Electrical and non-electrical safety spacing in PCB design
Anyone who has worked on PCB knows that there are many places in PCB design that need to consider safety spacing. Here, they are temporarily classified into two categories: one is electrical safety spacing, and the other is non-electrical safety spacing.
Electrical 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 pad aperture must not be less than 4mil if laser drilling is used. The aperture tolerance varies slightly depending on the board material, and can generally be controlled within 0.05mm, and the minimum pad width must not be less than 0.2mm.
3.Spacing between pads
As far as the processing capabilities of mainstream PCB manufacturers are concerned, the spacing between pads should not be less than 0.2mm.
4.Spacing between copper and board edge
The spacing between live copper and PCB board edge should not be less than 0.3mm. Set this spacing rule in the Design-Rules-Board outline page.
If it is a large area of copper plating, it is usually necessary 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 spreading the copper all the way to the board edge.
There are many ways to deal with this copper indentation, such as drawing a keepout layer on the board edge and then setting the distance between the copper plating and the keepout. Here is a simple method, which is to set different safety distances for copper objects. For example, if the safety distance of the whole board is set to 10mil, and the copper is set to 20mil, the board edge can be retracted by 20mil, and the dead copper that may appear in the device can be removed.
Non-electrical safety distance
1..Character width, height and spacing
No changes can be made to the text film during processing, except that 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, the processed printing will be blurred.
2.Via to via spacing
The via (VIA) to via spacing (hole edge to hole edge) should preferably be greater than 8mil.
3.Silk screen to pad distance
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 when tinning, thus affecting the installation 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 that the pad is tinned.
Of course, the specific situation is analyzed in detail during design. Sometimes the silk screen is deliberately made close to the pad, because when the two pads are close together, the silk screen in the middle can effectively prevent the solder connection from short-circuiting during welding, which is another matter.
4.3D height and horizontal spacing on the machine
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 between components, between the PCB finished product and the product shell, and in the spatial structure, and reserve a safe spacing for each target object to ensure that there is no conflict in space.
