Here’s the response I got from OSH Park:
Process-wise, it mostly doesn’t matter 95% of the time. In almost all cases, it’s VASTLY more important to consider design aspects, from noise, capacitance, and even just aesthetics. You want your board to work well and look good, and that’s the main thing.
In large-scale fabrication processes, there IS a situation where you’re best served by adding ground planes: It’s due to the desired “etch time”, and “copper balance”. If you have
No ground planes
Extremely large bare areas of the boards
… with long, thin traces
… and 5 mil clearances between some traces
LOTS of square inches of this order
Then you’re going to start seeing problems. A good example is a 10"x10" PCB, with a small circuit in the middle, running 5/5 differential pairs and maybe a couple other 5 mil traces from the center of the board to contacts at the edges, with no ground planes (we do actually see these from time to time). This board is likely to get really poor yields, with trace moves, eroded traces, or shorts.The problem here comes from A) Removing lots of copper takes a longer process etch time to let the chemicals do their thing and ensure clean removal and B) Wanting a shorter etch time to avoid over-etching the thin traces. There’s a spot where you simply can’t hit both requirements effectively, and so your process success rate drops. Modern fabs are pretty good at handling this, but it’s still hard on the process, and will be less consistent than a board with better copper balance.
The resolution if you add a ground plane to this board (even if it’s a disconnected fill far away from the circuit), it significantly reduces the need for a long etch time since you remove less copper. This brings the need for a long etch time down, and the process can be more easily realized with a shorter etch cycle.If you want more details on this, check out “thieving patterns”. This is what fabs add to production pcbs to ensure they A) Maintain a good, consistent copper balance, and B) don’t needlessly waste chemicals on parts of the production panel that don’t have designs on them. You’ll sometimes see these on larger designs going into production as well, to help minimize this type of effect.
Our process takes care of this internally, as we just make sure to generate sane copper balance across the whole of the production panel, which avoids this issue in most cases. The only time we still see it is 80+ square inch designs, which start becoming their own copper balance region we can’t effectively control or mitigate.
Given that I’m just driving two LEDs at a time I wouldn’t think the lack of copper fills would have any perceptible impact on its operation. But if there is a problem it would cost a whopping $6.85 to have three new boards fabricated.