How to design the PCB safety gap?

In PCB design, there are many places that need to consider the safety distance. Here, it is classified into two categories for the time being: one is electrical-related safety clearance, and the other is non-electrical-related safety clearance.


1. Electrical related safety distance
1. Spacing between wires

As far as the processing capabilities of mainstream PCB manufacturers are concerned, the minimum spacing between wires should not be less than 4mil. The minimum line distance is also the distance from line to line and line to pad. From a production point of view, the bigger the better if possible, the more common is 10mil.

2. Pad aperture and pad width

As far as the processing capabilities of mainstream PCB manufacturers are concerned, if the pad aperture is mechanically drilled, the minimum should not be less than 0.2mm, and if laser drilling is used, the minimum should not be less than 4mil. The aperture tolerance is slightly different depending on the plate, generally it can be controlled within 0.05mm, and the minimum pad width should not be less than 0.2mm.

3. The distance between the pad and the pad

As far as the processing capabilities of mainstream PCB manufacturers are concerned, the distance between pads and pads should not be less than 0.2mm.

4. The distance between the copper skin and the edge of the board

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

If it is a large area of ​​copper, it usually needs to be retracted from the edge of the board, generally set to 20mil. In the PCB design and manufacturing industry, under normal circumstances, due to the mechanical considerations of the finished circuit board, or to avoid curling or electrical short-circuiting due to the exposed copper skin on the edge of the board, engineers often spread copper on a large area The block is shrunk by 20 mils relative to the edge of the board, instead of spreading the copper to the edge of the board. There are many ways to deal with this kind of copper shrinkage, such as drawing a keepout layer on the edge of the board, and then setting the distance between the copper paving and the keepout. Here is a simple method to set different safety distances for copper paving objects. For example, the safety distance of the whole board is set to 10mil, and the copper paving is set to 20mil, and the effect of 20mil shrinkage of the board edge can be achieved. The dead copper that may appear in the device is removed.

2. Non-electrical safety clearance
1. Character width, height and spacing

The text film cannot be changed during processing, but the character line width of the D-CODE less than 0.22mm (8.66mil) is thickened to 0.22mm, that is, the character line width L=0.22mm (8.66mil), and the entire character Width=W1.0mm, the height of the entire character H=1.2mm, and the space between the characters D=0.2mm. When the text is smaller than the above standard, the processing and printing will be blurred.

2. Spacing between via hole and via hole (hole edge to hole edge)

The distance between vias (VIA) and vias (hole edge to hole edge) is preferably 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 is covered with the pad, the silk screen will not be tinned during the tinning, which will affect the component mounting. Generally, the board factory requires a space of 8mil to be reserved. If the PCB area is really limited, a 4mil pitch is barely acceptable. If the silk screen accidentally covers the pad during design, the board factory will automatically eliminate the part of the silk screen left on the pad during manufacturing to ensure that the pad is tinned.

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

4. 3D height and horizontal spacing on the mechanical structure

When mounting devices on the PCB, consider whether there will be conflicts with other mechanical structures in the horizontal direction and the height of the space. Therefore, when designing, it is necessary to fully consider the adaptability between the components, the PCB product and the product shell, and the space structure, and reserve a safe distance for each target object to ensure that there is no conflict in space.