How to design the safety spacing of PCB?

There are many places in PCB design that need to consider the safety spacing. Here, they are temporarily classified into two categories: one is electrical-related safety spacing, and the other is non-electrical-related safety spacing.

Electrical-related safety spacing

1.Spacing between wires

In terms of the processing capabilities of mainstream PCB manufacturers, the minimum spacing between wires must not be less than 4mil. The minimum line spacing is also the distance from line to line and from line to pad. From a production perspective, the larger the better if conditions permit, and 10mil is more common.

2.Pad aperture and pad width

In terms of the processing capabilities of mainstream PCB manufacturers, the minimum pad aperture must not be less than 0.2mm if mechanical drilling is used, and the minimum must not be less than 4mil if laser drilling is used. The aperture tolerance varies slightly depending on the board material, but can generally be controlled within 0.05mm, and the minimum pad width must not be less than 0.2mm.

3.Pad spacing

As far as the processing capabilities of mainstream PCB manufacturers are concerned, the pad spacing must not be less than 0.2mm.

4.Copper spacing from the board edge

The spacing between the live copper and the PCB board edge should not be less than 0.3mm. Set this spacing rule on the Design-Rules-Board outline page.

If a large area of ​​copper is laid, it usually needs to have an indentation distance from the board edge, which is generally set to 20mil. In the PCB design and manufacturing industry, generally speaking, for mechanical considerations of the finished circuit board, or to avoid curling or electrical short circuits caused by copper exposed on the board edge, engineers often indent large-area copper blocks by 20mil relative to the board edge, rather than laying the copper all the way to the board edge.

There are many ways to deal with this copper shrinkage, such as drawing a keepout layer on the edge of the board, and then setting the distance between the copper and the keepout. Here is a simple method, which is to set different safety distances for the copper objects. For example, the safety distance of the whole board is set to 10mil, and the copper is set to 20mil, which can achieve the effect of shrinking the board edge by 20mil, and also remove the dead copper that may appear in the device.

Non-electrical related safety spacing

1.Character width, height

And spacing text film cannot be changed during processing, but the character line width of D-CODE less than 0.22mm (8.66mil) is thickened to 0.22mm, that is, the character line width L=0.22mm (8.66mil).

The width of the entire character W=1.0mm, the height of the entire character H=1.2mm, and the spacing between characters D=0.2mm. When the text is smaller than the above standards, it will be blurred when processed and printed.

2.Via-to-via spacing

The via (VIA) to via spacing (hole edge to hole edge) should preferably be greater than 8mil.

3.Distance from silk screen to pad

The silk screen is not allowed to cover the pad. Because if the silk screen covers the pad, the silk screen will not be able to be tinned during tinning, which will affect the mounting of components. Generally, the board factory requires a spacing of 8mil. If the PCB board is really limited in area, a spacing of 4mil is barely acceptable. 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.

Of course, the specific situation is analyzed in the design. Sometimes the silk screen is deliberately made close to the pad, because when the two pads are very close, the silk screen in the middle can effectively prevent the solder connection from short-circuiting during welding. This situation is another matter.

4.Mechanical 3D height and horizontal spacing

When mounting components 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, we must fully consider the compatibility between components, between PCB products and product casings, and in spatial structures, reserve safe distances for each target object, and ensure that there is no conflict in space.