What are the requirements for PCB design spacing?

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

In PCB design, we divide these spacings into two categories:

Electrical safety spacing and non-electrical safety spacing

I. Electrical safety spacing

(1)Spacing between PCB design wires

This spacing needs to consider the production capacity. It is recommended that the spacing between traces should not be less than 4mil. The minimum line spacing is also the spacing between line to line and line to pad. So, from our production perspective, of course, the larger 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 shall not be less than 0.2mm, and if it is laser drilled, the minimum is recommended to be no less than 4mil. The aperture tolerance varies slightly depending on the board material, and can generally be controlled within 0.05mm, and the pad width shall not be less 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)Spacing between copper foil and board edge

The spacing between live copper foil and PCB board edge should not be less than 0.3mm. If it is a large area of copper, it is usually necessary to have an indentation distance from the board edge, which is generally set to 20mil.

In general, for mechanical considerations of the finished circuit board, or to avoid curling or electrical short circuits caused by copper foil exposed on the board edge, engineers often indent large area copper blocks by 20mil relative to the board edge, rather than spreading copper foil all the way to the board edge. There are many ways to deal with this copper foil indentation. For example, draw a keep out layer on the board edge, and then set the distance between copper foil and keep out.

2.Non-electrical safety spacing of PCB design

(1)Character width, height and spacing

Regarding the characters of PCB design silk screen, we generally use conventional values such as: 5/306/36mil, etc. Because when the text is too small, the processed printing will be blurred.

(2)The distance from the silk screen to the pad

The silk screen is not allowed to be on the pad, because if the silk screen covers the pad, the silk screen will not be tinned when tinning, which will affect the installation of components.

Generally, the board factory requires a spacing of 8mil. If the area of some PCB boards is really tight, we can barely accept a spacing of 4mil. Then, if the silk screen accidentally covers the pad during design, the board factory will automatically eliminate the silk screen part left on the pad during manufacturing to ensure the tinning of the pad. So we need to pay attention to it.

(3)3D height and horizontal spacing on the mechanical structure

When mounting the device on the PCB, it is necessary to consider whether there will be conflicts with other mechanical structures in the horizontal direction and spatial height. Therefore, when designing, it is necessary to fully consider the adaptability of the spatial structure between components, and between the PCB finished product and the product shell, and reserve a safe spacing for each target object.