Electrical safety distance
1. Spacing between wires
According to the production capacity of PCB manufacturers, the distance between traces and traces should not be less than 4 mil. The minimum line spacing is also the line-to-line and line-to-pad spacing. Well, from our production point of view, of course, the bigger the better under the conditions. The general 10 mil is more common.
2. Pad aperture and pad width:
According to the PCB manufacturer, the minimum hole diameter of the pad is not less than 0.2 mm if it is mechanically drilled, and it is not less than 4 mil if it is laser drilled. The aperture tolerance is slightly different depending on the plate. Generally it can be controlled within 0.05 mm. The minimum width of the pad shall not be less than 0.2 mm.
3. The distance between the pad and the pad:
According to the processing capabilities of PCB manufacturers, the distance between pads and pads should not be less than 0.2 mm.
4. The distance between the copper skin and the edge of the board:
The distance between the charged copper skin and the edge of the PCB board is preferably not less than 0.3 mm. If copper is laid on a large area, it is usually necessary to have a shrinkage distance from the edge of the board, which is generally set to 20 mil. In general, due to mechanical considerations of the finished circuit board, or to avoid the possibility of curling or electrical short circuit caused by the exposed copper strip on the edge of the board, engineers often shrink large-area copper blocks by 20 mil relative to the edge of the board. The copper skin is not always spread to the edge of the board. There are many ways to deal with this copper shrinkage. For example, draw the keepout layer on the edge of the board, and then set the distance between the copper and keepout.
Non-electrical safety distance
1. Character width and height and spacing:
Regarding the characters of silk screen, we generally use conventional values such as 5/30 6/36 MIL, etc. Because when the text is too small, the processing and printing will be blurred.
2. The distance from silk screen to pad:
Screen printing does not allow pads. If the silk screen is covered with pads, the tin will not be tinned when soldering, which will affect the placement of components. General board manufacturers require 8 mil spacing to be reserved. If it is because the area of some PCB boards is very close, the spacing of 4MIL is barely acceptable. Then, if the silk screen accidentally covers the pad during design, the board manufacturer will automatically eliminate the silk screen portion left on the pad during manufacturing to ensure the tin on the pad. So we need to pay attention.
3. 3D height and horizontal spacing on the mechanical structure:
When mounting the devices on the PCB, it is necessary to consider whether the horizontal direction and the space height will conflict with other mechanical structures. Therefore, when designing, it is necessary to fully consider the adaptability of the spatial structure between the components, as well as between the PCB product and the product shell, and reserve a safe distance for each target object.