Assembly and Reliability Studies on Reworked and Non-Reworked QFN Packages

Quad Flat No lead (QFN) packages are being used as alternative components to some of the large form-factor packages due to their smaller footprint, improved thermal and electrical performance. During the past few years, there has been a surge of published papers by end users and component manufacturers on the topic of assembly and reliability of the QFN packages. Both empirical and simulation data has been published, which has helped several manufacturers to develop design guidelines and assembly processes for these packages. There exists a gap in the current published work on the assembly and reliability of high I/O count QFNs on thick boards. In addition, reliability of the rework process on QFNs is not well documented. Therefore, objectives of this research were the following: • Evaluate the second level reliability in thermal cycling for reworked and non-reworked QFNs; • Evaluate the reliability of different sized QFNs on 125 mil thick boards and compare with existing literature; • Failure analysis and recommendations for reliability improvement; • Land pattern comparison; and • Rework process improvement. Studies have been conducted on different QFN packages between 32 pins and 164 pins on 125 mil thick boards with Organic Solder Preservative (OSP) surface finish. Experimental design includes the use of different package sizes, alternate thermal pad patterns, and use of tin-lead versus lead free processes. A subset of some of the package types has also been reworked using variations of the standard BGA rework processes to evaluate its reliability. All packages were daisy chained. Boards were subjected to thermal cycling test between 0 to 100 deg C for 6000 hours. Data is analyzed using Weibull distribution. Failure mechanisms have been analyzed using dye penetration tests and cross sections. The reliability results are discussed in terms of package construction, type of land patterns and rework. Recommendations for process improvement are delineated.