Circuits Assembly - April 2008 - (Page 54)

Protecting Parts ity to ESDS devices. We have observed device failures (<100V sensitive devices) in our case studies, even when the 12" rule was maintained in the application. In fact, all aspects of operator discipline will need to be addressed to fully protect these supersensitive devices.6 Based on personal experience with many case studies as a foundation, the attempt here is to provide suggestions on how to structure an ESD control program for fully protecting the handling of 20-100V sensitive devices. These are personal recommendations, based on many case studies involving subsequent device damage in electronics manufacturing environments with devices susceptible down to 20V. The suggestions have proven reliable in protecting devices and eliminating yield losses for such vulnerable devices. Redundant ESD controls become increasingly necessary. The suggestions below address <100V issues. 1. Humidity control: Humidity should be controlled to more than 40%. 2. ESD flooring: ESD floor (106 or less) should be implemented. 3. ESD carts: ESD transport carts with grounding mechanism should be used exclusively. However, no “open” (vulnerable) product should be permitted on the carts! All ESDS items should be housed in either closed static-dissipative totes (~108-9 ) or other static-dissipative packaging mediums. (Static-dissipative materials versus conductive materials are recommended to prevent rapid discharging of the ESDS items that may become inadvertently charged.) 4. ESD chairs: ESD chairs (106 ) with grounding mechanisms to the ESD floor should be used exclusively. 5. ESD work surfaces: Placing ESD mats on top of high charge generating work surfaces should be discontinued. All work surface laminates should be ESD-safe around and underneath the grounded ESD mat. 6. Totes and bins: Totes and bins for ESDS items should be made from static dissipative materials (~108-9 ). 7. ESD-safe tool handles and bottles: Use these ESD-safe alternatives exclusively. 8. Age-sensitive ESD packaging materials: Common static shielding bags, pink bags, etc. should be used only if 100% testing is accomplished, to verify they do not charge the ESDS item sliding on them to above its CDM rating. Preferably, use packaging that cannot degrade over time. (See Antistatic Materials section.) 9. Personnel grounding: All personnel should use wrist straps – connected to constant monitors – for the primary grounding mechanism. In addition, ESD footwear – preferably shoes (106 ) or sole grounders, not heel straps – should be required for secondary/backup grounding protection. 10. Shunt <100V ESDS devices: Wherever possible, <100V ESDS devices should be shunted (all leads shorted together) to provide additional ESD protection. This includes shunting the device when mounted into its board assembly. Inserting these devices in non-sliding mediums for storage and transportation is recommended over static shielding bags. Beware of potential contamination issues with certain conductive foams. 11. Ionizers: Ionizers should be located at every workstation where 54 Circuits Assembly APRIL 2008 <100V ESDS devices are handled. They should be balanced to take into account the most sensitive device in the facility. They should be monitored frequently. Even though all charge generators can be removed from an ESD workstation, some chargegenerating mechanisms invariably still exist, such as charging of devices and PCBs from hand coverings, plastic piece parts in the final assembly, etc. 12. Paperwork: Paperwork should be eliminated totally from the manufacturing areas. “Paperless” operations only! 13. ESD garments: ESD garments (106 ) should be used exclusively, with secure connection from the garment to the person (tight, conductive cuffs, etc.). 14. Common plastics in the product design: Plastic piece parts should be eliminated from the final product design, if possible. Non-charge generating alternatives should be used. 15. ESD discipline: Stringent ESD training and auditing is necessary to bring operator awareness to very high levels.5 Thorough, constant checking and measuring all ESD controls in use is required by in-house auditors. 16. Hand coverings: Check charging of product while using hand coverings.4 This is a difficult charging mechanism to eliminate, but it is crucial to be aware of the issue and eliminate it. 17. 18" rule: The distance rule (described earlier) should be increased to 18" minimum. 18. Taping operations: Be careful here, as most taping operations (even de-taping operations with “ESD-safe” tapes) cannot be controlled to safe levels via ionization.7 We view all taping operations as very high risk. Look for a way to eliminate them. 19. Insulators: Special attention should be given to eliminating all insulating materials at every ESD workstation. 20. Daily operator checks: All operating personnel should perform more rigorous ESD checks than typically occur, ensuring all the ESD controls work. The local operating personnel should accomplish daily checks (as a minimum) for charge generation with a static field meter. ■ References 1. G.T. Dangelmayer and Terry L. Welsher, “Class 0 ESD Trends: Implications for Back and Front End Processes,” A2C2 Magazine, May 2005. 2. International SEMATech, International Technology Roadmap for Semiconductors, 2003. 3. Dave Long, “7 Mandatory Considerations Before Handling Class 0 ESDS Devices” Conformity, September 2007. 4. R.J. Peirce, “The Most Common Causes of ESD Damage,” Evaluation Engineering, November 2002. 5. R.J. Peirce and Craig Zufelt, “Limitations of ESD Gloves and Finger Cots,” SMT, February 2007. 6. James Colnar and R.J. Peirce, “Improving ESD Program Discipline,” CIRCUITS ASSEMBLY, January 2007. 7. R.J. Peirce and Jay Shah, “Potential ESD Damage When Using Adhesive Tapes,” Evaluation Engineering, June 1996. Roger J. Peirce is director of technical services for Simco Ionization for Electronic Manufacture, an ITW Co. (esimco.com); rpeirce@esimco.com. circuitsassembly.com http://esimco.com http://circuitsassembly.com

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

Circuits Assembly - April 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Design and Modeling of High-Speed, High-Density 3-D CSPs and Memory Modules The ‘Big Brush Off’ Revisited Impact of Soldering Atmosphere on Solder Joint Formation Beyond Moore’s Law ESD Control For Class 0 ESDS Devices Growing Your Brand This Year’s Model Tech Tips Reflow Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - April 2008

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