We will encounter various safety spacing problems in ordinary life, such as the spacing between vias and pads, and the spacing between traces and traces, which are all things we should consider.
We divide these clearances into two categories: electrical safety clearances and non-electrical safety clearances
1. Electrical safety distance
a. The distance between the wires
This spacing needs to consider the production capacity. It is recommended that the spacing between traces should not be less than 4mil. The minimum line spacing is also the line-to-line and line-to-pad spacing. Then, from our production point of view, of course, the bigger the better under conditions. The general conventional 10mil is more common.
b. Pad aperture and pad width
According to the PCB manufacturer, the minimum diameter of the pad should not be less than 0.2mm if it is mechanically drilled, and it is recommended to be at least 4mil if it is laser drilled. The hole diameter tolerance is slightly different according to the different plates. Generally, it can be controlled within 0.05mm, and the minimum width of the pad should not be lower than 0.2mm.
c. Spacing between pads and pads
According to the processing capability of the PCB manufacturer, it is recommended that the spacing between the pads should not be less than 0.2mm.
d. The distance between the copper sheet and the edge of the board
The distance between the charged copper sheet and the edge of the PCB is preferably not less than 0.3mm. If it is a large area of copper, there is usually a shrinking distance from the edge of the board, which is generally set to 20mil.
Under normal circumstances, engineers often shrink large-area copper blocks by 20 mils relative to the edge of the board for mechanical considerations of the finished circuit board, or to avoid curling or electrical short-circuits that may be caused by the exposed copper skin on the edge of the board. Instead of laying copper all the way 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 copper and keepout.
2. Non-electrical safe distance
a. The width and height of the characters and spacing
For silk screen characters, 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.
b. The distance from the silk screen to the pad
Pads are not allowed on the silk screen, because if the silk screen covers the pads, the silk screen will not be able to be tinned during tinning, which will affect the mounting of components.
Generally, the board factory requires a spacing of 8mil to be reserved. If it is because the area of some PCB boards is really tight, we can barely accept the 4mil spacing. Then, if the silk screen accidentally covers the pad during the design, the board factory will automatically eliminate the silk screen part left on the pad during manufacture to ensure that the pad is tinned. So we need to pay attention.
c. 3D height and horizontal spacing on the mechanical structure
When mounting the components on the PCB, consider whether there will be conflicts with other mechanical structures in the horizontal direction and space height. Therefore, in the design, it is necessary to fully consider the adaptability of the spatial structure between the components, as well as between the finished PCB and the product shell, and reserve a safe distance for each target object.
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