Frequency of testing ESD Technical Elements in an EPA
Q: We have a number of carts, racks, and chairs in the clean room. When we do our weekly, monthly and semi-annual checks for ESD and grounding, do we do a statistical sample or do we check all of each item. Is there a standard that explains this?
A: You ask a very valid question and it just so happens that I received a similar question on this not long ago.
I wish there was one and only one ESD Bible that’s all inclusive and complete; from design to audit. The ESD Association has the ANSI/ESD S20.20-2007 document which gives us guidelines in establishing an ESD control program. But as far as a standard explaining when to audit each and every ESD technical element, I have yet to find a good source. The tables contained within 20.20 will give you “required limits” for various technical elements, but not a frequency of when to audit or test these systems. Between JEDEC Standard 625-A Table 2 of page 9 and other sources, I’ve put together these recommendations, but it’s up to you to implement them and tailor them to your unique processes;
Typical Facility Areas Requiring ESD Protection:
· Receiving
· Inspection
· Stores and warehouses
· Assembly
· Test and Inspection
· Research and Development
· Packaging
· Field Service Repair
· Offices and Laboratories
· Clean Rooms
Elements of Effective Training Programs
1. Successful training programs cover all affected employees
2. Effective training is comprehensive and consistent; Fundamentals of ESD, details of your facility’s ESD Control Program, and each person’s role.
3. Use a variety of training tools and techniques; equipment product sheets, 20.20, etc.
4. Test, certify and retrain; motivate and build employee pride and make this an ongoing process, provides opportunity for improved procedures, reminds and reinforces what they’ve learned.
5. Feedback, auditing, and measurement; use the tools you’ve been given- review, audit, analyze, feedback, and improve. Audits can be internal or 3rd party.
As far as Periodic Verification/Testing, you should include these ESD Materials/Equipment
A. Grounding/Bonding Systems < 1.0Ω impedance for equipment gnd conductor and should be checked weekly-visual and quarterly for gnd continuity
< 25Ω to equip gnd from auxiliary ground
< 1.0E9Ω for Equipotential ground Per ANSI/ESD S6.1-2005
< 2.0 Ω for tools and < 1.0 Ω for solder tip to ground, although this value can go to = 1.0E5Ω = 0.8E6Ω to 1.2E6Ω per ANSI/ESD S1.1 Required to check daily prior to each shift minimum or use constant monitor
C. Constant Monitors; Manufacturers specs/User defined; check quarterly
D. Footwear Method 1: <3.5E7Ω in combo with person, floor, footwear to gnd per STM97.1 or Method 2: <1.0E9Ω per 97.1 AND < 100 volts per 97.2 Required to check daily before each shift minimum
E. Flooring and floor mats 2.5E4Ω to 1.0E6Ω for Static Conductive and 1.0E6 Ω to 1.0E9 Ω for Static Dissipative, check quarterly, inspect gnd continuity weekly **Note, I like being near ESD conductive for flooring and floor mats, 1.0E6 ohms to less than 1.0E8 ohms for bench mats/work surfaces**
F. Personnel grounding with Garment; groundable point to gnd <3.5E7 Ω per STM2.1, visual check daily
G. Garments Sleeve to sleeve at 100 volts or auto ranging: Static Control < 1.0E11 Ω
Groundable Static Control <1.0E9 Ω, inspect after each wash; test per STM2.1
H. Air Ionizers < +- 150 Volts per STM 3.1; check charge decay performance and balance to <35volts semiannually unless auto calibrating.
I. Seating < 1.0E9 Ω per STM 12.1; check quarterly
J. Mobile Equipment/Carts/Shelving < 1.0E9 Ω per S4.1; Quarterly
K. Packaging; Manufacturers specs per S541, S11.31, etc.
L. Awareness/Signage; verification, User defined
Special considerations:
ESD carts racks and chairs being used in your EPA (ElectroStatic Discharge Protected Area) would obviously be used in conjunction with an ESD flooring system; albeit Static Conductive (2.5E4 Ω to 1.0E6 Ω from Point To Point and/or Resistance to Ground) or Static Dissipative (1.0E6 Ω to 1.0E9 Ω PTP and/or RTG) and that all personnel is utilizing the ESD flooring system by wearing ESD footwear as well.
More needs to be said about shelving per ESD TR20.20-Handbook: shelving systems are frequently included as part of an ESD protected workstation and are used to store both packaged and unpackaged ESD sensitive products. If that is the case, they should be treated as an ESD worksurface and should be properly covered with the proper resistance matting or esd protective material, grounded, and free of unnecessary static generators. If you have processes at different steps within your EPA Cleanroom, then you should have all of the shelves properly covered and grounded or properly marked as such.
Mobile equipment such as portable task carts, in-process storage carts, and mechanized skids that are used for heavier assemblies, can take advantage of the flooring system via ESD casters, drag chains (minimum of 12” coverage on ESD flooring system recommended), and cable and ball assemblies. The various surfaces can be measured in combination with the ESD flooring system to earth ground for compliance to 20.20 standards.
Seating. Another secondary grounding system that can take advantage of the ESD flooring system is seating. The resistance from a person seated in an ESD chair through the ESD flooring system to electrical ground should be less than 3.5E7 Ω if it is used as the “primary grounding source”, or less than 1.0E9 Ω as measured per ANSI/ESD STM 12.1 via ANSI/ESD S20.20-2007. You can achieve this with an ESD chair with ESD casters or an ordinary task chair with the conductive Chair Cover with drag chain. Personnel would be expected to wear the proper clean room smock for proper contact with the seat and chair back. For non-clean room conditions, cotton shirt and denim jeans are sufficient if it’s within personnel dress code.
That’s about all I can think of for now. Let me know if you have additional questions. It’s good to see people incorporating this.