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32 SMT Magazine • November 2016 nificant impact on the voiding and solder pro- trusion. Voiding depended more on the com- ponent type and design. The solder mask ring would help prevent solder from flowing down the via at the thermal pad. The solder mask ring results in some unsoldered area under the QFN component, which looks like more voiding at the QFN thermal pad. Acknowledgements The authors would like to thank Dennis Willie and Robert Pennings at Flex AEG Labs in Milpitas for their help in the assembly. Many thanks to Tu Tran and Francoise Sarra - zin at Flex AEG Milpitas lab for the cross-sec- tion images. SMT References 1. Quan Li, Lu Li, Richard Rowan, and Ma- hesh Shah, "PCB Layout Guidelines for PQFN/ QFN Style Packages Requiring Thermal Vias for Heat Dissipation," Freescale Application Note 3378. 2. Yang Boon Quek, "QFN Layout Guide- lines," Texas Instruments Application Report, SLOA122, July 2006. 3. Jennifer Nguyen, David Geiger and Dong- kai Shangguan, Ph. D., "Assembly Challenges of Bottom Terminated Components," Proceeding of IPC APEX EXPO, San Diego, CA, 2012. 4. Jennifer Nguyen, David Geiger and Mu- rad Kurwa "Voiding Mitigation of Bottom Ter- minated Components," Proceeding of SMTAI, 2013. 5. Derrick Herron, Dr. Yan Liu, and Dr. Ning- Cheng Lee, "Voiding Control for QFN Assem- bly," SMTA Pan Pacific, 2011. 6. Richard Coyle, Heather McCormick, Peter Read, Richard Popowich, and Debra Fleming, "Temperature Cycling Performance of a Quad Flat No Lead (QFN) Package Assembled with Multiple Pb-Free Solders" Proceedings of SMTAI conference, 2003. David Geiger is director for assembly and test technology at Flex. Anwar Mohammed is a senior director at Flex. *Jennifer Nguyen left Flex in August 2016. The National Institute of Informa- tion and Communications Technolo- gy has succeeded in the development of flexible optical design method for superconducting nanowire single- photon detectors (SSPDs or SNSPDs). This technique enables SSPDs with a broadband high detection efficiency reject a specific wavelength, and is effective for multidisciplinary ap- plications in fields such as the quantum cryptography, fluorescence spectroscopy, and remote sensing that require high efficiency over a precise spectral range and strong signal rejection at other wavelengths. This achievement appeared in the Scientific Re- ports. The reported results have been partially ob- tained as a part of JST-SENTAN pro- gram and AMED-SENTAN program from April 2015. The developed SSPD with the di- electric multilayer and the optical de- sign method can be applied for wide wavelength region between ultraviolet and mid-infrared, and thus provides an important ba- sis for development of application of SSPD to quan- tum cryptography, fluorescence spectroscopy, and re- mote sensing. In support of the SSPD measurements, the NICT team collaborated with Japan' s Osaka University and Scotland's University of Glasgow through the NICT internship scheme. Flexible Optical Design Method for Superconducting Nanowire Single-Photon Detectors THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY