What Are The Safety Clearance Issues Encountered In PCB Design

We will encounter a variety of safety clearance issues in PCB design, such as the clearance between vias and pads, the spacing between traces are all places we should consider. So today we divide these spacing requirements into two categories, one is electrical safety spacing, the other is non-electrical safety spacing.

1. Electrical safety distance

(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. From the perspective of our production, it is better to have a larger spacing under conditions. The general 10MIL is more common.

(2)Pad aperture and pad width

According to the PCB manufacturer, the minimum diameter of the pad diameter should be at least 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 board. Generally it can be controlled within 0.05mm. The width of the board must not be less than 0.2mm.

(3)The spacing between pads

According to the processing capacity of the PCB manufacturer, the distance between the 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. If copper is laid on a large area, it is usually necessary to have a shrinkage distance from the edge of the board, which is generally set to 20mil. Under normal circumstances, for mechanical considerations of the finished circuit board, or to avoid the possibility of curling or electrical short circuit caused by the copper strip exposed on the edge of the board, engineers often shrink large-area copper blocks by 20mil relative to the edge of the board. The copper skin is not always spread to the edge of the board. There are many ways to deal with copper shrinkage. For example, draw the keepout layer on the edge of the board, and then set the distance between the copper and keepout.

2. Non-electrical safety distance

(1) Width, height and spacing of characters

We generally use conventional values for characters printed on the screen. Because when the text is too small, it will be blurred after processing and printing.

(2)The distance from silk screen to pad

Screen printing is not allowed on the pad. If the silk screen is covered with pads, the tin will not be tinned when soldering, which will affect the placement of components. General board manufacturers require 8mil spacing to be reserved. If it is because the area of some PCB boards is very close, we can barely accept 4MIL. Then, if the silk screen accidentally covers the pad during design, the board manufacturer will automatically eliminate the silk screen portion left on the pad during manufacturing to ensure tin on the pad. So we need to pay attention to it.

(3)3D height and horizontal spacing on the mechanical structure

When mounting devices on the PCB, it is necessary to 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 of the spatial structure between the components, and between the finished PCB and the product shell, and reserve a safe distance for each target object.