi-layer printed board design must also pay attention to the anti-interference capability of the whole board. The general methods include:
A. Add a filter capacitor near the power supply and location of each IC. The capacity is generally 473 or 104.
B. For the sensitive signals on the printed board, the companion line shielding wire should be added separately, and the signal source should be cabled as little as possible.
C. Select a reasonable grounding point.