Download ESD Precautions for Handling Electronics

ESD Precautions for
Handling Electronics
R. Schnee (11/8/05)
Based on document by T. Perera
Procedures
• All CMOS chips (e.g. ADC or memory) should be transported to your
work station in insulating foam or ESD-safe plastic container.
• Before removing chips from insulator, put on the wrist strap connected to
the ESD mat. All work with CMOS chips should be done with the wrist
strap on. As an added precaution before first touching a chip, you should
touch a finger to a grounded metal surface such as the shell of one of the
BNC connectors on your RALboard.
• If using a DMM, its outside should be in contact with the ground of the
ESD mat, and both leads shorted to this ground before use.
General Principles
• The main cause for electrostatic charge on materials is triboelectric
generation, or the unequal sharing of surface charge when two materials
brought together and separated. Charge can also build up due to external
electric fields.
• It is easy to generate voltages > 100V by the simple act of walking over
a carpet. ESD occurs when the charged object is connected to ground or
an object at a different potential. No visible spark will be evident unless
the voltage is much larger, say 10000 V, but 100V ESD may destroy
chips.
• A device may experience ESD due to the human body or other object
discharging onto it, the device being charged up and discharging to the
human body or conductor, or due to varying external fields. For
reference, the human body may be modeled as a 100 pF capacitor with
the resistance of the contact between it and the device modeled as a 1.5
kOhm resistor.
• The guidelines above are based on the fact that a chip and its future
environment are usually at different potentials unless explicitly
connected together. For this reason, the chip must be brought to the
potential of its environment. This must be done slowly, to prevent large
currents from causing device failure.
• This slow discharge is accomplished through static dissipative materials.
ESD mats and wrist straps are made of these materials that have higher
resistance than conductors but lower resistance than insulators. The
surface resistance of a static dissipative material is in the range 105 -1012
Ohms/square (resistivity: 104-1011 Ohm-cm).
• The guidelines will also help maintain a device and its environment at
the same potential.
• A good website for further ESD information is:
http://www.esda.org/aboutesd.html
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