ESD and Static Control Questions

Q: I have read the White Paper 1: A Case for Lowering Component Level HBM/MM ESD Specifications and Requirements and found the ESD Control Programs and Resulting Data (Chapter 1, Page 20-23) particularly interesting.

Assuming a production environment with ESD flooring, footwear (and clothing), by the time a person walks to a workstation and sits down, the voltage of this persons should not exceed 500V (or even 100V as seen in Figure 3). That would mean even a seated operator in this case would not need to wear wrist strap, that theory would be correct right? After sitting down and this person sits on a stool (feet off the floor) with resistance to floor < 1.0×10exp9ohms, any HBM risk would be further reduced wouldn’t it?

A: Hello ****.  Nice try.  Even if you have an ESD flooring system and even if you have ESD footwear and even if you have an ESD task chair with ESD casters or an ordinary task chair with an ESD chair cover (very effective as well), ESD smock on… you STILL have to wear the wrist strap when seated at an ESD workstation.
�
The only time, per ANSI/ESD S20.20-2007 page 4, 8.2 Personnel Grounding, that personnel in the EPA (ESD Protected Area) should be without a wrist strap is when doing standing or walking about operations, and then two conditions must be met;
·         “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is less than 3.5E7 Ω…”
·         “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is greater than 3.5E7 Ω and less than 1.0E9 Ω and the BVG is less than 100 v per 97.2…”
�
This is what is said about seated personnel:
�
“When personnel are seated at ESD protective workstations, they shall be connected to the grounding / Equipotential bonding system via a wrist strap system.”
�
Hope this helps.   I guess you could say redundancy is good in the realm of ESD.  It’s the weak link in the chain that will cause an ESD event.  If someone lifts their ESD footwear from the ESD flooring system while seated, they can tribocharge to above 100 volts.  It takes only 0.3 seconds of charge time to exceed 20.20 requirements.  If personnel is seated and getting up to go to break, it seems best to stand up, remove the wrist strap from the wrist, carefully set it down and walk away from the ESD workstation.  Worst case is to take the wrist strap off while still seated, set it down, put your hand on the ESD workstation and near ESDS devices, then stand up out of the task chair before leaving the work station.  Under proper conditions and with good bench mats, clean ESD floors, ESD task chairs, etc. in place, no ESD event.  The problem with ESD events is that we cannot see, hear, feel them.�
�
The only alternative to not wearing a wrist strap while seated may be the used of a smock with a grounding coil cord attached to it.  You can see the footnotes on the 20.20 document at the bottom of page 4 for further details.
�
 We adhere to and meet or exceed requirements put forth in ANSI/ESD S20.20-2007 or IEC 61340-5-1, which assumes a target HBM of 100 volts and less.

Q: How does ESD Gloves, Aprons and Bunny Suit help for static discharge? WIll it not be mandatory to wear the wrist band, footwear and heel strap along with it?

A: People are a great generator of static electricity, among other things. The ESD gloves, aprons, etc. enable you to work and interface with sensitive components and equipment while protecting them from an ESD event as well as other things like contamination from human skin, street clothes, dirt, dust, etc. It will be mandatory for personnel ground to wear a wrist strap when in seated operations in a manufacturing environment or in the field. For personnel ground in an environment where you are standing or walking about, you do NOT have to wear a wrist strap if you have sufficient ESD footwear in conjunction with an ESD flooring system. The above requirements are per ANSI/ESD S20.20-1999 Table 1.

Q: Is it necessary to use the ESD garments? Is it not possible to ground the charges developed through a person’s clothing through the human body which is grounded through a wrist strap, footwear or heel strap while working at an EPA?

A: This depends who’s in charge.  If the ESD manager wants to require ESD personnel clothing and redundancy of ESD compliance for improved ESD control, they are the one to determine that.  More and more I’m seeing the use of ESD smocks outside of the cleanroom environment.  The company and their clients are reaping the benefits of increased quality control and reduced “out of box failures”.  Cloth is proven to be a better conductor of ESD charges than some types of clothing, but not everybody wears cotton.  Some people have dry skin and the ESD garments are a good idea for added protection and a requirement in some environments.

I’d like to add that when seated in an EPA, you must have your wrist strap on regardless of ESD Garments, ESD footwear, ESD flooring system, etc.  That’s a requirement of ANSI/ESD S20.20-2007 page 4 section 8.2 Personnel Grounding; “When personnel are seated at ESD protective workstations, they shall be connected to the grounding/equipotential bonding system via a wrist strap system.”

Further, per Table 2 note 2, “For situations where an ESD garment is used as part of the wrist strap grounding path, the total system resistance including the person, garment and grounding cord shall be less than 3.5E7 ohms.”

Q: Is it necessary to use an ESD Chair when the humany body is already grounded through a Wrist Strap, Footwear or Heel Strap when working in an EPA?

A: I think it’s a great idea.  I can be sitting at a workstation with an ESD flooring system, have a wrist strap on and when I get out of my chair, not generate more than 50 volts.  I may not generate more than 5 volts.  But what if I take my wrist strap off and jump out of my chair?  I can easily generate 100’s of volts.  Oh, by the way, jumping should not be allowed in an EPA.  If you have great ESD shoes or sole grounders and you keep at least one foot firmly planted on the ground at all times, then maybe your ESD chair is redundant, but still an added piece to the chain in your EPA system.

Q: We have an ESD tile floor in our testing lab. During a recent AS9100 audit we were asked why we don’t use wrist straps. Since our ISO9100 & 2 certified calibration provider only uses them when they have to certify the repair of a system and we don’t do that do we need more than the flooring? (booties, straps, etc.)

A: The answer to your question is, YES. In accordance to EOS/ESD standards, an ESD floor in conjunction with Heel Grounders, ESD Booties, ESD Foot Wear does NOT take the place of using Personnel grounded Wrist Straps at the workstation. So the basic ESD program would include the technician wearing a Wrist Strap.

Protective personnel grounding products such as; heel straps, booties and shoes working in conjunction with an ESD floor are designed for processes within your ESD program where the technician has to be mobile within an ESDPA, (ESD Protected Area) where the electrostatic sensitive device is not protected in an ESD protective container or shielded in some manner, e.g. ESD Bag, Bin, Tote, Box, Tray, Container, etc.

If your process has technical personnel setting or standing at a workstation they DO require grounding via a Wrist Strap. So one doesn’t necessarily take the place of the other, and both may be required depending on your personnel and their mobility throughout the plant.

If your technical staff removes their wrist strap to transport an ESD sensitive device or component outside the ESD protected area, the device also needs to be shielded as well as the technician being properly outfitted with ESD shoes, booties or heel straps. Within the ESDPA, they do require the additional shielding protection of the device, but it is always a good idea when practical.

I hope this helps address your question, please let us know if we can be of further assistance with any ESD questions or ESD protective product requirements you and your team may be sourcing.