Journal of SMT Article

Reliability Study and AF Modeling for SnAgCu Solder Joints and SnPb Solder Joints in QFN Packages

Authors: Dong Hyun Kim, Mudasir Ahmad, Sue Teng
Company: Cisco Systems, Inc.
Date Published: 1/31/2010   Volume: 23-1

Abstract: Leadless QFN (Quad Flat No-lead) packaging has become a popular choice in the application of wireless handset, power management and analog baseband due to its small form factor and improved thermal and electrical performance. The solder joint in QFN packaging makes a mechanical and electrical connection between the chip and board, so the solder joint reliability is one of the critical issues in the development of QFN packages. However, the solder joint reliability of QFN packages assembled on thick boards, such as 93mils or 125mils thick, has never been demonstrated. Also, it hasn’t been demonstrated that the conventional Coffin-Manson or Norris-Landzberg fatigue models accurately predict the AFs (Acceleration Factors) for the QFN packages. Therefore, the objective of this study is twofold: 1) to evaluate the 2nd level reliability of QFN packages on thick PCB assemblies and 2) to develop AF models for both SnPb and lead-free SnAgCu solder joints in QFN packages. To achieve these objectives, a DOE (Design Of Experiment) was developed using the following six parameters, and test vehicles were built and thermal-cycled.

Solder: SnPb vs. SnAgCu
Body: 10mm vs. 7mm vs. 5mm
Die to package ratio: small (18-28%) vs. large (40-47%)
Board thickness: 8 layer, 93mils vs. 16 layer, 125mils thick
Thermal profiles: 0-100C 30min cycle vs. -40-125C 1hr cycle
Exposed-pad via design: open vs. plugged

Thermal cycling test results showed that the AF between 0-100C and -40-125C thermal profiles is ~3.2 for SnAgCu solder joints and ~1.8 for SnPb solder joints. It was also found that solder joint reliability can be enhanced by making solder joints that have convex-shaped fillets, rather than an increased solder joint standoff. Using the characteristic cycles-to-failure, Coffin-Manson type AF models were developed for SnPb and lead-free SnAgCu solder joints in QFN packages and compared with conventional

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