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Designing a Safe Pitch for the PCB
2020-02-20
 
1 Space between wires
As far as the processing capacity of mainstream PCB manufacturers is concerned, the minimum distance between the wires must not be less than 4mil. The minimum line distance is also the line-to-line, line-to-pad distance. From a production perspective, the larger the better, the better, 10 mil is more common.

2 Pad aperture and pad width
As far as the processing capacity of mainstream PCB manufacturers is concerned, if the pad hole diameter is mechanically drilled, the minimum must not be less than 0.2mm, and if it is laser-drilled, the minimum must be no less than 4mil. The aperture tolerance is slightly different depending on the plate. Generally, it can be controlled within 0.05mm, and the minimum pad width must not be less than 0.2mm.

3 Pad-to-pad spacing
As far as the processing capabilities of mainstream PCB manufacturers are concerned, the distance between the pads and the pads must not be less than 0.2mm.

4 Distance between copper sheet and board edge
The distance between the charged copper skin and the edge of the PCB is preferably not less than 0.3mm. Set this spacing rule on the Design-Rules-Board outline page.
If it is large area copper, usually there is a shrinkage distance from the edge of the board, usually set to 20mil. In the PCB design and manufacturing industry, in general, due to mechanical considerations of the finished circuit board, or in order to avoid situations such as curling or electrical shorts caused by the bare copper on the board edge, engineers often place large areas of copper. The block shrinks 20mil relative to the edge of the board, instead of laying the copper skin all the way to the edge of the board.
There are many kinds of processing methods for copper skin shrinkage, such as drawing the keepout layer on the edge of the board, and then setting the distance between copper plating and keepout. A simple method is introduced here, that is, setting different safety distances for copper plating objects, such as setting the safety distance of the whole board to 10mil, and setting the copper plating to 20mil, you can achieve the effect of shrinking the board edge by 20mil. Removes possible dead copper in the device.

Non-electrically related safety clearance

01 Character width height and spacing
The text film cannot be changed in any way during processing, but the character line width with D-CODE less than 0.22mm (8.66mil) is bolded 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 space between the characters is D = 0.2mm. When the text is smaller than the above standards, the printing will be blurred.

02 Pitch from via to via
The distance from via (VIA) to via (hole edge to hole edge) is preferably greater than 8mil.

03 Screen-to-pad distance
Silk screen printing is not allowed to cover the pads. Because if the silk screen is covered with a pad, the silk screen cannot be tinned when the solder is applied, which affects the component mounting. Generally, the board factory requires to reserve 8mil spacing as well. If the PCB area is limited, the 4mil pitch is barely acceptable. If the silk screen accidentally covers the pad during the design, the board factory will automatically eliminate the silk screen part left on the pad during manufacturing to ensure the tin on the pad.
Of course, the specific situation is analyzed at design time. Sometimes the screen printing is deliberately made close to the pad, because when the two pads are close together, the screen printing in the middle can effectively prevent the short circuit of the solder connection during soldering.

04 Mechanical 3D height and horizontal spacing
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 between components, the finished PCB and the product shell, and the spatial structure, and reserve a safe distance for each target object to ensure that there is no conflict in space.
 
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