Circuits Assembly - April 2008 - (Page 50)

Protecting Parts ESD Control for Class 0 ESDS Devices By Roger Peirce How to protect devices between 20 and 100V. lass 0 ESD-sensitive (ESDS) devices (especially those sensitive below 100V) are exploding onto the scene, and with them comes a new set of ESD risks and challenges.1, 2, 3 (ESD Association industry standard S20.20 does not address ESD controls for devices with sensitivities less than 100V.) After years developing ESD control programs proven to work reliably for <100V sensitive devices in scores of facilities, this summarizes some of the problems associated with protecting these state-of-the-art devices with standard ESD controls. In addition, we suggest specific controls necessary for reliable protection for devices sensitive between 20 and 100V. Note: The device sensitivity voltage ratings described throughout are Human Body Model (HBM) ratings. Charged Device Model (CDM) and Machine Model (MM) susceptibilities can be substantially lower. We have witnessed the steady progression of increasing device sensitivities. In the past few years, scores of facilities have struggled with ESD damage directly resulting from limitations of the standard ESD controls used so effectively for the past 30 years. We commonly hear, “We’ve always had a good ESD control program in place, and never had any ESD losses – until we introduced our latest-generation device” (which often now can be less than 100V sensitive). As it turns out, many tried-and-true ESD control methods have serious limitations in conjunction with devices sensitive below 100V. Some major areas of con- C cern are listed below. (Note the charging values reviewed were determined using ESD Association standard measurement techniques.) Wrist strap grounding. When connected securely, voltage on personnel wearing wrist straps can be below 10V. However, many failure modes with wrist straps exist that can easily send that voltage number skyrocketing upward (loose connection, dry skin, powders, etc.). Without constant monitoring devices to alert operators of poor connection issues, our studies have shown voltage on personnel with wrist straps can average easily upwards of 50-100V in typical manufacturing environments, even when the wrist strap is good and the connection looks and feels secure. Constant monitors are a must for <100V programs. ESD footwear grounding. Our studies indicate many standard, commercially available floorgrounding implementations for the grounding of mobile personnel may not be sufficiently reliable to be the primary grounding system for handling <100V sensitive devices. It is not uncommon to perform voltage tests on personnel (with heel straps or ESD shoes properly connected and tested) walking on an excellent ESD floor (for example, floor tiles with resistance to ground of 106 ) and find that typical voltages on mobile personnel are 50-100V. (As with all the issues addressed here, these voltage numbers historically have not been a concern with device sensitivities well over that level. The advent of <100V sensitive devices now makes them major issues.) It is also noted that the ESD Association provides excellent standard measurement techniques to evaluate all aspects of mobile operator grounding. If an ESD circuitsassembly.com 50 Circuits Assembly APRIL 2008 http://circuitsassembly.com

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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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