8 Safety Distances Must Be Considered in PCB Design

A: Electrical Related Safety Distance

1. In terms of the processing capability of mainstream PCB manufacturers, the minimum distance between wires should not be less than 0.075mm. The minimum line distance refers to the minimum distance between the board sub line and the line, and between the line and the solder pad. From a production perspective, a larger line spacing is better, with a common one being 0.25mm.

2. In terms of the processing capability of mainstream PCB manufacturers, if the solder pad is mechanically drilled, the minimum hole diameter should not be less than 0.2mm; If it is laser drilling, the minimum aperture should not be less than 0.1mm. The aperture tolerance varies slightly depending on the type of plate, and can generally be controlled within 0.05mm. The minimum width of the solder pad should not be less than 0.2mm.

3. In terms of the processing capability of mainstream PCB manufacturers, the spacing between solder pads should not be less than 0.2mm.

4. The distance between the copper sheet and the edge of the PCB board is preferably not less than 0.3mm. You can set this spacing rule on the Design Rules Board outline page. If it is a large-scale copper laying, there usually needs to be an inward distance of 0.5mm from the edge of the board. In the PCB design and manufacturing industry, in general, due to mechanical considerations of the finished circuit board or to avoid situations such as curling or electrical short circuits caused by exposed copper foil on the board edge, engineers often shrink large-area copper blocks by 20mil relative to the board edge, rather than continuously laying copper foil to the board edge.

There are many ways to handle the shrinkage of copper skin, such as drawing a keep out layer on the edge of the board and setting the distance between copper laying and keep out. A simple method is introduced here, which is to set different safety distances for copper laying objects. For example, setting the safety distance for the entire board to 0.25mm and setting the copper laying distance to 0.5mm can achieve the effect of shrinking the board edge by 0.5mm, while also removing any dead copper that may occur inside the device.

B: Non Electrical Safety Distance

1. Character width, height, and spacing

Text film cannot be changed during processing, only the width of character lines with D-CODE less than 0.22mm (8.66mil) is bolded to 0.22mm, that is, the width of character lines L=0.22mm (8.66mil).

And 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 blurry when processed and printed.

2. The distance between through holes

The distance between via holes (VIA) and via holes (hole edge to hole edge) should preferably be greater than 8mil.

3. Distance from silk screen printing to solder pad

Silk screen printing is not allowed to cover solder pads. Because if the solder pads are covered with silk screen printing, the silk screen area will not be able to be soldered during soldering, which will affect the installation of electronic components. Generally, board factories require a reserved spacing of 8mil. If the PCB board has a limited area, achieving a spacing of 4mil is barely acceptable. If the silk screen accidentally covers the solder pad during design, the board factory will automatically eliminate the silk screen part left on the solder pad during manufacturing to ensure soldering on the solder pad.

Of course, specific situations will be analyzed during the design process. Sometimes the silk screen is intentionally placed close to the solder pads, because when two solder pads are in close proximity, the silk screen in the middle can effectively prevent short circuits in the solder connection during soldering. This situation is another matter.

4. Mechanical 3D height and horizontal spacing

When installing components on PCB, it is necessary to consider whether there will be conflicts with other mechanical structures in the horizontal direction and spatial height. Therefore, in the design process, full consideration should be given to the compatibility between components, between PCB finished products and product casings, and in terms of spatial structure. Safety distances should be reserved for each target object to ensure that there are no conflicts in space.