The Issues of Safety Spacing in PCB Design

Label:PCB Design, Safety Spacing, Silk Print

Oct 21, 202022502

The Issues of Safety Spacing in PCB Design

We will encounter various safety spacing issues in ordinary PCB design, such as the spacing between vias and pads, the spacing between traces and traces, etc., which should be considered. . So today we will divide these spacing requirements into two categories, one is: electrical safety spacing; the other is: non-electrical safety spacing.


Electrical Safety Spacing:


1.Spacing between wires


According to the production capacity of PCB manufacturers, the distance between traces should not be less than 4MIL. The minimum line spacing is also the line-to-line and line-to-pad spacing. So, from the perspective of our production, of course, the bigger the better if possible. The conventional 10MIL is very common.


2.Pad aperture and pad width


According to the PCB manufacturer, 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 hole diameter tolerance is slightly different depending on the plate. Generally, it can be controlled within 0.05mm. The minimum land width must not be lower than 0.2mm.


3.The distance between the pad and the pad


According to the processing capacity of the PCB manufacturer, the distance between the pad and the pad shall not be less than 0.2mm.


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

The distance between the charged copper skin and the PCB board edge is preferably not less than 0.3mm. If it is a large area of copper, it usually needs to be retracted from the board edge, generally set to 20mil. Under normal circumstances, due to the mechanical considerations of the finished circuit board, or to avoid curling or electrical shorts caused by the exposed copper skin on the edge of the board, engineers often shrink the large area copper block by 20mil relative to the edge of the board. The copper is not always spread to the edge of the board. There are many ways to deal with this kind of copper shrinkage. For example, draw a keepout layer on the edge of the board, and then set the distance between the copper paving and the keepout.


Non-electrical Safety Spacing


1.The width, height and spacing of characters


Regarding silk screen characters, we generally use conventional values such as 5/30 6/36 MIL, etc. Because when the text is too small, the processing and printing will be blurred.


2.Distance from silk screen to pad


The silk screen does not allow 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 it is because the area of some PCB boards is very tight, we can barely accept the 4MIL spacing. Then, 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. So we need to pay attention.


3.3D 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 space height. Therefore, when designing, we must fully consider the adaptability of the space structure between the components, and between the finished PCB and the product shell, and reserve a safe distance for each target object.


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