We will encounter a variety of safety spacing issues in the usual PCB design, such as the spacing between the over-hole and the pad, the spacing between the alignment and the alignment are places that we should take into account.
PCB design we divide these spacing into two categories:
Electrical safety spacing and non-electrical safety spacing.
I. The electrical safety spacing
1. PCB design spacing between wires
This spacing needs to be considered for production capacity, it is recommended that the spacing between the alignment and the alignment is not less than 4 mil. minimum line spacing, but also line to line, line to pad spacing. Well, from our production point of view, then, of course, is in the case of the conditions of the larger the better. General conventional 10mil is more common.
2. Pad aperture and pad width
According to PCB manufacturers, pad aperture if the mechanical drilling method, the minimum shall not be less than 0.2mm, if the laser drilling method, it is recommended that the minimum is not less than 4mil. and aperture tolerance according to the different boards slightly different, generally can be controlled within 0.05mm, the pad width of the minimum shall not be less than 0.2mm.
3. Spacing between pads and pads
According to the PCB manufacturer's processing capabilities, it is recommended that the spacing between pads and pads is not less than 0.2mm.
4. Spacing between the copper skin and the edge of the board
The spacing between the charged copper skin and the edge of the PCB board is best not less than 0.3mm, if a large area of copper, usually also with the board edge needs to have the shrinkage distance, generally set to 20mil.
In general, for the circuit board finished mechanical considerations, or to avoid the copper skin exposed in the board edge may cause the crimping or electrical shorts and other occurrences, engineers often lay a large area of copper block relative to the board edge shrinkage of 20mil, rather than has been the copper skin to the board edge. There are many ways to deal with this copper skin shrinkage. For example, the edge of the board to draw keepout layer, and then set the distance between the copper paving and keepout.
II. PCB design non-electrical safety spacing
1. Width and height of the characters and spacing
PCB design silkscreen characters we generally use conventional values such as: 5/306/36mil. Because when the text is too small, the processing of printed out will be blurred.
2. Screen printing to the pad distance
Silk screen is not allowed on the pad, because the silk screen if the cover on the pad, in the tin on the silk screen will not be able to tin, thus affecting the components mounted.
General board manufacturers require 8 mil spacing. If it is because some PCB board area is really very tight, we do 4mil spacing is barely acceptable. Then, if the silkscreen in the design accidentally over the pad, the board factory in the manufacture will automatically eliminate the silkscreen left on the pad part to ensure that the pad on the tin. So we need to pay attention to it.
3. Mechanical structure of the 3D height and horizontal spacing
PCB devices in the mounting to take into account the horizontal direction and space height will not conflict with other mechanical structures. Therefore, in the design, we should fully consider the components, as well as PCB finished products and products between the shell, the spatial structure of the adaptability of the target object to reserve a safe distance.
