Lightning is probably the most recognizable effect of static electricity. Generating millions and millions of voltage and we all know the damage lightning can do to property and people.
Another recognizable effect of static electricity is the shock you receive when you slid out of a car in dry weather conditions and feel a zap. The human body feels a shock when the voltage is higher than about 3,500 volts.
Walking over a carpet can generate 35,000 volts. The Electrostatic Discharge (ESD) from this voltage can cause pain. The discharge is not life threatening but it still hurts.
The static electricity that concerns the electronics industry is the discharge that you can neither feel nor see. Refer to the chart from MIL-STD-263B (page 21) for typical electrostatic voltages that can be generated by the things we do all the time. As the chart shows you can generate smaller amounts of static electricity and since we do not see or feel discharges at less than 3,500 volts, damage can occur to static sensitive devices without our knowledge.
(a) A Little Historical Background
In 1948 Bell Labs helped to invent the transistor and the electronic industry started to grow. The development of MOSFETs followed in 1962. RCA was the first company to use an IC in a television set in 1968. The first processor was introduced by Texas Instruments in 1973. Progress continued and IC’s started to get smaller and smaller and ESD control became harder and harder.
In the late 1970’s (1978 to be closer) it became apparent that newly designed MOSFET’s were failing for no apparent reason. A lot of work went into determining why they were failing and the damage trail led to the problem being static electricity.
This was the beginning of learning how to control these charges. There were many non-believers in the late 70’s and early 80’s that felt that a static charge could not damage or destroy integrated circuits. Even today there are some engineers that believe static is an overblown issue.
Today most companies recognize the value of a complete static control program.
Implementing a static control program may cost a few dollars
(peso’s) in the beginning but in the long term can save money, boost
reliability of product, and enhance a companies reputation in the field.
(b) Static Electricity and IC’s
Devices have various levels of sensitivity. A chart from MIL-STD-1686C and reprinted in the ESD Awareness Guide from ESD Systems will give you a partial list of various devices and their sensitivity levels.
A revised level of sensitivity for ESD sensitive components is shown in the ESD Association document ESD-STM5.1-1998 (Electrostatic Discharge (ESD) Sensitivity Testing) on page 2. It is reprinted here because it is important information when you determine the level of your ESD program
HMB ESDS Component Classification
|
Class |
Voltage Range |
|
0 |
<250 |
|
1A |
250 to <500 |
|
1B |
500 to <1000 |
|
1C |
1000 to < 2000 |
|
2 |
2000 to < 4000 |
|
3A |
4000 to < 8000 |
|
3B |
> or + 8000 |
(c)
Observations
· Static electricity at low voltage levels is real even though you can not feel or see it.
· Integrated circuits have different levels of sensitivity.
· A systematic static control program can add to your companies bottom line (profits).