Circuits Assembly - October 2008 - (Page 27)

Medical Electronics Assembly Table 1. Guidelines for Dealing with Obsolete Components Issues Timing Resolutions • Sound working relationship with component vendor • Sustained communications • Knowledge of substitute components • Procurement maintains close liaison • EMS provider understands component vendor’s product lines • Anticipate product or pricing changes • Track component vendor’s business success and failures • Look for signs of component delivery problems • Keep good records of alternate component suppliers Price changes Dubious vendors medical electronics assembly can assist in finding other components that can replace obsolete ones, SnPb or Pb-free, perfectly. An EMS provider can avoid major OEM surprises by maintaining periodic alerts warning a component’s life is coming to an end, for example. At the same time, the OEM can be counseled on tradeoffs and options. For instance, the OEM can either design out a particular component and test new prototypes. Or, it can (try to) buy sufficient inventory of soon-to-be obsolete components to cover production for years to come. The key is to rely on a knowledgeable EMS provider. The main service here is disciplined communications to keep the OEM updated on obsolete component schedules. There are three basic guidelines for helping OEMs avoid product catastrophes related to obsolete or hard-to-get parts (Table 1). First is timing. This demands a sound working relationship between component supplier and EMS provider. Communication between these two is critical because a particular design is based on assumptions a specific component will be available over the long run. In some cases, if demand for that component doesn’t materialize, the component vendor may ease it out of production. If that happens, it is imperative the component vendor and EMS provider settle on the right substitute. A close working relationship could mean advance notification of such component changes or substitutions, saving time and expenses later. Second, major component price changes at times, in effect, put a component out of reach. Price increases occur for any number of reasons and subsequently, pricing structure changes. Component manufacturers should provide EMS providers advance notice that for the ensuing three months, for example, they are going to do whatever they can to sustain certain pricing levels. But, by a certain given date, component pricing is expected to increase by a certain percentage, for example. Third is dubious vendors. A small, underfunded supplier may suddenly cease production. In cases like this, it is vital to have sound EMS provider design and assembly support to make this issue transparent to an OEM. If one supplier is no longer available, others with similar components must be so the OEM’s PCB assembly does not miss a beat. New RoHS Rules EU’s latest RoHS Directive addendum is expected to include medical electronics. Some OEMs are well en route to understanding the various assembly changes involved with Pb-free. But for those who waited, the road to Pb-free assembly may be bumpy and costly. circuitsassembly.com ! Figure 3. This Pb-free profile of a solder paste shows the higher temperature range, in this case more than 250°C. As stated, one issue deals with component manufacturers all but ending SnPb production in favor of Pb-free versions. For the reluctant medical electronics OEM, this means hastily engineered, expensive and time-consuming re-spins with Pb-free components and boards. Other assembly-related areas of concern include PCB materials and surface finishes, solder joint reliability, incorrect thermal profiles, hybrid lead and Pb-free assemblies, and using separate PCB design layout, fabrication and assembly contractors. SnPb materials don’t necessarily translate to Pb-free. For example, using materials like FR-4 with an inappropriate Tg for Pb-free assemblies can severely damage the PCBs, as the Pb-free reflow temperature profile ranges up to 255°-260°C, much higher than SnPb’s peak of 230°-235°C (Figure 3). Also, board surface finishes must be able to withstand these higher reflow temperatures. Pb-free finishes include electroless nickel immersion gold (ENIG), immersion silver, immersion tin, organic solderability protectants (OSP) and a special Pb-free brand of hot-air solder leveling (HASL). Solder joint and assembly reliability are achieved during assembly if trained manufacturing engineers carefully implement several process requirements. A number of variables must be considered. Included are alloy melting temperature, an alloy’s wetting characteristics, surface tension properties, solder balling and bridging, cosmetic effects of flux at higher reflow temperature and several others. Applying a wrong thermal profile without carefully considering PCB materials, surface finishes or the correct Pb-free solder can have catastrophic effects during assembly. Care also must be taken to correctly implement leaded and Pb-free components on hybrid assemblies. n Ed.: For more on Pb-free surface finishes, see Better Manufacturing, pg. 20. Zulki Khan is president and founder of NexLogic Technologies Inc. (nexlogic.com); zk@ nexlogic.com. Circuits Assembly OCTOBER 2008 27 http://www.nexlogic.com http://www.zk@nexlogic.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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