First – Requirements for PCB spacing
1. Spacing between conductors: the minimum pattern spacing is also pattern to pattern, and the distance between lines and pads shall not be less than 4MIL. From the perspective of production, the bigger the better if conditions permit. Generally, 10 MIL is common.
2. Pad hole diameter and pad width: according to the situation of PCB manufacturer, if the pad hole diameter is mechanically drilled, the minimum shall not be less than 0.2mm; If laser drilling is used, the minimum shall not be less than 4mil. The hole diameter tolerance is slightly different according to different plates, and can be controlled within 0.05mm generally; The minimum pad width shall not be less than 0.2mm.
3. Spacing between pads: According to the processing capacity of PCB manufacturers, the spacing shall not be less than 0.2MM. 4. The distance between the copper sheet and the plate edge should be no less than 0.3mm. In case of large-area copper laying, there is usually an inward distance from the plate edge, which is generally set as 20mil.
– Non electrical safety distance
1. Width, height and spacing of characters: For characters printed on silk screen, conventional values such as 5/30 and 6/36 MIL are generally used. Because when the text is too small, the processing and printing will be blurred.
2. Distance from silk screen to pad: silk screen is not allowed to mount pad. Because if the solder pad is covered with the silk screen, the silk screen cannot be coated with tin, which affects the assembly of components. Generally, the PCB manufacturer requires to reserve a space of 8mil. If the area of some PCB boards is very close, the spacing of 4MIL is acceptable. If the silk screen accidentally covers the bonding pad during design, the PCB manufacturer will automatically eliminate the silk screen left on the bonding pad during manufacturing to ensure tin on the bonding pad.
3. 3D height and horizontal spacing on mechanical structure: When mounting components on PCB, consider whether the horizontal direction and space height will conflict with other mechanical structures. Therefore, during the design, it is necessary to fully consider the adaptability of the space structure between components, as well as between the finished PCB and the product shell, and reserve a safe space for each target object. The above are some spacing requirements for PCB design.
Requirements for via of high-density and high-speed multilayer PCB (HDI)
It is generally divided into three categories, namely blind hole, buried hole and through hole
Embedded hole: refers to the connection hole located in the inner layer of the printed circuit board, which will not extend to the surface of the printed circuit board.
Through hole: This hole passes through the whole circuit board and can be used for internal interconnection or as the installation and positioning hole of components.
Blind hole: It is located on the top and bottom surfaces of the printed circuit board, with a certain depth, and is used to connect the surface pattern and the inner pattern below.
With the increasingly high speed and miniaturization of high-end products, the continuous improvement of semiconductor integrated circuit integration and speed, the technical requirements for printed boards are higher. The wires on the PCB are thinner and narrower, the wiring density is higher and higher, and the holes on the PCB are smaller and smaller.
Using laser blind hole as the main micro through hole is one of the key technologies of HDI. The laser blind hole with small aperture and many holes is an effective way to achieve high wire density of HDI board. As there are many laser blind holes as contact points in HDI boards, the reliability of laser blind holes directly determines the reliability of products.
Shape of hole copper
Key indicators include: copper thickness of corner, copper thickness of hole wall, hole filling height (bottom copper thickness), diameter value, etc.
Stack-up design requirements
1. Each routing layer must have an adjacent reference layer (power supply or stratum);
2. The adjacent main power supply layer and stratum shall be kept at a minimum distance to provide large coupling capacitance
An example of the 4Layer is as follows
SIG-GND（PWR）-PWR （GND）-SIG; 2. GND-SIG（PWR）-SIG（PWR）-GND
The layer spacing will become very large, which is not only bad for impedance control, interlayer coupling and shielding; In particular, the large spacing between the power supply layers reduces the capacitance of the board, which is not conducive to filtering noise.