I was hoping someone would point to a nice document or IPC standard, but so far I am unlucky.
Let me first comment on some of the other reactions:
@chitransh92 I do know that fabricators tend to modify the design without notifying the designer. Perhaps it is more often that they split larger shapes into smaller ones. However, a coverage of 60...80% is mentioned sometimes by others as well.
@WhoKnewKnows Exactly, but do you know of any guidelines or standards for the shapes, maximum sizes, coverage, etc.?
@Lakshamana Balakrishnan I do not see how that calculator can help...
@robertferanec Perfect footprint, but unfortunately Library Expert does not create smaller paste mask 'pads' for big pads.
Now my own set of ideas.
The shape and size of the paste mask opening depends on many parameters:
1) Thickness of the stencil
2) Type of solder used
3) PCB finish used
4) SMD or NSMD (Solder Mask Defined)
5) Production method of the stencil
For thermal pads there are also the via's which may or may not be filled. Size of the hole, solder mask, etc.
Some manufacturers spend more time on how to produce there products then others, but at the same time also limiting the options of stencil thickness and solder type.
One of the interesting documents I have found on the paste mask is:
https://smtnet.com/library/files/upl...Guidelines.pdfUnfortunately, no mentioning of dividing large pads into smaller sections.