ESD control for class 0 ESDS devices: how to protect devices between 20 and 100V.

Class 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 concern 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 floor-grounding 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 [10.sup.6] [ohm]) 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 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."

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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 attention 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 proximity 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 ([10.sup.6] [ohm] 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 (~[10.sup.8-9] [ohm]) 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 ([10.sup.6] [ohm]) 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 (~[10.sup.8-9] [ohm]).

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 ([10.sup.6] [ohm]) 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 <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 charge-generating 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 ([10.sup.6] [ohm]) 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); [e-mail].