In the ordinary PCB design, we will encounter various safety spacing problems, such as the spacing between vias and pads, and the spacing between routing lines.

In PCB design, we divide these distances into two categories:

ElectrICal safety distance

Non electrical safety distance

1、 Electrical safety distance

1. Spacing between PCB design conductors

   
   

It is recommended that the distance between the wires 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, of course, the bigger the better if conditions permit. The conventional 10mil is common.

2. Pad hole diameter and pad width

According to the PCB manufacturer, if the bonding pad hole diameter is mechanically drilLED, the minimum shall not be less than 0.2mm. If laser drilling is used, it is recommended that the minimum shall not be less than 4mil. The hole diameter tolerance is slightly different according to different plates, which can be controlled within 0.05mm generally, and the minimum pad width shall not be less than 0.2mm.

3. Spacing between pads

According to the processing capacity of PCB manufacturers, it is recommended that the space between pads should not be less than 0.2mm.

4. Spacing between copper sheet and plate edge

The distance between the electrified copper sheet and the PCB board edge should be no less than 0.3mm. If it is a large area of copper laying, it is usually required to have an inside distance with the board edge, which is generally set as 20mil.

In general, for the mechanical consideration of the finished circuit board, or to avoid the curling or electrical short circuit caused by the exposure of the copper sheet on the board edge, the engineer often shrinks the large area of copper paving block by 20 mils relative to the board edge, instead of laying the copper sheet to the board edge all the time. There are many ways to deal with the copper skin shrinkage. For example, draw a keepout layer on the board edge, and then set the distance between copper paving and keepout.

PCB design

2、 PCB design non electrical safety distance

1. Width, height and spacing of characters

For the characters of PCB design silk screen, we generally use conventional values such as 5/306/36mil. Because when the text is too SMAll, the processing and printing will be blurred.

2. Distance from silk screen to bonding pad

The bonding pad is not allowed for the silk screen, because if the silk screen is covered with the bonding pad, the silk screen cannot be coated with tin, which affects the assembly of PCB components.

Generally, 8 mil spacing is required to be reserved in the plate factory. If it is because the area of some PCB boards is very close, the 4mil spacing is barely acceptable. If the silk screen accidentally covers the bonding pad during design, the board factory will automatically eliminate the silk screen left on the bonding pad during manufacturing to ensure tin on the bonding pad. So we need to pay attention.

3. 3D height and horizontal spacing on mechanical structure

When mounting PCB components, it is necessary to consider whether the horizontal direction and space height will conflict with other mechanical structures. Therefore, in the design, it is necessary to fully consider the adaptability of space structure between components, as well as between PCB finished products and product shells, and reserve a safe space for each target object.