Hi Gregg - it depends on the specific underfill product you are using. 
Some underfills like to stay put and some underfills love to run thru the 
vias. And covering the vias with kapton tape can actually make things 
worst instead of acting like a dam.  I recommend you run a quick test to 
assess the potential issue.

Dave Hillman
Rockwell Collins
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[TN] Are open vias in underfill area a problem?






Are there any concerns with .008" dia vias beneath a BGA requiring 
underfill?  Would these holes be sufficiently small to prevent "leakage" 
to the other side of the PWB?  The underfill material used will be Loctite 
3563.  I don't care if the vias fill, just don't want material draining 
out the other side.

Thanks,

Gregg

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