PCB design requires attention to numerous safety distances, including trace spacing, text spacing, and pad spacing. These considerations can generally be categorized into two types: electrical safety distances and non-electrical safety distances.
01 Electrical Safety Distances
Trace-to-Trace Spacing
For mainstream PCB manufacturers, the minimum spacing between traces must not be less than 0.075mm. The minimum trace spacing refers to the smallest distance between traces or between a trace and a pad. From a production perspective, larger spacing is better, with 0.127mm being a common standard.
Pad Hole Diameter and Pad Width
If the pad uses mechanical drilling, the minimum hole diameter should be no less than 0.2mm; for laser drilling, the minimum hole diameter is 0.1mm. The hole diameter tolerance varies slightly depending on the material, typically controlled within 0.05mm, and the minimum pad width should not be less than 0.2mm.
Pad-to-Pad Spacing
The minimum spacing between pads must not be less than 0.2mm for most mainstream PCB manufacturers.
Copper-to-Board Edge Spacing
The spacing between live copper areas and the PCB edge should be no less than 0.3mm. This can be configured in the Design > Rules > Board Outline settings.
For large copper areas, a setback distance from the board edge is typically required, usually set to 0.2mm. To avoid issues such as copper exposure on the board edge, which may lead to warping or electrical shorts, engineers often inset copper areas by 8 mils (~0.2mm) from the edge instead of extending copper to the edge.
Simplified Method for Copper Setback: Set the general safety distance for the board to 0.25mm and the copper safety distance to 0.5mm. This ensures a 0.5mm setback for copper from the edge and eliminates dead copper within components.
02 Non-Electrical Safety Distances
Text Width, Height, and Spacing
Text film cannot be altered during processing, but character line widths smaller than 0.22mm (8.66 mils) are thickened to 0.22mm. The standard character dimensions are:
- Line width (L): 0.22mm (8.66 mils)
- Character width (W): 1.0mm
- Character height (H): 1.2mm
- Spacing between characters (D): 0.2mm
Text smaller than these dimensions will appear blurry after processing.
Via-to-Via Spacing
The spacing between vias (edge to edge) should be greater than 8 mils.
Silkscreen-to-Pad Spacing
Silkscreen markings must not overlap pads. Overlapping the silkscreen will prevent the solder from adhering properly during the soldering process, affecting component placement. A spacing of 8 mils is recommended; 4 mils is acceptable in space-constrained designs. If the silkscreen accidentally covers the pad, the manufacturer will automatically remove the overlapping portion to ensure soldering quality.
In some cases, silkscreen may intentionally be placed near pads to prevent solder bridging between closely spaced pads.
Mechanical 3D Height and Horizontal Clearance
When placing components on the PCB, ensure that there are no conflicts with other mechanical structures in both horizontal and vertical directions. Consider spacing between components, the PCB, and the product casing. Reserve sufficient clearance to prevent spatial interference.
By adhering to these safety distances, you can ensure the manufacturability, reliability, and functionality of your PCB design.
