Electrostatic Discharge (ESD) is sometimes described as the spark which occurs when you walk across a carpet and touch a door knob. However, much smaller events occur every time. Below about 3000 V they are not felt.
Typical Electrostatic Voltages Generated by a Variety of Activities are as Follows:
- Walking across untreated vinyl floor, 250 to 12,000 volts
- Moving a common plastic bag at a work bench, 1,200 to 20,000 volts
Typical Voltages Required to Damage Electronic Devices are as Follows:
- MOSFET 100 to 200 volts
- JFET 140 to 10,000 volts
- CMOS 250 to 2,000 volts
- Schottky diodes, TTL300 to 2,500 volts
- Bi-polar transistors 380 to 10,000 volts
- SCR 680 to 1000 volts
It can be seen from the voltages shown above that common activities generate voltages great enough to damage electronic devices. Frequently the voltages are below the minimum which a person can feel, so damage will occur with no way for the person to see or feel the spark.
Static charges are left on the surface when two surfaces are separated or when one is brushed past another. Merely picking up a circuit board from a table causes the board and the table to become charged. One will become negatively charged and the other will become positively charged. Simply pulling tape off a roll or lifting a label off the backing paper or so called “liner” will cause a static charge. Of special interest to the board assembler, the masking tape used to protect the conductive contacts from solder could carry a damaging charge to the board. Using static dissipative tape has been found to offer protection.
ESD is caused by a sequence of events. The first event is the accumulation of charge on the surfaces of insulators such as plastics. These charges cannot move when there is no path to ground. Hence they are called static charges. The second event is discharge through a conductor such as the metal leads on a circuit board to a grounded metal or the relatively conductive skin of a person.
An ESD event causes rapid charge movement and heating of the insulating gates in the sensitive components. This damage can cause the device to completely stop functioning, in which case it is so-called catastrophic damage. Or the damage can be partial. Or damage can be so slight that it is not immediately evident in tests but it shows up later, in which case it is called “latent” damage. Latent damage is usually the most costly as the equipment fails during customer use, necessitating the return of the board and replacement of the component. It has been estimated that such repair cost averages upward of 100 times the cost of the component.
ESD events and damage can be prevented. Prevention begins with training everyone about the causes, effects, and methods of prevention.
Effective measure include grounding everyone who comes close to electrosensitive devices. Using grounded static dissipative work surfaces. When not working on the devices, store them and transport them in static dissipative bags. Install ionizers in the work stations to make the air conductive and thereby reduce the accumulated charges not otherwise controlled. Use static dissipating gloves, wrist straps, heel grounders, static dissipative work surfaces, and target area ionizers, anti-static masking tape and anti-static labels in every area where the devices are approached and handled. Finally, periodic retraining and auditing are necessary to assure the program remains effective.
People interested in participating in the development of ESD standard test methods or handling procedures should contact THE ESD Association headquarters at:
7902 Turin Road, Suite 4
Rome, NY 13440-2069
Phone: (315) 339-6937
Fax: (315) 339-6793
About The Author:
Paul Henkel is the Founder of Electronic Imaging Materials, Inc. in Keene, New Hampshire. Paul recently retired and his son Alex Henkel is now the President of the company.
Electronic Imaging Materials is a company Specializing in Label Materials for Printing Barcodes.