How is the PCB safety spacing designed?

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

Electrical-related safety spacing

1 Wire Spacing

For the processing capability of mainstream PCB manufacturers, the distance between the wires and the wires must be at least 4 mils. The minimum line spacing is also the line-to-line, line-to-pad distance. From a production point of view, the larger the better, the more common is 10mil.

2 Pad Aperture and Pad width

As far as the processing capability of mainstream PCB manufacturers is concerned, if the hole diameter of the pad is mechanically drilled, the minimum is not less than 0.2mm. If laser drilling is used, the minimum is not less than 4mil. The aperture tolerance is slightly different depending on the plate. Generally, the control can be within 0.05mm, and the pad width should be at least 0.2mm.

3 Pad to Pad Spacing

For the processing capability of mainstream PCB manufacturers, the distance between the pads and the pads must not be less than 0.2mm.

4 Copper and the Edge of the Board Spacing

The distance between the charged copper skin and the edge of the PCB board is preferably not less than 0.3 mm. Set this spacing rule on the Design-Rules-Boardoutline page.

If it is a large area of copper, usually it needs to have a retraction distance from the edge of the board, generally set to 20mil. In the PCB design and manufacturing industries, engineers often use large-area copper considering the mechanical aspects of the finished board, or to avoid curling or electrical shorting due to bare copper on the edge of the board. Instead of always spreading the copper to the edge of the board.

There are many ways to handle this copper skin 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 an easy way to set different safe distances for copper objects. For example, the safe clearance of the whole board is set to 10mil, and the copper plating is set to 20mil, which can achieve the effect of 20mils of the edge inside shrink. The dead copper that may appear in the device is removed at the same time.

Non-electrical-related safety spacing

1 Character Width Height and Spacing

The text film can't be changed during processing, just bold character line width which its D-CODE is less than 0.22mm(8.66mil) to 0.22mm, that is, the character line width L=0.22mm (8.66mil).

The width of the entire character W = 1.0 mm, the height of the entire character H = 1.2 mm, and the spacing between characters D = 0.2 mm. When the text is smaller than the above standard, the processing and printing will be blurred.

2 Via to Via Spacing

The via to via pitch (hole to hole side) is preferably greater than 8 mils.

3 Silk Screen to Pad Distance

Silk screens are not allowed to cover the pads. If the silk screen is covered with a pad, the silk screen will not be tinned when the tin is applied, which affects the component mounting. Generally, the factory requires a reserved space of 8 mils. If the PCB board is really limited in area, it is barely acceptable to achieve a 4 mil pitch. If the silk screen is accidentally overlaid on the pad during design, the board will automatically remove the silkscreen portion left on the pad during manufacture to ensure tin on the pad.

Of course, the specific situation is analyzed in the design. Sometimes the silk screen is intentionally placed close to the pad, because when the two pads are close together, the middle screen printing can effectively prevent the solder connection from short-circuiting during soldering. This is another case.

4 Mechanical 3D height and Horizontal Spacing

When mounting the device on the PCB, it should be considered whether there will be conflicts with other mechanical structures in the horizontal direction and the space height. Therefore, in the design, full consideration should be given to the compatibility between components, PCB products and product shells and space structure, to reserve a safe spacing for each target object, to ensure that there is no conflict in space.