Question 1

Why do internal traces need to be wider than external traces for the same current?

Accepted Answer

Internal traces are embedded in FR-4 dielectric, which is a poor thermal conductor (k ≈ 0.25 W/m·K). External traces benefit from convective cooling to air, dissipating heat much more effectively. The IPC-2221 internal coefficient (k=0.024) is half the external coefficient (k=0.048), resulting in roughly double the required cross-sectional area.

Question 2

Is IPC-2221 still accurate for modern PCB designs?

Accepted Answer

IPC-2221 is conservative and widely accepted but based on data from the 1950s. IPC-2152 (2009) provides updated thermal models based on modern simulation and testing. For designs with heavy copper (>2 oz), high layer counts, or thermal vias, IPC-2152 gives more accurate results. IPC-2221 remains a safe starting point for most standard designs.