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:
ESD Association
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.,
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. |
