Monthly Archives: December 2008

19 Dec

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; Read More →

05 Dec

What is Static Electricity?

Q: What is Static Electricity?

A: Static Electricity can be defined as an electrical charge at rest.  ESD is the transfer of static charges between bodies or objects at different electrical potentials. This may be caused by either direct contact or by induction of an electrostatic field.  Static Charges are generated when two materials are rubbed together. This term is called Triboelectric Generation or Tribocharging

Factors that Influence Tribocharging:

  • Intimacy of Contact Speed of Separation
  • Conductivity of Materials
  • Triboelectric Series Position
  • Relative Humidity Read More →
05 Dec

The difference between an ESD Control Program and Plan

Q: I’m having a hard time understanding the difference between an ESD Control “Program” and an ESD Control Program “Plan”… Can someone explain what is required? Do I need two documents, one “Program” and one “Plan”, do I need to develop a single ESD document? I’m not sure what the difference between a program and a plan is.

A: It is my understanding that the program is ANSI/ESD S20.20-2007’s way of providing requirements to establish, implement, and maintain an ESD program.

It is my understanding that the “Plan” is the organization’s scope within that “Program” that conforms to their own internal requirements while evolving with changes to technical elements as time goes by.

05 Dec

ESD Warning Signs/Labels and which one to use

Q: What is the difference between the symbols used in ESD warning signs / labels: yellow hand w/ black background; black hand w/ yellow background; attention; triange/hand with arch; warning; etc. and how do you decide which one to use?

A: As per ANSI/ESD S8.1-1993, there are two types of ESD awareness symbols identified.

The ESD Susceptibility Symbol:

ESD Susceptibility Symbol

ESD Susceptibility Symbol

  • Consists of the Triangle, the reaching hand with slash through it.
  • This means ESD sensitive devices or assemblies are present.
  • DO NOT TOUCH or Handle Properly.
  • If the device sensitivity is known, it can be added to the label.

ESD Protective Symbol

ESD Protective Symbol

ESD Protective Symbol

  • Consists of the Triangle, the reaching hand and an Arc around the Triangle.
  • This indicates ESD Protective Material such as chairs, mats, and wrist straps.

Another common ESD symbol for safety would be;

Common Point Ground

  • Common Point Ground

    Common Point Ground

  • This symbol looks like a bull’s eye and may be black and yellow.
  • This represents connecting ESD control materials or equipment to electrical or 3rd wire ground.
  • You can consult the standard S6.1 from the ESD Association for more on this.

On our website we have various ESD Labels, ESD Warning Signs and ESD Control Area Signs and Posters for your EPA (ESD Protected Area). The labels can be used on bags, boxes, and totes. The Caution Label symbol is still popular and is represented by a circle with three arrows entering into it.

The Attention Labels utilize the ESD Susceptibility Symbol and are represented by four different types:

  1. Small ESD attention labels- Destructible type or tamper-evident and is good for closing small bags containing ESDS components
  2. MIL 129 Label – Also destructible type or tamper-evident label
  3. ESD Attention Label – a label that also allows writing space
  4. Dry Packaging & ESD Susceptible Label

ESD Control Area Signs and Posters.

ESD Control Area Poster

ESD Control Area Poster

Black on Yellow or Yellow on Black print is just used for ease of reading and the MIL 129 label is the only one that I know that uses a black hand in a yellow triangle instead of a yellow hand in a black triangle.

05 Dec

The difference between Low Static and Static Dissipative

Q: What is the difference between Low static 3.5kv carpeting and static dissipative carpeting? When used on walls is 3.5kv carpeting ok in electronic equipment rooms?

A: When people refer to 3.5 kV carpeting I believe they are referring to the threshold voltage that people can feel as a nuisance static shock. We deal primarily with manufacture, test, assembly, and application environments where the end-user is protecting expensive electronic components, explosives, assemblies, etc. and the threshold for their needs is down to 100 volts and less.

I believe the 3.5 kV carpeting is considered to be somewhat antistatic (resists or has reduced tribocharging abilities) and is treated topically with some temporary chemical. These types of carpet do not satisfy our needs to provide long-term solutions for the commercial, industrial, and even consumer electronics industry.

I’d like to find out more about what you’re using the carpet for. Are you using it on the walls to deaden noise or create some special environment for audio design? If you need some kind of ESD protection, can you find out what your voltage threshold is- or what is the highest acceptable voltage that your environment can tolerate?

We offer ESD carpet in broadloom form and in tile form in both static conductive (typically around 2.5e04 or 25,000 ohms to 1e06 or 1 Meg Ohm) and static dissipative (1E06 – 1E09 ohm). As the resistance increases, the generated charge dissipates less rapidly to the point that a charge potential exists somewhere in the system and an ESD event occurs. This ESD event may occur without the end user knowing, but it may damage or destroy sensitive devices. Having a textile with a resistance in the static conductive range will discharge this charge potential more rapidly and work to prevent a charge from getting too high in the first place. Different textiles tribocharge at different rates and increase to different potentials, depending which textiles are making contact with and separating from them. Many carpets perform fairly well compared to other textiles in a humid environment. The humid environment may knock the created voltage down from 10’s of thousands of volts to thousands or hundreds of volts, but not low enough to prevent ESD Sensitive Devices from getting damaged or destroyed.

I hope I’ve touched on some of your questions but need to know more about your current application to help you better.

05 Dec

ESD Compliance: Test the ESD Devices, and test the testers

Q: How does a wrist bandcum footwear ground tester differ from the work station monitor? Can we not use only the work station monitor when it can monitor the worthiness of wrist strap even? And, Can we not check the worthiness of a wrist strap with a normal multimeter

A: I think that I understand your question here.  One way or another, a company needs to say what they are going to do in terms of ESD compliance.  They need to then do what they say they will do and document it.  They need to test the ESD devices.  They then need to test the testers.  And they need to show records of such in a pass/fail document and to prove that they are committed to quality and do not accept anything into the environment than will lead to a failure.

A wrist strap/footwear tester is proven and tested.  It is convenient to monitor and record the compliance to a whole shift of people in a given EPA.  But what if someone tests their wrist strap that morning and it fails at 10:25 that morning.  They won’t know it failed until later that day. 

The advantage of the constant monitor is that it not only alarms the moment it fails but it also forces the user to keep it on (wrist strap) while seated at the ESD workstation.  That’s fine.  But you need to test the testers periodically.  And to document those tests.  I hope I understood your question and answered it accordingly.

I suppose you could check the worthiness of a wrist strap with a normal multimeter.  I have a device that measures the connection from a connected 4mm, 7mm, or 10mm snap, through the coil cord and 1M Ω resistor, and two conductive cylinders that measure a stretched out wrist strap.  With it, I can measure the cuff only, the cuff and cord, or the cord only.  It cost me (us) a small fortune.  I have a Fluke 123 Industrial Scopemeter and a Fluke 77.  They are great meters, up to about 40M Ω or 4.0E7.  If you have wrist straps that are conductive enough, then great.  Good luck.  You may need a Megohmmer.

05 Dec

ESDA Specifications

Q: May I know the standard test procedures and parameters (ie: surface resistivity/surface to or person to ground resistance, decay time or any other parameter if applicable) and recommended values for confirming the worthiness of the following ESD items?

  1. ALL ESD PACKAGING ITEM
  2. ESD FLOOR AND WORK SURFACES
  3. ESD CLOTHINGS
  4. ESD FOOT WEARS
  5. ESD HANDLING ITEM viz., BRUSHES / BINS / PCB RACKS

A: Please consult your copy of ANSI/ESD S20.20-1999 from the ESDA at http://www.esda.org/.  It has specific documents for different technical elements.  For example, S1.1 for wrist straps, 2.1 for garments, 3.2 for ionization, 4.1 for worksurfaces-resistive characterization, 4.2 for worksurfaces- charge dissipation, 5.1 for Human Body Model, 5.2 for Machine Model, 5.3 for Charged Device Model, 6.1 for grounding, 7.1 for resistive characterization of materials-flooring materials, 8.1 for symbols-ESD awareness, 9.1 for footwear-resistive characterization, 10.1 automated handling, s11.11 surface resistance of static dissipative planar materials, 11.12 for EDS items-volume resistance of…, 11.2 for Triboelectric charge accumulation testing, 11.31 for bags, 12.1 for seating, 13.1 for electrical potential from soldering/desoldering hand tools, STM 97.1 for floor materials and footwear-resistance measurement in combination with a person, and STM 97.2 for floor materials and footwear-voltage measurement on a person just to name a few.

I could spend a lot of time digging up actual values for these specific items that we provide, but I haven’t put anything like that together all in one place.  I will work on that and perhaps we can post that info all in one place.  But keep in mind.  We do not determine the parameters, we merely work to comply with them.

05 Dec

People are a great generator of static electricity

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.

05 Dec

ESD Garments in addition to Wrist Straps and Footwear/Grounders

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.”

05 Dec

Using an ESD Chair when already protected

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.