ESD Systems’ ESD Technical Newsletter
Issue 2, August 1998: Volume 1
Reference:
http://www.esdsystems.com/newsletters/v1issue2.htmSender : ESD SYSTEMS, 19 Brigham Street #9, Marlboro, MA 01752-3170
Phone : 508-485-7390
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editor@esdsystems.comIN THIS ISSUE:
WELCOME TO OUR 2nd EDITION ESD E-NEWSLETTER
We at ESD Systems would like to extend our thanks for your support and enthusiasm for this Newsletter. This is a free newsletter scheduled with monthly releases, which started with Issue 1, under Volume 1. This newsletter will compliment our U.S mailed newsletter, but be distinctly different from it. It will focus more on the technical issues regarding ESD Control in the manufacturing and field arenas. Hold the presses; we would like to incorporate our readers feedback and suggestions into our future issues. Please send your comments to:
newsletter@esdsystems.com20TH ANNUAL EOS/ESD SYMPOSIUM, 1998
The EOS/ESD 20th annual Symposium will be held October 4th - 8th in Reno Nevada. This Symposium is an excellent opportunity to learn more about ESD Control from tutorials, technical papers, workshops, speakers, presentations, exhibitors, etc.
We encourage you to stop by the ESD Systems exhibit to learn more about our wide-variety of Electrostatic Discharge (ESD) Control products. This is a great opportunity to meet our ESD Systems’ team. Our Booth number is 106. Look forward to seeing you there.
For more information about the EOS/ESD Symposium, contact:
1998 EOS/ESD Symposium
c/o ESD Association
7900 Turin Rd. Bldg. 3, Suite 2
Rome, NY 13440-2069
Phone: (315) 339-6937
Fax: (315) 339-6739
ESD Q&A CORNER
The following questions and answers are selected from our FAQ WEB Page:
http://www.esdsystems.com/default.asp?PageNo=QANDAINDEX concerning Grounding.Q1:
We are operating out of an older building that has a 3 phase and Neutral AC system. The equipment ground is bonded to the Neutral Bus via the EMT and armored conduit running throughout the building. Can we use the conduit as an ESD grounding point or do we have to install a separate earth ground system for our ESD system? - Anonymous, Long Island City, NYA1:
Yes, you can use the conduit as an ESD grounding point. The main purpose of an ESD ground or "common" point ground is to bring to "common" or the same potential all conductors in the critical working area so as to eliminate potential differences between conductors. Once the potential difference between conductors is eliminated, the ability to generate an ESD event between these conductors is minimized. YOU STILL RUN THE RISK OF AN ESD EVENT, even when all conductors in the work area are grounded. This is because a foreign conductor can still be introduced at a different potential to your ESD ground and generate an ESD event to your grounded conductor(s). This includes a visiting engineer (ungrounded human body) delivering a needed component to you (grounded human body), IC devices sliding out of a protective tube onto your grounded table mat, a circuit board transported onto a conductive tray, etc.Q2a:
We are currently looking for alternatives to hard wiring our wrist strap sockets and table matting, and were hoping if you could tell us if it is sufficient to do the following and still provide a safe ESD environment for PBA manufacturing?"Attach copper strips to the feet of the production tables, then hard wire the wrist strap socket and table matting to the table frame. (Our floor is static dissipative, on average, to between 10to7 & 10to9 ohms per square.) This will allow our tables to be more flexible, as the table doesn't necessarily have to have power to be earthed." – Clayt, Australian Electronic Manufacturing Services, Sidney, Australia
A2a:
I would not recommend wiring your table mats and wrist straps to the ESD floor in (place of) of a hard ground. Your floor's resistance of 100 Megohms to 1,000 Megohms would not protect you adequately as a "ground substitute" for your wrist strap or table mat. The recommended grounding (for common point grounds, table matting, and wrist straps) is a hard ground that does not exceed 1.0 Ohm according to the ANSI/EOS/ESD-S6.1-1991 under section 4.4.1.One of the biggest concerns to not using a hard ground is the increase in your electronic time constant /RC (discharge time). We have a white paper available on our Web Site that better discusses discharge times http://www.esdsystems.com/whitepapers/wp_discharg.asp.
You don't need to have power at the benches to run an "Earth" ground to them. A common ground bus can be used and wired to these benches. An electrician can easily help you with this. You will want to be careful of ground loops for possible electrical noise problems to both sensitive electronic equipment and ESD grounded operators.
Q2b:
If possible could u please explain the below sentence in a little more detail."You will want to be careful of ground loops for possible electrical noise problems both to sensitive electronic equipment and ESD grounded operators."
A2b:
This really depends on the Class of the devices you are handling. If you are handling Class 2 or higher devices (> 4,000 Volts), then this should not be an issue for you. Below is a scenario with a very sensitive Class 0 device.When working with Class 0 ESD sensitive devices, i.e., < 250 Volts (ESD STM5.1-1998), some of these devices could be sensitive to as little as 10 Volts. A ten Volt ESD (electrostatic discharge) can easily be set up from ground loops. If a ground point has more than one path to bus bar, you can create "small circuits" or loops within the grounding system. These "loops" can create potential differences from millivolts to several volts (beyond 10 Volts). If an ESD Sensitive device comes between to points in the grounding system where the potential difference is say 10 Volts, the device is at risk.
In addition to ground loops, a grounding system can also act as a large antenna. If any of the ground connection points are ohmic (say because of a bad solder joint or a loose mechanical connection) you can create standing waves (depending on the length of the conductors) or other types of electrical noise which can be delivered through your ground into the devices being handled (if a direct connection is made).
Common Point grounding as well as a good physical layout helps to reduce ground loops. The physical layout of the grounding system should have a main ground trunk with tributaries coming off in parallel to this main trunk to provide the common point grounds at your workstations. Hard grounds to these common point grounds should then be employed.
Another reason for "hard" grounds (direct connections with essentially no ohmic characteristics) versus "soft" grounds (direct connections with ohmic characteristics typically at 1 M Ohm) is to eliminate any potential problems (no pun intended) associated with an ohmic potential (voltage) difference in the ground connection. Usually a 1 M Ohm load is OK in a ground cord because of the typical discharge times and the associated with this RC electronic time constant. The 1 M Ohm load is a safety feature to the operator to reduce the current they may encounter when grounded and handling a live circuit (120 VAC).
PRODUCT UPDATES
ESD Systems’ DELUXE COMBO TESTER, Item # 41204.
All the same basic features of the standard Combo Tester with the following extras:
Sole Grounder increases contact area between your foot and floor surface
We’re sure you’ll love the features of these new grounders, and we encourage you to give them a try! Call customer service to receive a sample or request Drawing 24800.
ESD Training Paddles easily generate high static voltages
Ideal for Demonstrating Electrostatic Principles
Item #36080 is ideal for training, and we encourage you to give them a try! For more information and experiments to perform with the paddles, request Tech Brief PS-2079.
Dr. ZAP
Topics: Grounding Definitions and Failure Mechanisms
These definitions are found on our On-Line ESD Dictionary found at:
http://www.esdsystems.com/default.asp?PageNo=WP, sources include the ESD Association who’s temporary page is found at: http://www.borg.com/~eosesd/Ground
- A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of earth.Ground Cord
- A ground cord connects two conductors together to bring at the same potential, usually ground. Most of the wrist strap ground cords are coiled to retain excess cord length when not in use. Standard ground cords have a built in 1-megohm resistor for operator safety.Hard Ground
- A connection to ground through a wire or other conductor that has a negligible resistance to ground.Foot Grounder
- A ground strap used for mobility that connects the bodies foot to ground (ESD floor) via a conductive tab between the sock and insole connected to a conductive outer wear (rubber cup) that is positioned on the bottom sole of the shoe. Foot grounders come in different styles: heel grounders, toe grounders, ball (of the foot) grounders and sole (combination heel and ball) grounders.Equipment Ground
- The ground point at which the equipment-grounding conductor is bonded to any piece of equipment, at the equipment end of the conductor.Auxiliary Ground
- A separate supplemental grounding conductor for uses other than general equipment grounding.FAILURE MECHANISMS:
Three major failure mechanisms for hard failures have been experimentally noted for semiconductor devices.
THERAMAL BREAKDOWN: Is caused by the injection of an electrical transient (such as an ESD pulse) of sufficient magnitude and duration to initiate secondary thermal breakdown and subsequently to melt a portion of the junction. If the ESD pulse has enough energy, the thermal runaway will cause the silicon in the hot spot to melt, short-circuiting the junction and failing the device.
DIELECTRIC BREAKDOWN: Occurs when a potential difference is applied across a dielectric region that is in excess of the region’s inherent breakdown characteristics. The result is a puncture of the dielectric. And once this oxide layer has been punctured, very little extra energy is required to create a short circuit. This form of failure is primarily due to voltage and results in either total or limited degradation of the device, depending upon the energy of the pulse.
METALIZATION MELT: Occurs when ESD transients increase the device temperature sufficiently to melt or fuse bond wires. Metalization melt is frequently a secondary failure mechanism. It occurs when a second breakdown or gate oxide punch through results in a short circuit which then draws enough current to melt the metalization. For devices with closely spaced, thin metal electrodes metalization melt will cause degraded performance.
How serious is this static problem?
Well, many people feel that static is no great problem because with minimal static control procedures, they are able to keep the device failure rates to 0.5%. However, with 20 devices per board, this equates to 10% of the boards being defective and with 5 boards per system, a system failure rate of 50%.
Fortunately, the solution is not difficult. It is not necessary to ban visitors from the work area. In fact it would be impossible to eliminate all the potential carriers of static charge. Instead, all it takes is for everyone (from the top on down) to observe an ESD control program.
Need your own copy? Want to subscribe to this Newsletter? All you or your colleague(s) need to do is simply fill out the subscription form at
http://www.esdsystems.com/forms/esdmail.aspThis Newsletter is never sent unsolicited. To unsubscribe from this mailing, simply reply to this e-mail and include in the subject field the following:
UNSUBSCRIBE esdnlThis is a free monthly newsletter, which specializes on issues in static control in the workplace.
Let us know what you think. Tell us what you would like to see in future issues. Want to contribute articles or other related information to our Newsletter? Send your comments to
editor@esdsystems.comCopyright © 1998, Desco Industries, Inc., Employee Owned
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