Solder Joint Reliability of QFN Packages

Luke England, Yong Liu
Richard Qian, Ji-Hwan Kim
Fairchild Semiconductor

QFN packages have become mainstream designs for mobile applications. As more applications adopt the QFN style packages, I/O count requirements are increasing. The typical method for increasing pin count in a QFN package is to increase the body size to accommodate the additional lead fingers. This is undesirable though, as mobile device users are pushing for smaller package sizes. By using a dual row design, more lead fingers can be added in the same overall body size. This increases the overall performance to package size ratio.

Previous studies published on dual row QFN packages focus mainly on design considerations for manufacturing. [1-3] Since the current design uses standard lead frame processing techniques, no additional processing strategies are needed compared to single row QFN production.

This study focused on the board level solder joint reliability of a 28 lead dual row QFN package. Prior to manufacturing, a mechanical modeling DOE was performed for various dual row QFN footprints to estimate the solder joint lifetime through temperature cycle testing. The modeling was followed by prototype manufacturing of daisy chain units. The daisy chain devices were subjected to temperature cycle testing according to JEDEC specifications. Testing was continued until the complete lifetime estimation curve could be obtained. It was determined that a dual row design can actually improve solder joint reliability performance when compared to a single row design of similar body size.

Since there are no lead fingers in the immediate corners of the package for the dual row design, which is typically the highest stress area of the package during testing, the overall solder joint lifetime can be increased. Although the typical failed lead fingers on the dual row package were still the farthest distance away from the package center, these lead fingers are not located in the package corners. Final results show that the dual row QFN package has good performance through temperature cycle testing, with a performance increase over standard single row QFN packages.

Mechanical modeling of two dual row QFN design configurations (staggered and inline) for a given x-y package size was performed. The staggered configuration was predicted to have better thermal cycle performance for two main reasons:

•The staggered configuration simply has more pins/leadfingers within the given package size.

•The inline configuration has only two leadfingers on each end, while the staggered configuration has three leadfingers on each end.