QFN DFN Package PCB Layout Footprints

[fullmoon6661]

I learned from Renesas guideline which was referenced by Trinamic that for QFN/DFN packages, pad sizes should be 1:1 for the 'inner' side of the footprints, but longer for the outside.


I see conflicting recommendation from other manufacturers:
NXP (link):

Maxim Integrated (link link):


How it is supposedly the correct one?

If it is 1:1 for 'inner' side, but the package pads have tolerances, should I follow the Nominal size, or the Max size for the PCB footprint pads?

[jpanhalt]

It could be clearer.  See Capture1, which gives as Option 1 a pad the same width as the max.  Unfortunately, Renesas doesn't give guidelines for how to pick each option.  Capture2 clearly shows the pads wider.  Since two of the three recommend a wider pad, and the outlier (Renesas) gives wider as an option, that seems to be the way to go.  Maybe with the narrower pad one is expected to have a larger aperture in the solder mask (or no mask).

References:
Options (Capture1):
https://www.renesas.com/us/en/document/psc/20-qfn-package-outline-drawing-40-x-40-x-075-mm-body-050mm-pitch-ncg20

Pad wider (Capture2):
https://www.renesas.com/us/en/document/oth/qfn-mounting-manual

EDIT: It is frustrating dealing with the messy way images are handled here.  I intended to have Capture1 before Capture2.  Unfortunately, the first image uploaded created a huge blank space.  So, I uploaded Capture1 a second time.  It posted OK as the third image.  Then deleted the first, so now it's the second.  That's a real pain and requires multiple edits.

[fullmoon6661]

Thank you for the effort you put into this @jpanhalt! It does make sense that way. Other manufacturers I mentioned also seem to recommend the PCB pads just tiny bit wider, or same width as the Max.

[thm_w]

For the outside pads, extending significantly from the QFN IC makes it easier to hand solder, hand rework and inspect the solder fillet, but reduces the board density. Its a trade off depending on what is more important to you.

The one thing thats considered superior though are the rounded pads from NXP, no reason not to use those.

[fullmoon6661]

Indeed it's nice to have elongated outer side pads for those purposes.

I came across this rounded pads recommendation quite often which I think is from IPC-7351A. Now I wonder if I should round the PCB pads even when the package pads are not rounded?