In everyday PCB design, we encounter various clearance requirements – for example, clearances between vias and pads, traces and traces, and many other aspects that must be considered. Today, we will divide these clearance requirements into two categories: Electrical Clearances and Non‑Electrical Clearances.
Electrical Clearances
1. Trace‑to‑Trace Spacing
According to PCB manufacturing capabilities, the spacing between traces should not be less than 4 mil. Trace spacing also applies to trace‑to‑trace and trace‑to‑pad distances. From a production perspective, larger spacings are always better when conditions permit. A common standard is 10 mil.
2. Pad Hole Diameter and Pad Width
According to PCB manufacturers, the pad hole diameter must not be less than 0.2 mm for mechanical drilling, or less than 4 mil for laser drilling. The hole‑size tolerance varies slightly depending on the laminate material, but is generally controllable within 0.05 mm. The pad width must not be less than 0.2 mm.
3. Pad‑to‑Pad Spacing
According to PCB manufacturing capabilities, the spacing between pads must not be less than 0.2 mm.
4. Copper‑Pour‑to‑Board‑Edge Spacing
The spacing between live copper and the PCB edge should be no less than 0.3 mm. For large copper pours, it is common to set an inward clearance of 20 mil from the board edge. This is usually done for mechanical considerations, to avoid copper exposure at the board edge that could cause curling or electrical shorts. Engineers often set the copper pour to be recessed 20 mil from the board edge, rather than extending it all the way to the edge. Various methods are used to achieve this, such as drawing a keep‑out layer along the board edge and setting the copper‑pour clearance from the keep‑out.
Non‑Electrical Clearances
1. Silkscreen Text Width, Height, and Spacing
For silkscreen text, common values are 5/30 mil, 6/36 mil, etc. If the text is too small, the printed result will be blurred and illegible.
2. Silkscreen‑to‑Pad Spacing
Silkscreen must not overlap pads. If silkscreen covers a pad, the pad will not accept solder during soldering, affecting component placement. PCB manufacturers generally require a minimum clearance of 8 mil. If board space is very tight, a clearance of 4 mil may be acceptable as a compromise. If silkscreen accidentally covers a pad during design, the manufacturer will automatically remove the silkscreen portion on the pad to ensure solderability – this is something to be aware of.
3. Mechanical 3D Height and Horizontal Clearance
When assembling components on the PCB, consideration must be given to potential conflicts with other mechanical structures in both the horizontal and vertical dimensions. Therefore, during design, adequate clearance must be reserved between components and between the finished PCB and the product enclosure to ensure proper mechanical fit.
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