Category Archives: Question & Answers

Now is the Time for ESD Control Programs to be Improved

Originally Published by InCompliance Magazine- September 2012
“By Fred Tenzer and Gene Felder”

ElectroStatic Discharge (ESD) is the hidden enemy within your factory. You cannot feel or see most ESD events but they can cause electronic components to fail or cause mysterious and annoying problems. There are two types of ESD damage: 1) Catastrophic failures, and 2) Latent defects. By definition, normal quality control inspections are able to identify catastrophic failures, but are not able to detect latent defects.

In general, the ESD susceptibility of modern electronics are more sensitive to ElectroStatic Discharge; that is the withstand voltages are lower. This is due to the drive for miniaturization particularly with electronic devices operating faster. Thus the semiconductor circuitry is getting smaller. For example Intel began selling its 32 nm processors in 2010 that would be 0.032 micrometer equal to 0.000032 millimeter or 0.00000128 inch.

For more information on ESD and the direction of electronics manufacturing, see the articles listed below.

Evaluation Engineering Magazine November 2001 article “ESD Control Program Development” “As the drive for miniaturization has reduced the width of electronic device structures to as small as 0.10 micrometer (equal to 0.0001 millimeter or 0.000004 inch), electronic components are being manufactured with increased ElectroStatic Discharge (ESD) susceptibility.”

www.ESDA.org, the ESD Association’s latest White Paper “Electrostatic Discharge (ESD) Technology Roadmap – Revised April 2010” forecasts increased ESD sensitivities continuing the recent “trend, the ICs became even more sensitive to ESD events in the years between 2005 and 2009. Therefore, the prevailing trend is circuit performance at the expense of ESD protection levels.” The White Paper’s conclusions are:

“With devices becoming more sensitive through 2010-2015 and beyond, it is imperative that companies begin to scrutinize the ESD capabilities of their handling processes. Factory ESD control is expected to play an ever-increasing critical role as the industry is flooded with even more HBM and CDM sensitive designs. For people handling ESD sensitive devices, personnel grounding systems must be designed to limit body voltages to less than 100 volts.

To protect against metal-to-device discharges, all conductive elements that contact ESD sensitive devices must be grounded.

To limit the possibilities of a field induced CDM ESD event, users of ESD sensitive devices should ensure that the maximum voltage induced on their devices is kept below 50 volts.

To limit CDM ESD events, device pins should be contacted with static-dissipative material instead of metal wherever possible.”

InCompliance Magazine May 2010 article by Dr. Terry L. Welsher The “Real” Cost of ESD Damage which includes “Recent data and experience reported by several companies and laboratories now suggest that many failures previously classified as EOS may instead be the result of ESD failures due to Charged Board Events (CBE). … Some companies have estimated that about 50% of failures originally designated as EOS were actually CBE or CDE.”

Charleswater

For additonal technical information Click Here

To read the rest of the article go to Now is the Time for ESD Control Programs to be Improved

Images of ESD Damage

Seeing ElectroStatic Discharge (ESD) damage is basically impossible. Damage to semiconductor device structure is NOT visible at ordinary magnifications of an optical microscope. If the microscope is capable of 1000X-1500X magnifications, you just might be able to “see” something. The method used, only occasionally as there is considerable expense, is by delayering and etch enhancement producing high magnification photographs using a scanning electron micrograph (SEM). See Images of ESD Damage, photos of Human Body Model (HBM) ESD damage provided by Hi-Rel Laboratories, Inc. at 6116 N Freya, Spokane, Washington 99217 (509-325-5800 or www.hrlabs.com). Used with their permission.

Ground Trolleys in Series With Cords Having 1 Megohm Resistor

Question:
I am confused on ESD connections. I understand that you cannot clip a cord from a wrist strap to the edge of an ESD protective mat. I know that the cord for wrist straps has 1 megohm of resistance and that the point to connect the wrist strap cord should have less than 1 ohm to ground.

I have people using 2 meter high metal rolling trolleys. I believe the preferred method is to connect each trolley directly to a common ground point. Can several trolleys, say 3 to 5 be connected in series with 1 megohm cords with the final cord connected to a ground point or earth bonding point? From what I have read this would only mean about 5 megohms to ground at the furthest cart. I read somewhere something about staying under 35 megohms to ground. Now I cannot find that.

Answer:
The compliance verification limit per EN 61340-5-1 for wrist strap system is less than 3,5 x 10^7 ohms listed in Personnel grounding requirements Table 2. The required limit per ANSI/ESD S20.20 for both working surfaces and mobile equipment (trolleys) is less than 1 x 10^9 ohms resistance to ground listed in EPA requirements Table 3.

It does not do much good to have an operator test their wrist strap using an integrated wrist strap tester as describe in IEC 61340-5-1 Clause A.1, but then work where the matting resistance is added resistance in the path-to-ground.

Although not recommended you could attach the cord for the wrist band to the matting material, and daisy chain the mats and/or trolleys if you could demonstrate that you reliably meet the required limits.

Per CLC/TR 61340-5-2:2008 User guide Wrist Strap subclause 4.7.2.2.2 “NOTE Many wrist strap users have been observed to clip the wrist cord to the edge of an ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 3.5 x 10^7 ohms limit required by IEC 61340-5-1.”

Regarding daisy-chaining “All working surfaces need to be capable of being grounded. EPA grounding of instruments or surfaces by chaining, or by placing items in series should not be used, since in the event of a broken connection the risk of floating items and ESD damage will be unnecessarily increased.” [EN 61340-5-2 clause 5.2.2 Working surfaces and storage racks] Note that both of these recommendations are not requirements.

Regarding ground cords, the grounding conductors (wires) from wrist straps, working surfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common ground point or common connection point is acceptable and recommended.

However, “A nominal 1 megohm resistor is commonly used in wrist straps and to ground work surfaces. In the event of an operator touching an energized conductor, for the normal mains electricity supplies this resistor will limit the current flowing through the person to less than 0,5 mA.” [User Guide IEC 61340-5-2-1999 subclause 5.1.1]

Charleswater sells ground cords with and without current limiting resistors – Click Here

What happens if you staple ESD Bags shut?

Question:

What happens if you staple ESD Bags shut? Does that damage the ESD Bag’s effectiveness? What if the ESD Bag is heat sealed shut & a staple on the seam is used to attach paperwork?

Answer:

Charleswater ESD Shielding Bags have a layer of metalized film which creates continuous conductive enclosure or Faraday Cage to provide electrostatic shielding protecting the ESD sensitive devices placed inside the Bag. The use of stapling to close ESD Bags is counter productive and not recommended. The metal staple provides a conductive path from the outside of the ESD Bag to the inside. The use of a metal staple would undermine the effectiveness of the ESD Bag making a conductive path for charges outside the Bag to charge outside the Bag to charge or discharge to ESD sensitive components inside the Bag.

To close the ESD Bag, it is recommended to heat seal, or use Charleswater ESD Labels after the opening of the bag has been folded over.

To view Charleswater ESD Labels Click Here

Or to view Charleswater Antistatic Tape Click Here

Carefully locating the staple to only the seam of the Charleswater Statshield® Bag would theoretically make it part of the “continuous conductive enclosure” and be acceptable. However, we are not aware of any end user using this method and cannot recommend it. The staple would be an exposed conductor that could charge or discharge to ESD sensitive devices.

To ask an ESD Question Click Here.

ESD Control Programs Should be Improved

ElectroStatic Discharge (ESD) is the hidden enemy within your factory. You cannot feel or see most ESD events but they can cause electronic components to fail or cause mysterious and annoying problems. There are two types of ESD damage: 1) Catastrophic failures, and 2) Latent defects. By definition, normal quality control inspections are able to identify catastrophic failures, but are not able to detect latent defects.

In general, the ESD susceptibility of modern electronics are more sensitive to ElectroStatic Discharge; that is the withstand voltages are lower. This is due to the drive for miniaturization particularly with electronic devices operating faster. Thus the semiconductor circuitry is getting smaller.

See November 2001 Evaluation Engineering Magazine article “ESD Control Program Development” “As the drive for miniaturization has reduced the width of electronic device structures to as small as 0.10 micrometer (equal to 0.0001 millimeter or 0.000004 inch), electronic components are being manufactured with increased ElectroStatic Discharge (ESD) susceptibility.”

What’s happening currently? Intel began selling its 32 nm processors in 2010 that would be 0.032 micrometer equal to 0.000032 millimeter or 0.00000128 inch.

See www.ESDA.org, the ESD Association’s latest White Paper “Electrostatic Discharge (ESD) Technology Roadmap – Revised April 2010” forecasts increased ESD sensitivities continuing the recent “trend, the ICs became even more sensitive to ESD events in the years between 2005 and 2009. Therefore, the prevailing trend is circuit performance at the expense of ESD protection levels.” The White Paper’s conclusions are:

“With devices becoming more sensitive through 2010-2015 and beyond, it is imperative that companies begin to scrutinize the ESD capabilities of their handling processes. Factory ESD control is expected to play an ever-increasing critical role as the industry is flooded with even more HBM and CDM sensitive designs. For people handling ESD sensitive devices, personnel grounding systems must be designed to limit body voltages to less than 100 volts.

To protect against metal-to-device discharges, all conductive elements that contact ESD sensitive devices must be grounded.

To limit the possibilities of a field induced CDM ESD event, users of ESD sensitive devices should ensure that the maximum voltage induced on their devices is kept below 50 volts.

To limit CDM ESD events, device pins should be contacted with static-dissipative material instead of metal wherever possible.”

See InCompliance Magazine May 2010 article by Dr. Terry L. Welsher The “Real” Cost of ESD Damage which includes “Recent data and experience reported by several companies and laboratories now suggest that many failures previously classified as EOS may instead be the result of ESD failures due to Charged Board Events (CBE). … Some companies have estimated that about 50% of failures originally designated as EOS were actually CBE or CDE.”

Charleswater

For additonal ESD information Click Here