How to design PCB safety spacing?

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How to design PCB safety spacing? [Copy link]

There are many places in PCB design that need to consider safety spacing. Here, they are temporarily classified into two categories: one is electrical related safety spacing, and the other is non-electrical related safety spacing.

Electrical related safety distance

1. Wire spacing

In terms of the processing capabilities of mainstream PCB manufacturers, the minimum spacing between wires must not be less than 4 mils. The minimum line spacing is also the distance between wires and wires and between wires and pads. From a production perspective, the larger the better if conditions permit, and 10 mils is more common.

2Pad aperture and pad width

In terms of the processing capabilities of mainstream PCB manufacturers, the minimum pad aperture should not be less than 0.2mm if mechanical drilling is used, and should not be less than 4mil if laser drilling is used. The aperture tolerance varies slightly depending on the board material, but can generally be controlled within 0.05mm, and the minimum pad width should not be less than 0.2mm.

3. Pad to pad spacing

As far as the processing capabilities of mainstream PCB manufacturers are concerned, the spacing between pads must not be less than 0.2mm.

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

The distance between the live copper and the edge of the PCB should not be less than 0.3mm. Set this distance rule on the Design-Rules-Boardoutline page.

If copper is laid over a large area, it is usually necessary to have a retracted distance from the edge of the board, which is generally set to 20 mils. In the PCB design and manufacturing industry, generally speaking, for mechanical considerations of the finished circuit board, or to avoid curling or electrical short circuits caused by copper exposed at the edge of the board, engineers often retract the large copper block by 20 mils relative to the edge of the board, rather than laying the copper all the way to the edge of the board.

There are many ways to deal with this copper shrinkage, such as drawing a keepout layer on the board edge and then setting the distance between the copper and the keepout. Here is a simple method, which is to set different safety distances for the copper objects. For example, if the safety distance of the whole board is set to 10mil, and the copper is set to 20mil, the board edge can be shrunk by 20mil, and dead copper that may appear in the device can also be removed.

Non-electrical safety distance

01

Character width, height and spacing

The text film cannot be changed during processing, except that the character line widths with D-CODE less than 0.22mm (8.66mil) are thickened to 0.22mm, that is, the character line width L = 0.22mm (8.66mil).

The width of the entire character is W = 1.0mm, the height of the entire character is H = 1.2mm, and the spacing between characters is D = 0.2mm. When the text is smaller than the above standards, it will be blurred when processed and printed.

02

Via to via spacing

The via to via spacing (hole edge to hole edge) should be greater than 8 mil.

03

Distance from silk screen to pad

Silk screen printing is not allowed to cover the pad. Because if the silk screen printing covers the pad, the silk screen printing area will not be tinned during tinning, which will affect the mounting of components. Generally, the board factory requires a spacing of 8 mils. If the PCB board is really limited in area, a spacing of 4 mils is barely acceptable. If the silk screen printing accidentally covers the pad during design, the board factory will automatically remove the silk screen part left on the pad during manufacturing to ensure the tinning of the pad.

Of course, the specific situation is analyzed in the design. Sometimes the silk screen is deliberately placed close to the pad, because when two pads are close together, the silk screen in the middle can effectively prevent the solder connection from short-circuiting during welding. This situation is another matter.

04

3D height and horizontal spacing on the machine

When mounting components on PCB, it is necessary to consider whether there will be conflicts with other mechanical structures in the horizontal direction and spatial height. Therefore, when designing, it is necessary to fully consider the adaptability between components, between PCB products and product shells, and in spatial structures, and reserve safe spacing for each target object to ensure that there is no conflict in space.

This content is originally created by EEWORLD forum user Zhong Xinhua . If you want to reprint or use it for commercial purposes, you must obtain the author's consent and indicate the source

慵懒的虫

慵懒的虫

Lazy Bug published on 2019-7-10 15:10

小潜艇

In addition to basic principles, drawing a board is more of a process of compromise and trade-off among various factors.

小阿

I'm learning how to draw PCB.

qwqwqw2088

Set different safety distances for copper objects. For example, if the safety distance of the entire board is set to 10mil, and the copper is set to 20mil, the effect of shrinking the board edge by 20mil can be achieved.

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