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Assume you want a trace to be able to carry 20 amps of current on 1oz copper with a temperature rise of 20. Using an online calculator this yields 12.60mm trace width.
My question is, can you split that width among bottom and top layers (6.30mm each) and stitch together those layers using vias? If so, is there any special considerations with this method?
1 - Yes. With attention to detail by both you and the board house, you can move a hundred amps with +/-10% sharing.
2a - The internal traces will run warmer than the external ones. Some online calculators address this.
2b - Pay attention to the vias; they have a large impact on how evenly the current is spread among the layers. Depending on the board stackup, you might have 2 oz copper traces but only 1 oz copper in the vias. If you want to get picky about it, a via is a very short trace whose width is equal to the hole circumference. Use the hole diameter minus one plated copper thickness as the diameter. This is equivalent to the average of the inside and outside circumferences of the copper tube.
Also, while it is good to spread out some vias along the trace run, the clusters of vias around the ends (or any high-current branches) have the biggest impact on current sharing.
At the very least, you need to do some derating as the two traces are not going to share current exactly 50-50. When we parallel connector pins, we add 25% to the calculated number of pins and round up. I think we do the same thing with wires. So as a SWAG I would add 25% to your calculated trace widths.
I don't think we've ever done that. We would either increase the trace width appropriately, or go with heavier copper, which would be 2 oz Cu in your case.
The current carrying capacity of a trace is determined by the cross sectional area and the length. Another factor to consider is temperature rise.
If you have one trace that is 1000mil x 50mil, they will have the same resistance as two traces with dimensions of 1000mil x 25mil.
So yes, you can spit a 12.6mm trace into two 6.3mm traces. One other thing to consider is that if vias are used for stiching, they also need to be able to handle the current, and vias also add small amounts of resistance. So use a current carrying calculator for vias and make sure you have a sufficent number of vias and size of vias.
