I have a PCB that has 2 traces on one corner who's width only supports a ~10a load. I need support for a 20a load. Actually, it really only needs to support a 15amp load but I'm trying to build in some healthy breathing room.
Can I just solder a wire between the two leads in parallel with the trace? The terminal block and the relay both support 20amps and these are the only parts touched by the load. I know it's messy but I figure it's not as messy as having to wire all of this up off the PCB.
Yes. Fixing production errors by using jumper wire is a time honored tradition by those blessed with hindsight.
For Power rails or low speed GPIO (Think Push Buttons inputs or LED outputs), a simple wire would work. As a precaution, you could cut the trace, and carry the full load on the jumper you are adding. If the jumper fails, the load would try to go over the 10A trace, and can cause issues (i.e. fire).
High Speed Signaling or Protocols with tight tolerances, RF signal traces, Traces that require uninterrupted ground/vcc planes under them, matched impedance, etc, require significantly more thought into it.
Then again, some have wider tolerances than stated. There are many hobbyist projects/hacks that add high speed USB ports to a board with a CPU that supports it, using nothing but regular wires and protoboard, not to spec.
That is one option if your design can handle the additional (capacitive/inductive) impedance created by the wire.
Another option if the trace has no solder mask over it is to build up a large amount of solder along the trace in order to increase its ampacity.
I would recommend cleaning the trace and end connections with sandpaper, laying clean copper wire on top of the trace between the terminals, then cover the top of the wire with solder, from one terminal to the other, making sure the solder flows between the wire and the trace.
