Evaluation of QFN Technology with Optically Inspectable Solder ConnectionsAuthors: Dwight Daniels, Andrew Mawer, Paul Galles and WH Chan
Company: NXP Semiconductors
Date Published: 9/17/2017 Conference: SMTA International
Another area of considerable development has been with the QFN package assembly process itself. QFNs have always been molded on a 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 of these 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 the fact 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 results in only the joints under the package being inspectable by X-ray in a similar manner to BGA. However, BGA does not rely as much as QFN 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. This paper will go over leadframe and package manufacturing processes that can be used to potentially make the portion of the leadframe that is exposed on the side of the package to be solderable. This can lead to consistent solder joint fillet formation on the side of the QFN package that is easily inspectable including with Automated Optical Inspection (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” (IJ) or “wettable flanks” (WF). This paper will present 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 will be discussed.
QFN, Plastic QFN, Quad Flat Pack No-Leads, Bottom Termination Components, BTC, Inspectable Joints, Wettable Flanks, Punched QFN, Sawn QFN, Step-Cut, Dimpled, Automated Optical Inspection, AOI, Solder Joint Reliability, Board-Level Reliability.
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