A certain quantity of these traces is connected straight to the ground-plane.
What happens when an unpowered board (and completely disconnected from any peripheral) is on a grounded antistatic mat?
Will the ground-plane reach equipotential with the mat, because the GND traces make contact with the mat?
Or nothing happens, because both the V+ traces AND the GND traces touch the mat?
What happens when an unpowered board (and completely disconnected from any peripheral) is on a grounded antistatic mat?
All potentials on the board settle to zero (ok maybe not the battery and RTC circuit) but they would float to whatever ground is (which is close to 0V) It would be almost like attaching a 1MΩ resistor to every point on the board and then to ground. If there was air in between the board and the mat it would be even higher.
simulate this circuit – Schematic created using CircuitLab
Will the ground-plane reach equipotential with the mat, because the GND traces make contact with the mat?
Eventually it will, depends on how much charge exists on the motherboard. which should be really low.
The mat is 1MΩ (or more, I just measured mine and across the whole mat is 10MΩ to the button, yes I realize its not super accurate, but its close) to ground, most mats are made of dissipative material so this also won't affect the high speed signaling (noticeably anyway) if you run the motherboard right on the mat.
We do have ESD tables and they are 1MΩ also but underneath that thin 1MΩ layer there is a conductive metal layer. I would not run a motherboard on this because of a risk of capactivly coupling the signals to ground or to each other. This probably wouldn't affect the operation of the board but it could so I would not do it.
Most anti-static mats are static dissipative. The mat will not short out any traces, and unless you have very, very high impedance signals it should be fine.
If you have a conductive surface, things change. Some bags that are delivered with electronics are coated in a thin layer of conductor. Some people suggest putting the motherboard on this bag during testing, but this is not advisable, as it can short out connections.
Remember that the mat has a pretty high resistance to earth, many megaohms. Earthed wrist straps are typically connected to earth through 1 Mohm.
This is to leak static away rather than short it hard to earth. Shorted static is likely to damage something its charged up on, such as when a board with a static charge on is dropped down on the mat. It also makes people jump. So the mat is not a sheet of earthed tinfoil. This is also for safety, as the consequences of an electric shock from the mains or wherever would be much worse if people were connected straight to earth by their earth strap or the mat they're leaning on.
Therefore the tracks on your board that touch the mat are touching a large resistor to earth. I would say that the impedance between V+ and earth on your board there is pretty low because of a lot of decoupling capacitors across the rail and a lot of conduction paths, some resistive. So the effect of touching either rail to the mat produces a series circuit with the impedance to earth much higher than the impedance between the tracks. Nothing dramatic happens.
