There are many safe spacing in PCB design that demands to be considered, including wire spacing, character spacing, pad spacing and so on. Here, it is tentatively classified into two categories: one is electrically related safety spacing, and the other is non-electrically related safety spacing.
Electrically Related Safety Spacing
1. The spacing between wires. According to the processing capacity of mainstream PCB manufacturers, and the spacing between wires shall not be less than 0.075mm. Minimum line spacing which refers to the minimum distance from the board to the line and from the line to the pad. From the angle of production, the larger the wire spacing, the better, and the common wire spacing is 0.25mm.
2. Pad hole diameter and pad width. As per the processing capacity of mainstream PCB manufacturers, the hole diameter of the pad shall not be less than 0.2mm if it is mechanically drilled, and 0.1mm if it is laser drilled. The hole diameter tolerance is slightly different according to different pad plates, and the tolerance can generally be controlled within 0.05mm. The pad width shall not be less than 0.2mm.
3. The spacing between pads. Based on the processing capacity of mainstream PCB manufacturers, and the spacing between pads shall not be less than 0.2mm.
4. The clearance between the copper sheet and the board edge. The clearance between charged copper sheet and the PCB board edge should not be less than 0.3mm. This spacing rule can be set on the Design-Rules-Board outline page. If a large-area copper is laid, an inward shrinkage distance from the board edge is usually required, which is generally set to 0.5mm. In the PCB design and manufacturing industry, in general, due to the mechanical considerations of the finished circuit board, or in order to avoid the curling or electrical short circuit caused by the exposed copper on the edge of the board, engineers often shrink the large area copper by 20mil relative to the board edge, instead of laying the copper all the way to the board edge.
There are many ways to deal with this kind of copper indentation, such as drawing a keepout layer on the board edge, and then setting the clearance between the copper sheet and the keepout. A simple way is introduced and that is to set different safety clearance for the copper sheet. For example, the safety spacing of the whole board is set as 0.25mm, and the safety clearance of copper sheet is set as 0.5mm, which can achieve the effect of 0.5mm inward shrinkage of the board edge, and also remove possible dead copper in the device.
Non-Electrically Related Safety Spacing
1. Character width, height, and spacing
The text film cannot be changed in any way, but the width of the character line with the D-CODE less than 0.22mm (8.66 mil) is increased to 0.22mm, that is, the character line width L=0.22mm (8.66mil).
The width of the whole character is W=1.0mm, the height of the whole character is H=1.2mm, and the spacing between the characters is D=0.2mm. When the text is less than the above standard, the processed and printed text will be blurred.
2. The spacing from via to via
The spacing from via to via (hole edge to hole edge) should preferably be greater than 8mil.
3. The silkscreen to pad distance
Silkscreen is not allowed to be stamped with pads. Because if the pad is covered with silkscreen, the pad cannot be soldered, which will affect the component assembly. Generally, the board factory requires a spacing of 8mil to be reserved. If the PCB board is really limited, it may be accepted to achieve a spacing of 4mil. If the silkscreen is accidentally covered with the pad during the design, the board factory will automatically eliminate the part of the silkscreen on the pad during production to ensure that the pad is soldered.
Of course, the specific situation is analyzed on a case-by-case basis during design. Sometimes the silkscreen is deliberately close to the pad, because when the two pads are close together, the silkscreen between the two pads can effectively prevent the solder connection from short-circuit during soldering, which is another case.
4. 3D height and horizontal spacing on the machine
When the device on the PCB is assembled, whether there will be conflicts with other mechanical structures in the horizontal direction and space height should be considered. Therefore, in the design, it is necessary to fully consider the adaptability between components, between PCB products and product shells, and in the space structure, and reserve a safe spacing for each target object to ensure that there is no conflict in space.
In conclusion, the above data is only used as a reference, so that it can guide safety values design, and the above data does not fully represent the industry standard.
In PCB design, safety distance is an important parameter. Engineers need to set the electrical and non-electrical safety distances according to actual needs and manufacturing processes to ensure the performance, reliability, and production of the circuit board.
