Circuits Assembly - October 2008 - (Page 33)

Components an acceptable joint. Figure 9 (online) shows a solder fillet inspection sheet prior to internal quality control, taking measures for all the aforementioned factors. Figure 10 (online) shows the inspection sheet following QC. The white areas of the table are unacceptable fillets; the yellow areas are acceptable fillets. Fillet yields increased from 85.2% to better Figure 4. Profile used for QFNs. than 99.7%. Occasionally, 100% good fillets are achieved. Figure 11 (online) shows typical results. By controlling the solder paste, reflow and board quality, it is possible to achieve toe fillets with a QFN, which Figure 5A. Misaligned solder Figure 5B. Excessively large solFigure 6. Variable pad length should translate into better mask. der mask openings. created by the solder mask. device reliability. Testing of QFNs with fillets that pass these criteria were subjected to 1000 hr. of burn-in at 70°C, followed by 600 thermoshock cycles between -40° and +85°C with no failures and no degradation in electrical performance. Results were achieved with SnPb solders, but the same principles should improve parts soldered with Pb-free solders. n Figure 7a.(left), b (center) and c. Examples of desirable solder masking. Acknowledgments The authors wish to acknowledge Ed Konopka for his useful review and proofreading of this paper. References Figure 8a. (left) and b. Inspection results. 1. IPC-A-610D, Acceptability of Electronic Assemblies, February 2005. 2. Carsem application note, “Comprehensive Users Guide,” April 2002. 3. Duane Benson, “How to Succeed the First Time with Ultra-Small QFN Packages,” Microwave Engineering Online, Oct. 31, 2007. Joseph G. Ameen is manufacturing process engineer and Gilson V. Geralde is in drafting at Herley-CTI Inc. (herley-cti.com); jameen@herley-cti.com. reflow and board quality are controlled, it is possible to achieve nearly 100% filleting on the toes, while maintaining a good ground connection. Our internal inspection criteria are shown in Figure 8. A fillet that climbs the toe at least 50%, is not a thin coating on the toe, and is not excessive in volume (i.e., does not intrude into the space between adjacent I/Os) is considered circuitsassembly.com Circuits Assembly OCTOBER 2008 33 http://www.herley-cti.com http://www.circuitsassembly.com

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Circuits Assembly - September 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing 'Checking Up' on Medical Electronics Solder Ball Attachment Using Laser Soldering Improving QFN Reliability Reflow Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Alternative Energies Eastern Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - October 2008

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