When designing a QFN footprint, I've read from (reputation limited, only 2) sources...
...that the QFN should sit approximately 50-75um (2-3 mil) high off the board for reliable solder joints. I assume that this "standoff" height is created by the solder joints themselves.
So far I have gathered:
- Use ENIG or OSP finish. This provides the most flat surface (necessary for applying the stencil, paste and placing the package).
- For the center pad (a.k.a. exposed pad, EP, thermal pad), use a SMD (solder mask defined) pad.
- For outside (a.k.a perimeter) pads, use non-SMD pads. NSMD pads will result in a pad that is most accurate to your design because copper etching has a tighter tolerance than solder mask.
- Extend perimeter pads towards center of package 0.05mm and away from package ~0.2-0.6mm. This provides a more reliable solder joint and potentially allows for easy rework since the pad will extend outside the package.
- For QFNs at a pitch of 0.5mm and below, keep PCB footprint pad width equal to package pad width. For 0.65mm and above.
- For QFNs at a pitch of 0.5mm and above, you can define the solder mask between pads. For QFNs at a pitch of 0.4mm and below, define an area that keeps mask out for each row of pads on the QFN.
There are many variables here.
The answer I am looking for would define a stencil thickness and aperture opening relative to pad size and pitch. Does the center pad or do the perimeter pads have the most effect on standoff height?
How important is the standoff height?
Are the datasheets simply warning us not to float the part with too much solder in the middle?
Past experiences in production environments as well as prototype (DIY, applying stencil and paste by hand) would be incredible.
What design parameters influence the "standoff" (or solder joint) height?