Reference plane gaps

A trace crossing a break in the plane beneath it; the return current has to detour, and the loop that creates radiates.

Missing return vias

A signal changing layer with no nearby via joining the two reference planes, so the return current has no path across.

Plane stitching

Areas where two ground planes overlap with no via joining them within λ/20; between stitching points the planes form a cavity that resonates.

Edge stitching

Stretches of board edge where a ground plane reaches the outline with no via fence; an unstitched plane edge radiates like a slot antenna.

Stackup

Signal layers with no adjacent reference plane, and adjacent signal layers with no plane between them to stop broadside coupling.

Decoupling capacitors

IC supply pins far from a capacitor to ground, and decoupling capacitors whose ground pad has no via nearby. The loop through them sets the frequency above which they stop decoupling.

Thin power traces

Supply copper squeezed through a thin trace between two wide pieces, so all the current goes through it; and, when a net group gives a net its current, traces narrower than IPC-2221 asks for.

Via stubs

The unused length of a through via below the layer a signal leaves on, which resonates.

Test point stubs

Test points on fast nets reached by a branch off the route instead of sitting on it. The branch is an open stub that makes the signal ring and radiates.

Length matching

Members of a matched group whose delay differs from their reference by more than the budget. Measured in time, not millimeters: an inner-layer millimeter and an outer-layer one are about 25% apart.

Impedance

Computed characteristic impedance against a target, and discontinuities along a net where width or reference plane changes.

Long nets

A net long enough against the wavelength to couple to free space efficiently.

Copper near the board edge

Traces or planes close to the edge, where fields are not contained.

Differential pair routing

Pair halves that run apart, and one half running beside another net's pour more than the other. Either way the halves stop matching and part of the signal turns into common-mode current, which radiates.

Floating copper

Pour islands connected to nothing, which pick up and re-radiate with no path to ground.

Crystals and oscillators

Signals routed under a crystal, and crystals close to the board edge or to a connector, where a cable becomes their antenna.

Copper under antennas

Traces, vias or pours of any net, ground included, in the area of an antenna or an RF module's printed antenna, on any layer. They couple into the antenna and detune it.

Power input filtering

Power entering through a connector with no capacitor near it, so the loads' switching current flows out along the supply cable and a surge meets nothing on the way in.

Switching regulator nodes

Switch nodes with more copper or track than the current needs. The node swings the full input voltage in nanoseconds, and its copper couples that into everything nearby.

ESD protection at connectors

Lines leaving the board through an edge connector with no clamp on them, clamps placed far from the connector or after the IC they protect, clamps with no short path to ground, and other signals routed under a clamp.

Shield and chassis ground

Connector shells and plated mounting holes connected to nothing, and chassis nets with no path to the board's ground, so a discharge to the enclosure has to cross the circuit.

Reset lines

Reset inputs held only by a pull-up with no filter capacitor, or with the capacitor far from the pin, where a fast transient on the trace resets the board.

Cable resonance

Cables assigned to connectors that are at their best as antennas at a frequency this board produces. A cable's resonances depend on the cable, not the layout, so this needs no solve, and it decides more about a radiated result than the board does.