Safety distance issues encountered in PCB design

We encounter various safety spacing issues in normal PCB design, such as the spacing between vias and pads, the spacing between traces, etc., which we should consider.

We divide these spacings into two categories:

Electrical safety distance

Non-electrical safety distances

1. Electrical safety distance

1. Spacing between wires

This spacing needs to consider the production capacity of the PCB manufacturer. 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. So, from our production perspective, of course, the bigger the better if conditions permit. Generally, the conventional 10mil is more common.

2. Pad aperture and pad width

According to PCB manufacturers, if the pad aperture is mechanically drilled, the minimum diameter should not be less than 0.2mm. If laser drilling is used, it is recommended that the minimum diameter be not less than 4mil. The aperture tolerance varies slightly depending on the board material. It can generally be controlled within 0.05mm, and the minimum pad width must not be lower than 0.2mm.

3. The spacing between pads

According to the processing capabilities of the PCB manufacturer, it is recommended that the distance between pads should not be less than 0.2mm.

4. The distance between the copper sheet and the edge of the board

The distance between the live copper sheet and the edge of the PCB is preferably not less than 0.3mm. If copper is laid on a large area, there is usually a shrinkage distance from the edge of the board, which is generally set to 20mil.

Under normal circumstances, for mechanical considerations of the finished circuit board, or to avoid curling or electrical short circuits that may be caused by exposed copper on the edge of the board, engineers often shrink large-area copper blocks by 20 mil relative to the edge of the board. Do not spread the copper all the way to the edge of the board. There are many ways to deal with this copper shrinkage. For example, draw a keepout layer on the edge of the board, and then set the distance between copper laying and keepout.

2. Non-electrical safety distances

1. Character width, height and spacing

Regarding silk screen characters, we generally use conventional values such as: 5/30 6/36 mil, etc. Because when the text is too small, it will be blurry when printed.

2. The distance from silk screen to solder pad

Silk screen printing is not allowed to cover the soldering pads, because if the silk screen covers the soldering pads, the silk screen printing area will not be tinned during tin application, thus affecting the mounting of components.

Generally, board manufacturers require a spacing of 8mil. If the area of some PCB boards is really close, the spacing of 4mil is barely acceptable. Then, if the silk screen accidentally covers the pad during design, the board manufacturer will automatically eliminate the part of the silk screen left on the pad during manufacturing to ensure that the pad is tinned. So we need to pay attention to it.

3. D height and horizontal spacing on the mechanical structure

When mounting components on the PCB, consider whether they will conflict with other mechanical structures in the horizontal direction and height. Therefore, when designing, it is necessary to fully consider the adaptability of the spatial structure between 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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