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It is when there is an inductance from the ground pin on the IC to a point that is the common ground for the whole system or circuit. When the IC switches, current flows through this impedance and makes the ground reference of the circuitry inside the IC become higher potential than the reference for the outside world.
The cure is to make this inductance very small by putting the ground pin to the PC ground plane layer with the minimum length of trace to the via and making the trace wide and the via a large diameter.
This causes other problems, like reducing the routing options, on the PC board and so a compromise with the trace and via widths is made.
A virtual pin is simply a balance point between the pins of a net. It is not a real pin, and may not even be a drilled via when the routing task is complete but is merely a location to hold the place of branching interconnect points. Therefore, you use a virtual pin as a terminal to define a fromto tree so that delay, length, width, and clearance rules may be set to control impedance and minimize skew.
In my opinion, the ground bouncing mainly aroused by the inductor between the Power and Gnd. So it is necessary to lower the inductance of the power plane and ground plane. The best method is to shorten the length of power trace and ground trace.
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