On most MCU's and other IC's they have multiple pins assigned to GND/VSS (excluding AGND or other specific function related grounds). Do I need to attach each one to ground or can I leave one or two floating. Is there any risk to that or is it down to the IC I'm using?
Reason I ask is that I have an annoying GND pin on a MCU that I just can't route too on a double loaded board and I really don't want to go Blind via.
Connecting all ground pins is extremely important to ensure signal integrity. A classic problem with amateur projects and designs is that they don't include enough ground pins on a connector and/or they group all the ground pins together - whereas they should be evenly distributed.
Each ground pin has associated inductance, and a changing current induces a changing voltage which can corrupt the signal. V=L
di/
dt, so a 1mA signal turning on or off in 1ns though a lead with 1nH inductance will induce a 1V drop! Having N parallel grounds reduces the inductance by a factor of N - and hence induced voltage.
Having a ground next to a signal lead also reduces reflections due to impedance variations in the transmission lines.
Reducing the loop area reduces EMI, but that is secondary to the other considerations.
The inductance (and various parasitic capacitances and V-vs-I graphs) is specified in the associated IBIS file (warning IBIS files are large and need careful interpretation).