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February 2013

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From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Tue, 26 Feb 2013 22:17:05 +0000
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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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