PCB copper pour: clearance, thermal relief and how to avoid isolated islands
A copper pour fills the unused area of a layer with copper, usually connected to ground. Done well it helps the board; done carelessly it causes problems in production.
What a copper pour is
A copper pour (also called copper fill, copper area or zone) is a large copper area that fills the free space of a layer around the traces and pads. It is normally connected to one net, most often ground, and it keeps a set clearance to everything of a different net.
Why use one
- Return path and noise: a solid ground pour next to a signal gives its return current a short path (see ground and return path layout).
- Current and heat: a pour carries more current than a thin trace and spreads heat (see trace width and current).
- Even copper: large empty areas next to heavy copper make a board warp during soldering; spreading copper evenly keeps it flat (see the stack-up guide).
- Less etching: less copper has to be removed from the board.
Clearance
Give the pour at least the same clearance as your other copper-to-copper spacing, and more where you can, for example 0.3 mm or more around traces and pads of another net. Keep the pour and every other copper area away from the board edge and from V-cut lines: copper that reaches the edge is a problem for the milling or scoring (DFM checklist). The limits for trace width and spacing are in the design rules.
Thermal relief on pads
If a pad of a through-hole part is connected to the pour over its whole circumference, the pour drains the heat of the soldering iron or the wave, and the joint is hard to solder. A thermal relief connects the pad through a few narrow spokes instead. Use it for through-hole pads and large pads; small via connections to a plane are usually left solid, because they do not need soldering.
Isolated islands
When the pour is cut by traces or the outline, it can leave small copper islands that connect to nothing. They do not help; they can act as antennas and sometimes peel off. Remove islands with the "remove dead copper" or "remove islands" option of your CAD tool, or tie them to ground with a via. Run the design rule check after you fill the zones.
Solid or hatched
A solid pour is the better choice for almost every rigid board: lower impedance, simpler to check, no extra work. A hatched (grid) pour is rarely needed; it gives a weaker return path and more small gaps to check.
Before you send the files
Fill all zones again before you plot the Gerber files, otherwise they can hold old copper (the steps for KiCad are in KiCad design rules for a PCB manufacturer). Then upload the archive in the price calculator.
Frequently asked questions
Is a copper pour necessary?
No, but a ground pour helps most boards: shorter return paths, more current capacity and flatter boards. It is common on 2-layer boards.
What is thermal relief?
A connection between a pad and the pour through a few narrow spokes, so the pad does not drain the heat during soldering and the joint is easy to make.
What are copper islands and how do I remove them?
Isolated copper areas connected to no net. Use the option of your CAD tool that removes dead copper, or connect them to ground with a via.
How far should the pour stay from the board edge?
Keep copper away from the outline and from V-cut lines; check the edge clearance in the DFM checklist before you order.
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