Just saw a web knock-down of the Barnes and Noble eBook reader. The larger
Chip Scale Packages appear to have underfill applied to each corner, which
has me intrigued. See

 

http://www.techrepublic.com/photos/cracking-open-the-2011-barnes-noble-nook-
e-book-reader-wi-fi/6243892?seq=35
<http://www.techrepublic.com/photos/cracking-open-the-2011-barnes-noble-nook
-e-book-reader-wi-fi/6243892?seq=35&tag=content;thumbnail-view-selector>
&tag=content;thumbnail-view-selector

 

Does anyone know the criteria for this? Underfill is applied in flipchip
assembly to prevent the silicon cracking because of the thermal mismatch in
CTE between glass (3ppm/degree C) and pcb (FR4 at about 16ppm/degreeC). I
guess the CSP might have a similar issue - no doubt the silicon chip inside
the package also gets stressed as the package stresses, which implies that
there isn't a lead frame to give compliance, or a FR4 laminate interposer
like in bga packages that doesn't need stress relief.

 

Cheers........

 

Andy Kowalewski

AdvantagePCB Pty Ltd

+61 (2) 9499 4853

 



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