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 constantly—below about 3000 V they are not felt.
Typical Electrostatic Voltages Generated by a Variety of Activities:
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:
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 without anyone’s knowledge.
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 and the other 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. The damage can be partial or so slight that it is not immediately evident in tests but shows up later, in which case it is called “latent” damage.
Latent damage tends to be 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 repairs cost upwards 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 measures include:
Grounding everyone who comes close to electrosensitive devices and 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 reduce accumulated charges not otherwise controlled.
The use of 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 devices are approached and handled.
Periodic retraining and auditing to assure your program remains effective.
If you are interested in participating in the development of ESD standard test methods or handling procedures, contact the ESD Association headquarters at:
7900 Turin Road, Building 3
Rome, NY 13440-2069
Phone: (315) 339-6937
Fax: (315) 339-6793