IPC-2221B Conductor Spacing: The Table Everyone Reads Wrong

One voltage, five different answers

Take 50 V between two conductors. What spacing does IPC-2221B require?

• B1, internal conductors: 0.1 mm
• B2, external uncoated, sea level to 3050 m: 0.6 mm
• B3, external with permanent polymer coating: 0.13 mm

Same voltage, a 6× spread. The row you pick matters more than the voltage does over most of the practical range.

That is the whole point of the table, and it is where most mistakes happen — a design gets checked against B2 when it is coated, and ends up with six times the spacing it needed, or checked against B3 when the coating was never actually specified in the fab notes.

Why coating buys so much

An uncoated external conductor sits in air, exposed to whatever lands on it. The failure mechanism is not usually dielectric breakdown of the air gap — it is surface tracking. Contamination and humidity form a conductive film across the laminate surface, and the discharge follows that film rather than jumping the gap.

That is why B2 is so much more demanding than B1. An internal conductor is encapsulated in cured laminate: there is no surface for contamination to land on and no humidity path. The only mechanism left is bulk dielectric breakdown, which needs far less distance.

A permanent polymer coating (B3) restores most of that. It seals the surface, so you are back to something closer to a bulk mechanism. Conformal coating (B4) goes further still.

The consequence: on an uncoated board, the surface is the weak point, not the air. Cleanliness after assembly matters as much as geometry. Flux residue under a no-clean process is a conductive contaminant if it is not fully reflowed.

Altitude

Air gets thinner and its breakdown strength drops with it. Above 3050 m, IPC applies a derating factor to the uncoated external spacing.

This catches avionics and high-altitude equipment, but also anything shipped by air in an unpressurised hold and anything qualified at altitude. If your product spec names an operating altitude above 3 km, the B2 numbers in the table are not the numbers you need.

Coated conductors are much less affected, because the mechanism is no longer surface discharge through air. Coating is often the cheaper answer to an altitude requirement than opening up every clearance on the board.

What the table is not

IPC-2221B gives you creepage and clearance for functional insulation on a bare board. It is not a safety standard.

If you have mains on the board, or any requirement for reinforced or basic insulation between a hazardous voltage and a user-accessible circuit, IEC 60664-1 and the relevant product standard govern — and their numbers are considerably larger. They also bring in pollution degree, material group (CTI), and the distinction between creepage and clearance as separate quantities.

IPC-2221B collapses those into one number for one purpose. Using it where a safety standard applies is a compliance failure, not a conservative approximation.

Practical guidance

Decide the coating before you route, not after. It changes clearances by a factor of four or more, and retrofitting coating onto a design that assumed it will not help if the fab notes never said so. Put the condition in the fab notes explicitly. "Conformal coat per IPC-CC-830" is a specification. "Coated" is not. Apply the spacing to the worst-case voltage, not the nominal. A 48 V rail during a load dump or an inductive kick can be several times nominal, and the spacing has to survive the transient. Check both sides of a via. A via carrying a high-voltage net has an annular ring on every layer, and the spacing to neighbouring copper has to hold on the internal layers too — where the B1 numbers apply and the checking tools sometimes do not look. Do not rely on solder mask as a coating. Solder mask is not a permanent polymer coating in the IPC-2221B sense. It is porous, thin over conductor edges, and not qualified for the purpose.

The minimum conductor spacing calculator returns all the IPC-2221B conditions side by side for a given voltage, with altitude derating, so you can see the cost of each choice before committing.