Solder Mask free Area advantages and recommendations

[xzswq21]

Hello
I'm designing a 4-layers PCB for a high speed ADC/ FPGA.
there are many 0.8mm pitch connectors/LFCSP/0201/0402/0603/0805/1206/..... packages on the board.
the surface finish is ENIG.
I have seen some people set a solder mask free area around the pads.

what's the advantage of solder mask free area around a pad?
how much the suitable size should I set for such a PCB?

Thanks

[DerekG]

what's the advantage of solder mask free area around a pad?

The solder mask adds some height above the board. Whilst 0.8mm is not fine pitch, when the pads get small & the associated clearances between them small, the solder paste stencil needs to be quite thin to avoid laying down too much solder which may lead to bridges between the pads. Under these conditions, the solder mask height may lead to a degradation in both the solder pad adhesion area & the associated mechanical strength of the joint.

[vzoole]

Solder mask size has to be bigger than the PAD because of manufacturing tolerances.
If they are the same size and the layer align is not perfect there could be solder mask on the PAD.

There is some nice info:
https://www.eurocircuits.com/pcb-design-guidelines/#soldermask