Sampath,

I can not give you hard numbers, but maybe this will help.

Regarding fillet height - the taller/thicker/more massive it is, the greater
the likelihood it will crack/separate from the substrate when exposed to
thermal cycling.  To me, a crack is a potential point for moisture, etc.
ingress, therefore potentially equals a bad thing - subject to the products
use requirements.

Depending upon the effective volume (height and extension from device), a
grossly excessive underfill fillet may begin to show cracking at 1500/1800
cycles, whereas a more 'delicate' fillet will go to 2500 or more.  (thinking
of -40/+125°C cycles, generic, traditional, underfill.  Unfilled,
flux/underfills [no flow underfills] will begin fillet cracking earlier
[800-1000 cycles] because of no filler to moderate the expansion/shrinkage
of the underfill during cycling.  

Also note that all of the above generic comments are highly dependent upon
the specific underfills and substrate surfaces being tested.

My suggestion is that you immediately contact the potential underfill
suppliers in your area to find out what they feel best suits your substrate,
device passivation and screening requirements.  

Virtually all of the underfill suppliers have products 'tweaked' for the
various device passivations and substrate (PWB, kapton, or ceramic) surfaces.

Hope this helps somewhat

Steven Creswick - CTS RF Integrated Modules

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