PCB Clearance Standards: Understanding IPC-2221
How to prevent arcing and ensure high-voltage safety.
When designing high-voltage circuits, the spacing between traces (clearance) is a matter of safety, not just performance. If traces are too close together, the air between them can ionize, leading to a spark or 'arc' that destroys the board. The IPC-2221 standard defines the minimum distances required to prevent these failures.
Clearance vs. Creepage
Clearance is the shortest distance through air between two conductors. Creepage is the shortest distance along the surface of the insulating material. High-voltage designs must account for both. While IPC-2221 focuses on clearance, standards like IEC 60664 provide deeper guidance on creepage, which is affected by 'pollution degree' (dust and humidity).
IPC-2221 Spacing Categories
The standard breaks down spacing requirements into categories: B1 (Internal layers), B2 (External bare conductors, sea level), B4 (External coated conductors), and others for high altitude. Coated boards (using solder mask or conformal coating) allow for much tighter spacing because the coating acts as a dielectric barrier with higher breakdown strength than air.
The Effect of Altitude on Breakdown Voltage
As altitude increases, air becomes less dense and its dielectric strength decreases (Paschen's Law). A PCB that works perfectly at sea level might arc in a high-altitude aircraft or a satellite. Designers must significantly increase clearances for products intended for aerospace or high-altitude industrial use.
Implementing Clearance in Your CAD Tool
Don't just rely on a single 'global' clearance rule. Modern CAD tools allow you to set 'net-class' specific rules. High-voltage nets (like AC mains) should have their own class with much larger clearance requirements than low-voltage logic signals. Use our clearance calculator to determine the exact values for your specific voltage and assembly type.
FAQ
What is the minimum clearance for 240V AC?
According to IPC-2221 (B2 - External bare), the minimum clearance is approximately 1.25 mm. However, safety standards like UL often require even larger distances for consumer products.
Does solder mask count as a coating for IPC-2221?
Technically, yes, but its thickness can be inconsistent over trace edges. Many conservative designs treat solder-masked areas as 'bare' (B2) and only use 'coated' (B4) rules for boards with high-quality conformal coating.
How do I increase creepage without moving components?
A common trick is to mill a slot (air gap) in the PCB between the high-voltage pads. Since the air gap has no surface, the creepage distance is forced to follow the slot, effectively increasing the path length.