.....and much easier to rework.

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Steve Gregory
Sent: Wednesday, June 08, 2011 6:06 PM
To: [log in to unmask]
Subject: Re: [DC] Underfill for chip scale packages

Hi Andy,

Corner bonding, not underfill. It helps with surviving drop shock
without 
having to go through all the trouble of underfilling. Corner bonding is
lots 
easier to do...

Steve

-----Original Message----- 
From: Andy Kowalewski
Sent: Wednesday, June 08, 2011 6:53 PM
To: [log in to unmask]
Subject: [DC] Underfill for chip scale packages

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-n
ook-
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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