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Electrical and non-electrical safety clearances in PCB design.

September 08, 2020



Electrical related safety clearance

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 larger 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, the minimum pad aperture should not be less than 0.2mm if it is mechanically drilled, and it should not be less than 4mil if it is laser-drilled. The aperture tolerance is slightly different depending on the plate, generally can be controlled within 0.05mm, the minimum width of the pad 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 sheet and the board edge

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 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 way 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, which can achieve the effect of 20mil shrinkage of the board edge, and also The dead copper that may appear in the device is removed.

Non-electrically related safety clearance

1. Character width, height and spacing

No changes can be made to the text film during processing, except that the width of the character line whose D-CODE is 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 is W = 1.0 mm, the height of the entire character is H = 1.2 mm, and the distance between the characters is D = 0.2 mm. When the text is smaller than the above standard, the processing and printing will be blurred.

2. Spacing between vias and vias

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 when it is tinned, which will affect the mounting of components. 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, specific conditions are analyzed in 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. Mechanical 3D height and horizontal spacing

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, between the finished PCB 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.