In PCB design, safe spacing is crucial at various points. These considerations can be categorized into two main types: electrical-related safety spacing and non-electrical-related safety spacing.
Electrical-Related Safety Spacing
- 1. Wire Spacing
- 2. Pad Aperture and Pad Width
- 0.2 mm (if mechanically drilled)
- 4 mils (if laser drilled)
- 3. Pad-to-Pad Spacing
- 4. Copper-to-Edge Spacing
For the capabilities of mainstream PCB manufacturers, the minimum distance between wires should be at least 4 mils. This minimum spacing also applies to line-to-line and line-to-pad distances. From a production perspective, the larger the spacing, the better. A more common value is 10 mils.
For most PCB manufacturers, the minimum hole diameter for pads is:
The aperture tolerance can vary slightly depending on the board material, but it is generally within 0.05 mm. The minimum pad width should be 0.2 mm.
The distance between pads should not be less than 0.2 mm, according to the processing capabilities of most PCB manufacturers.
The distance between copper and the edge of the PCB should ideally be at least 0.3 mm. This spacing can be set on the Design-Rules-Boardoutline page.
For large copper areas, it is often necessary to leave a retraction distance from the board’s edge, typically set to 20 mils. This precaution helps avoid issues like curling or electrical shorts caused by bare copper at the board’s edge.
Various techniques can manage copper retraction, such as drawing a keepout layer near the board edge and defining the clearance between copper and the keepout area. For example, setting the overall safe clearance to 10 mils and copper plating to 20 mils will result in a 20-mil shrinkage from the edge while simultaneously eliminating any dead copper near devices.
Non-Electrical-Related Safety Spacing
- 1. Character Width, Height, and Spacing
- 2. Via-to-Via Spacing
- 3. Silk Screen to Pad Distance
- 4. Mechanical 3D Height and Horizontal Spacing
Text cannot be modified during the manufacturing process. The minimum line width for bold characters with a D-CODE of less than 0.22 mm (8.66 mils) should be set to 0.22 mm. The character width (W) should be at least 1.0 mm, the height (H) should be at least 1.2 mm, and the spacing (D) between characters should be no less than 0.2 mm. If text is smaller than these values, the processing or printing may become unclear.
The via-to-via pitch (center-to-center) should be no less than 8 mils.
Silk screens should never cover pads. If the silk screen overlaps a pad, the pad will not receive solder, which can interfere with component mounting. A typical factory requirement is an 8 mil clearance. In cases of limited PCB area, a 4 mil clearance may be acceptable. If the silk screen accidentally overlaps the pad during design, the PCB manufacturer will remove the silk screen from the pad during production to ensure proper soldering.
In some cases, designers intentionally place the silk screen close to pads to prevent solder bridges when pads are very close together. This design choice can help avoid short circuits during soldering.
When mounting components on the PCB, it’s essential to consider any potential conflicts with other mechanical structures in terms of horizontal and vertical space. Designers must ensure that components, the PCB itself, and the product casing are compatible, reserving sufficient space to prevent any spatial conflicts.
If you have any questions about PCB or PCBA, feel free to contact me at info@wellcircuits.com.
