Hi Wayne - another good one is the SMTAI Conference paper/ppt by Scott
Nelson of Harris. The IPC has also published the IPC-7093 standard which
covers QFNs as part of the BTC family. Loads of good information in the
standard.
Dave
From: Wayne Thayer <[log in to unmask]>
To: <[log in to unmask]>
Date: 02/26/2013 04:17 PM
Subject: Re: [TN] QFN Reliability
Sent by: TechNet <[log in to unmask]>
Nice presentation at
http://www.dfrsolutions.com/uploads/courses/2010-08_SMTA_FL.pdf
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Monday, February 25, 2013 4:13 PM
To: [log in to unmask]
Subject: Re: [TN] QFN Reliability
Thank you, good input.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Geert Willems
Sent: Monday, February 25, 2013 3:08 PM
To: [log in to unmask]
Subject: [TN] QFN Reliability
QFN packages pose a major solder joint reliability risk and more now than
a couple of years ago. The reason is that today highly quartz filled (85%)
low CTE molding compounds having CTE's typically in the range of 7 to
10ppm have found wide application. For the lowest CTE mold compounds the
CTE is comparable to that of ceramics (CTE=6.7ppm Al2O3) used in LCCCs and
CBGA. This creates for nearly all plastic packages a significant thermal
mismatch with the PCB (CTE=14-18ppm). Unfortunately, the lowest CTE mold
compounds are often used for QFN packages which are because of their lack
of significant stand-off the most critical packages regarding solder joint
reliability. Additionally, for QFN, the Si die contributes considerably
to the mismatch. Details of package size, package thickness, die size,
die thickness, PCB thickness determine the lifetime. Werner Engelmaier's
strain formula for leadless packages (IPC-D-279) cannot be used because it
only considers shear strain and it does not take into account the
flexibility of package and PCB. That is where the major difference lies
between QFN and LCCC, the CTE of nowadays mold compounds is comparable to
that of ceramics but the elasticity is an order of magnitude larger making
the flexibility of the package along with that of the PCB an important
contributor to solder joint lifetime.
More info on low CTE mold compounds and their impact on solder joint
reliability (BGA, TSOP, QFN) as well as on wire bond reliability: iNEMI
free webinar, February 27th, 2013, 8h30 GMT, www.inemi.org.
Geert Willems
imec - Center of Electronics Design & Manufacturing Leuven, Belgium
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