Circuits Assembly - October 2008 - (Page 23)

Better Manufacturing wider use. Here, an organic coating of 8-20 µin is applied over etched bare copper. During reflow, the coating dissipates where flux and heat are present, leaving the solder in contact with the copper board surface. It offers a planar board surface, is low cost, easily reworkable and widely available, as most fabricators have the means to apply OSP. On the other hand, the shelf life is relatively short. That’s not a problem for highvolume, quickturn applications, but not ideal when shelf life requirements exceed 12 months. While initially able to sustain up to three thermal excursions (reflow and/or wave solder), this might diminish toward the end of its shelf life. Also, the OSP coating is not conductive, and this must be taken into consideration when soldering test points. ImAg. Immersion silver, with a very planar surface of 3-12 µin of silver codeposited with an organic inhibitor, presents an alternative surface finish less expensive than gold, but with a longer shelf life than OSP. Silver is an extremely conductive metal and is sometimes favored for improved signal integrity in high RF and microwave applications. But, like your silverware, it will tarnish over time. This can occur during assembly, though it is often cosmetic and does not affect solderability or joint integrity. A more serious problem is silver migration. Silver ions, under certain conditions, may migrate from cathode to anode, forming dendrites that reduce resistance (and may even, in some extreme cases, form a short). We don’t have as extensive experience with ImAg as with other finishes, so the jury is still out, but it does show a great deal of potential and has been adapted to many applications. ImSn. Platers do not like to do large volumes of immersion tin because of the difficulties in disposing of the wastes, principally thiourea or other nasty chemicals required to get the tin to bond to copper. Moreover, ImSn has shown poor performance in multiple heat cycle tests. Throw in the prospect of tin whiskers, and ImSn becomes less attractive. There is no panacea surface finish (oh, how we long for those simpler times of through-hole). But there are a plethora of choices and, as with everything else SMT, the selection is applicationdriven. Shelf life, surface planarity, conductivity and certainly cost are the key factors. Remember, we’re all in this together. n Au.: Special thanks to Jim Hall of ITM Consulting for his contributions to this column. Your Answer to the Your Answer to the Factory Automation Puzzle Factory Automation Puzzle NEA Offers it All Since 1988, NEA, Inc. has been providing the industry with certified remanufactured assembly and automated test equipCertified Equipment ment which operates as well as or Sales/Service/Installs better than new with the latest upgrades available and at an affordable price. But did you know that NEA is also a comprehensive, turnkey operation providing world-class service, installs, long-term operator/maintenance/programming training, regular and emergency service, complete spare parts, feeders, and the best warranty in the industry? Spare Parts/Feeders Contact NEA, Inc. today to find out more about how this all-in-one partnership can benefit you, your company Long-term Training and your bottom line. Tel: 800.225.1632 www.neainc.com circuitsassembly.com Circuits Assembly OCTOBER 2008 23 http://www.neainc.com http://www.neainc.com http://www.pkaymetal.com http://www.pkaymetal.com http://www.circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - October 2008

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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