We will encounter various safety spacing issues in ordinary PCB design, such as the clearance between vias and pads, and the clearance between traces and traces, which are all things we should consider.
In PCB design, we divide these spacing into two categories:
I. PCB design-Electrical safety clearance
1. PCB design spacing between lines
This clearance needs to consider production capacity, and it is recommended that the clearance between the traces is not less than 4mil. The minimum conductor spacing is also the conductor-to-conductor and conductor-to-pad spacing. So, from the perspective of production, if conditions permit, the bigger, the better. Generally, the conventional 10mil is relatively common.
2. Via size and pad width
According to the PCB manufacturer, if the Via is mechanically drilled, the minimum diameter should not be less than 0.2mm. If using laser drilling, it is recommended that the minimum diameter not less than 4mil. The Via tolerance is slightly different depending on the board material, generally can be controlled within 0.05mm, and the minimum pad width must not be small than 0.2mm.
3. The spacing between the pad and the pad
According to the processing capability of the PCB manufacturer, it is recommended that the clearance between the pads is not less than 0.2mm.
4. The clearance between the copper and the edge of board
The clearance between the copper and the PCB board edge is preferably not less than 0.3mm. If it is a large area of copper plane, there is usually a pullback clearance between the copper and the board edge, generally set to 20mil.
Under normal circumstances, due to considerations of the finished circuit board machinery, or to avoid copper foil curling or electrical circuit-short that may be caused by the exposed copper plane on the edge of the board, engineers often set a 20mil pullback clearance between the large-area copper and the edge of the board, will not spread the copper to the edge of the board. There are several ways to deal with the pullback of the copper plane. For example, draw a keep-out layer on the edge of the board, and then set a distance between the large-area copper and the keep-out layer.
II. PCB design non-electrical safety clearance
1. Silk screen width and height and spacing
Regarding the PCB design silkscreen, we generally use conventional values such as 5/30mil or 6/36mil and so on. Because when the legend is too small, the finished PCB silkscreen will be blurred.
2. The distance from the silk screen to the pad
The silk screen is not allowed to printed on the pad, because if does, the silk screen on soldering PAD will affect the component mounting.
Generally, the board factory requires a space of 8mil to be reserved. If the area of some PCB boards is really tight, the 4mil pitch is barely acceptable. Then, if the silk screen accidentally covers the pad during design, the board factory will automatically eliminate the silk screen on the pad during DFM, to ensure that the pad can be wetted by solder during SMT process. Thus, we need to pay attention.
3. 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 the height of the shell. Therefore, when designing, it is necessary to fully consider the adaptability of the space structure between the components, and between the finished PCB and the product shell, and reserve a safety distance for each target object.
