In the ordinary PCB design, we will encounter all kinds of safe spacing problems, such as the spacing between vias and pads, the spacing between traces, and so on, which we should consider.
In PCB design, we divide these spaces into two categories:
Electrical spacing
Non-electrical safety spacing
I. PCB design electrical safety spacing
1. spacing between 1.PCB design wires
This spacing needs to take into account the production capacity of PCB manufacturers. 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 the perspective of our production, of course, the bigger the better if conditions permit. General 10mil is quite common.
2. Pad aperture and pad width
According to the PCB manufacturer, if the pad diameter is drilled mechanically, the minimum diameter shall not be less than 0.2mm If it is drilled by laser, it is recommended that the minimum diameter shall not be less than 4mil. The aperture tolerance is slightly different according to different plates, which can generally be controlled within 0.05mm, and the minimum width of the pad should not be lower than 0.2mm.
3. Spacing between pads
According to the processing capacity of PCB manufacturers, it is recommended that the spacing between pads should not be less than 0.2mm
4. The distance between the copper skin and the edge of the plate
The distance between the charged copper skin and the PCB board edge should be no less than 0.3mm If the copper is laid in a large area, it is usually necessary to have an inward distance from the board edge, which is generally set to 20mil.
Generally, for the mechanical consideration of the finished circuit board, or to avoid curling or electrical short circuit caused by the exposed copper skin at the board edge, engineers often shrink the large-area copper block by 20mil relative to the board edge, instead of laying the copper skin all the way to the board edge. There are many ways to deal with this kind of copper skin shrinkage. For example, draw the keepout layer at the edge of the board, and then set the distance between the copper plating and keepout.
Second, PCB design non-electrical safety spacing
1. Width, height and spacing of characters
We generally use conventional values such as 5/306/36mil for the characters of PCB design screen printing. Because the text is too small, it will be blurred when printed.
2. Distance from screen printing to pad
Pad is not allowed in silk screen printing, because if pad is covered in silk screen printing, tin will not be applied to the silk screen when tin is applied, thus affecting the mounting of components.
Generally, the board factory requires a spacing of 8mil. If it’s because some PCB boards are really compact, it’s barely acceptable for us to achieve a spacing of 4mil. 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 to ensure the tin on the pad. So we need to pay attention.
3. 3D height and horizontal spacing on mechanical structure
When mounting PCB devices, it should be considered whether there will be any conflict with other mechanical structures in the horizontal direction and space height. Therefore, in the design, we should fully consider the adaptability of space structure between components, as well as between the finished PCB and the product shell, and reserve a safe distance for each target object.
