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摘要

"Evaluation of QFN Technology with Optically Inspectable Solder Connections" Authors: Dwight Daniels, Andrew Mawer, Paul Galles and W.H. Chan; dwight.daniels@nxp.com. Abstract: In quad flat no-lead (QFN) package assembly, QFNs have always been molded on multi-up (matrix) leadframes. For QFNs where each individual unit has its own mold cavity, a punching process is typically used to singulate units from the leadframe. For QFNs where multiple units are molded in a single cavity, a saw process is used to singulate individual units. Both processes can result in leadframe bare copper on the exterior perimeter of the part being exposed. One of the perceived disadvantages of QFN had been that on packages with this exposed copper, which generally gets oxidized prior to SMT reflow, it does not consistently wet with solder in typical SMT processes. Lack of a QFN side fillet means only the joints under the package are inspectable by x-ray, similar to BGAs. However, BGAs do not rely as much as QFNs on a very consistent solder paste printing process, since most of the volume of a BGA solder joint is supplied by the volume of BGA sphere. The authors review leadframe and package manufacturing processes to potentially make solderable the portion of the leadframe exposed on the side of the package. This can lead to consistent solder joint fillet formation on the side of the QFN package that is easily inspectable, including with AOI. The two most common ways to achieve at least a partially wettable lead on the sides of the QFN are the use of "dimpled" leadframes, or by step-cutting the package so that part of the edge of the lead gets electroplated with Sn. QFNs that promote the formation of side fillets are referred to as QFNs with "inspectable joints" or "wettable flanks." This paper presents a case study on step-cut QFNs that are able to form these optically inspectable joints. PCB footprint design, stencil design, SMT processes, inspection data, along with reliability data, are discussed. (SMTA International, September 2017)