Circuits Assembly - April 2008 - (Page 52)

Protecting Parts tion may be necessary to ensure scheduled maintenance (cleaning of the emitter pins, etc.) is performed. Improper maintenance can result in dangerously degraded performance – even to the point of causing device damage. Transport carts. Electronics manufacturers around the world transport unprotected ESDS items throughout their facilities on “ESD carts” that connect the conductive or static dissipative cart to the grounded ESD floor via drag chains or conductive wheels. It is a very common mode of product transportation. Our studies again show cart charging of 50-75V is common with this approach, even when the “best” carts, drag chains and ESD floors are employed. That 50-75V level (never a problem before) can be a killer issue with <100V ESDS devices. ESD chairs. Similarly to the transport cart discussion above, many companies permit operating personnel to handle ESDS items where the only grounding device is via their ESD chair (i.e., no wrist strap or foot grounding device). Our case studies show voltages on personnel sliding in their ESD chairs to be typically in the 75-150V range (with some chair designs faring better than others) – again using high quality chairs, drag chains/conductive wheels, and ESD floors. Wrist strap grounding is recommended at all times for <100V ESDS devices. Hand coverings. Operators in electronics and semiconductor manufacturing environments frequently use gloves, finger cots and other types of hand coverings to prevent grease and oils on their bare hands from contaminating ESD-sensitive devices, PCBs and final systems. Most facilities select “ESD-safe” versions of these hand coverings, expecting that the ESD hazards associated with regular, insulative hand coverings (plastics, latex, vinyl, etc.) will be eliminated. However, when outfitted operators handle PCBs and ESDS devices, they have been shown to be very likely to cause subsequent charging (many thousands of volts is common!) on those boards and parts (even when worn by grounded operators).5 For safe handling of Class 0 devices, this potential issue needs to be addressed. Insulator distance discipline. S20.20 properly warns against bringing charged objects into close proximity with ESDS items to avoid possible field induction failure modes. That specification calls for a set distance to be maintained by operator discipline. For <100V Class 0 devices, our case studies indicate the distance should be increased substantially to fully protect these more sensitive devices. Many facilities have implemented a 6" or 12" rule to separate charge-generating materials in close proximcircuitsassembly.com floor is used, it is important to conduct those voltage studies on personnel as they walk – and ensure the resulting levels do not exceed the level of the most sensitive component handled. Very good results can be achieved with high quality floors and footwear, but testing is crucial. Not all implementations result in “very good results.” Antistatic materials. All sorts of effective “antistatic” materials (materials that reduce triboelectric charge generation) are in use. Examples of common antistatic materials include pink poly bags, pink antistatic foams and bubble pack, inside layers of static shielding bags, clear IC shipping tubes, clear antistatic plastic “clamshell” containers, document holders, etc. Antistatic materials are designed to keep devices sliding against them from charging. These materials do this well; however, they do not reduce the charging to 0V, even under their best performance. Many studies have shown typical charging on assemblies sliding out of high quality (new) static shielding bags was 20-30V, even when all conditions were optimum! Similar numbers (20-30V) were experienced on devices in antistatic IC tubes, parts sliding in pink antistatic bags, etc. (In addition, as the chemically loaded antistatic materials lose their properties over time, more severe charging occurs routinely. In fact, one of the most serious expenses documented during the past 20 years has been device failures resulting from contact with aged antistatic materials.4) Ionizers. Some ionizers common in electronics manufacturing facilities are designed to maintain a balance of only 150V. S20.20 recommends limiting offsets to below 50V. These relatively large typical voltage offsets can become a liability with <100V ESDS devices. Facility ESD coordinators should ensure their ionizers meet the Class 0 device requirements. In addition, more atten52 Circuits Assembly APRIL 2008 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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.