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Avoid the void
Keywords: Voiding, Bottom Terminated Component, Automotive
Perhaps the single biggest challenge in PCB assembly today is voiding under bottom terminated components such as QFN’s (quad flat-pack, no lead), D-Paks, and LGA’s (land grid arrays). Many bottom terminated components like the QFN have a large thermal pad on the bottom side which offers excellent thermal and electrical grounding properties. However, effectively soldering the component can be a challenge. Many customers such as automotive are requesting less than 10% voids to improve relaibiltiy. The large deposit of printed solder paste required to solder the thermal pad typically induces flux entrapment and subsequent voiding. Large voids and/or a high number of voids cause decreased thermal conductivity, and lower the mechanical strength of the resulting solder joints. The large solder paste deposit may also cause the component to float and open up the electrical connections around the perimeter of the component. Another factor has been, the transition to lead-free technology, which has been marked by the use of various SAC type alloys. Most applications for surface mount assembly use SAC305 (96.5Sn/3.0Ag/0.5Cu) with a melting temperature range of 217-220°C. The higher temperature required for soldering and the increased surface tension of the SAC alloy exacerbate this voiding issue. This paper will focus on the techniques for optimizing the assembly process for QFN components. Best practices for lead-free reflow profiling, stencil aperture design, solder paste volume control, and the importance of flux chemistry to minimize void formation will be discussed
Ed Briggs, Senior Technical Engineer Automotive
Indium Corporation
Clinton, NY

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