PCB Tech - Safety distance in PCB design

We will encounter a variety of safe spacing problems in the normal PCB design, such as the spacing between vias and pads, the spacing between wiring and wiring, etc., which should be considered. So today, we will divide these requirements into two categories: electrical safety spacing and non electrical safety spacing.

Safety distance in PCB design

A, Electrical safety distance:

1. Spacing between conductors:

According to the PCB production capacity of ipcb company, the distance between lines shall not be less than 2mil. Line spacing is also the distance between lines and pads. Well, from the point of view of our production, of course, the bigger the better under the conditions. General routine 4mil is more common.

2. Pad aperture and pad width:

According to the PCB production capacity of ipcb company, if mechanical drilling method is adopted, the pad aperture shall not be less than 0.15mm; if laser drilling method is adopted, it shall not be less than 3mil. The hole diameter tolerance is slightly different according to the different plates. Generally, it can be controlled within 0.05mm. The pad width shall not be less than 0.2mm.

3. Spacing between pads:

According to the PCB production capacity of ipcb company, the distance between pads shall not be less than 0.2mm.

4. Distance between copper sheet and plate edge:

The distance between the live copper sheet and the PCB edge shall not be less than 0.3mm. If the copper is laid in a large area, there should be an internal contraction distance from the board edge, which is generally set as 20MIL. In general, for the mechanical consideration of the finished circuit board, or to avoid the curling or electrical short circuit caused by the exposed copper sheet on the board edge, engineers often shrink the large-area copper block by 20MIL relative to the board edge, instead of laying the copper sheet to the board edge all the time. There are many ways to deal with the shrinkage of copper skin. For example, the keepout layer is drawn on the edge of the plate, and then the distance between copper laying and keepout is set.

B,Non electrical safety distance:

1. Width, height and spacing of characters:

As for the characters in silk screen printing, we usually use conventional values such as 5 / 30 6 / 36 mil. Because when the text is too small, the processing and printing will be blurred.

2. Distance from silk screen to pad:

The pad is not allowed for screen printing. Because if the pad is covered with silk screen printing, the screen printing will not be tin coated when tin is applied, which will affect the installation of components. Generally, 8 mil spacing is required to be reserved in the plate plant. If it is because the area of some PCB boards is very close, the spacing of 4mil is barely acceptable. Then, if the screen printing covers the pad carelessly in the design, the plate 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 installing the components on PCB, it is necessary to consider whether there will be conflicts with other mechanical structures in horizontal direction and space height. Therefore, in the design, it is necessary to fully consider the adaptability of space structure between components, PCB products and product shell, and reserve a safe distance for each target object.