DFM Rule Fundamentals

DFM rules are the fab's physical limits expressed as geometry. Knowing the handful that cause most escapes lets you design manufacturable boards from the start instead of reacting to a rejection.

Trace and space

Every fab has a minimum trace width and spacing driven by its etch process and your copper weight. Heavier copper etches with more undercut, so it needs wider spacing. Design above the minimum, not exactly at it — margin absorbs process variation.

Annular ring

Annular ring is the copper collar around a drilled hole. Too little, and layer-to-layer misregistration causes the drill to break out of the pad. Class 3 tightens this well beyond Class 2.

Rule	Class 2 typical	Class 3 typical
Annular ring	0.05 mm	0.075 mm+
Trace/space	0.10 mm	0.10 mm
Mask sliver	0.10 mm	0.10 mm+

Slivers and acid traps

Acute copper angles create slivers that under-etch or lift, and acute solder-mask gaps trap etchant. Avoid sharp intersections and keep mask webs above the minimum between fine-pitch pads.

Aspect ratio

A hole's aspect ratio (board thickness ÷ drill diameter) governs whether plating can throw evenly down the barrel. High aspect ratios — thick boards with small drills — risk voids and thin plating. Microvias sidestep this by spanning only one dielectric layer.

Isolated and hanging copper

Dangling traces, orphaned pours, and antenna stubs cause plating and RF problems. A DFM run flags them so they are removed before tooling.

Engineering noteThese rules are fab-specific. The same design can be perfectly manufacturable at one plant and marginal at another — always check against the deck for the fabricator that will actually build the board.

Master these fundamentals and the majority of first-spin rejections simply never happen.