Why not simply use a very thin veneer of a good tacky flux on the pads, with no solder paste? First you apply the flux to that part, then print solder paste on the rest of the boards, place all the parts including the ubga, and reflow. Remove the apertures in the stencil for that part.
You can try this by taping off the apertures on the stencil for the ubga and do it on one or two CCAs.
The tacky flux will hold the ubga in place quite well, but it does take careful handling. Something like Alpha WS1208 or similar. Just use the solder balls on the ubga to solder it in place. Additional solder is not needed. If that works buy a new stencil minus the ubga apertures.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vargas, Stephen M
Sent: Wednesday, March 06, 2019 3:56 PM
To: [log in to unmask]
Subject: [TN] Solder Paste Size

Good afternoon:

    We've got an assembly down here where I need to screen-print down to an aperture size of 6mils round (uBGA). I am using a type 4 solder paste and I'm specifying a 4 mil thick (fine grain stainless steel) stencil with a Nano coat. Boards have ENIG finish. Just wondering what success (or lack of) others are having with this type of application. Might I need to consider a move to type 5 solder paste? Anything else I should be considering? Thanks in advance.

Regards,
Steve Vargas