For a 20 A current path on 1 oz copper, using a 20°C temperature-rise target, an online calculator suggests a trace width of about 24.8 mm.
Can that width be effectively split between the top and bottom layers (for example, ~12.4 mm on each layer) and connected with a via array so that both layers share the current?
If so, what practical considerations should be taken into account regarding current distribution, via sizing and quantity, thermal performance, voltage drop, and layout symmetry?
Yes, this is a common approach. Splitting the current between top and bottom copper can reduce the required trace width on each layer, provided the layers are tied together with enough vias.
The vias are usually the limiting factor. Make sure the via array has sufficient current-carrying capacity, since the current must transfer between layers through those plated barrels. It’s worth calculating the resistance and current capacity of the vias, not just the traces.
Also, pay attention to where the current enters and leaves the conductors. The via clusters near source, load, and branch points often have a bigger impact on current sharing than vias distributed along the middle of the trace. A symmetrical layout generally helps achieve more even current distribution.
One thing to keep in mind is that even with a symmetrical layout, the current won’t divide perfectly between layers. Small differences in path resistance, via placement, and connection geometry can skew the split.
There’s also a thermal asymmetry to account for: internal traces run warmer than external ones for the same current, so the two layers won’t reach the same temperature even if the current is split evenly. That makes the internal layer the limiting conductor. Sizing both layers to exactly half the original width assumes a perfect 50/50 split under identical thermal conditions, neither of which is guaranteed in practice. Some margin on the width is usually worth it.
One aspect worth considering is the voltage drop introduced by the via array itself. Even if the traces are sized correctly, the vias become part of the current path and contribute their own resistance. At 20 A, even a few milliohms can result in a noticeable voltage drop and additional heating.
It can be useful to estimate the resistance of a single via based on its barrel length, plated copper thickness, and finished hole diameter, then calculate the equivalent resistance of the via array as parallel conductors. Adding that to the trace resistance gives a more realistic estimate of the total DC path resistance.
A useful check is to look at the current bottlenecks, not just the long trace sections. Even if the parallel traces are adequately sized, the current often has to converge into a connector pin, terminal, fuse, or component pad. Those transition regions can become the hottest part of the path if the copper necks down too quickly.v Where possible, widen the copper locally with pours or teardrops around high-current pads and transitions. This helps reduce localized resistance and heating at the points where the current path is most constrained.