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Electrostatic Discharge (ESD) P
recautions
5. Electrostatic Discharge (ESD) Precautions
Static electricity is generated when two materials move against one another. The voltage
generated depends on the materials generating the electricity, the speed of movement,
humidity and rate of discharge. All man made materials generate static, such as plastic coffee
cups, plastic bags, binders and folders, all of which are likely to be within the working area.
5.1 What is Static Electricity?
Static electricity is generated very easily, and is only felt by us when we discharge the built up
charge rapidly by touching a grounded object such as a door handle. The voltages felt by us
are as low as 3kV, but only 20V is necessary to damage some components. Voltages as high
as 35kV and current spikes of 40A have been known.
The damage to the component, or assembly can be immediate or latent. Latent damage is
not immediately obvious but can lead to the circuitry subsequently failing or becoming erratic.
5.2 Protective Measures
Measures must be taken to ensure that all charges generated are safely discharged before
they build up to a dangerous level.
The use of dissipative mats for the work surface and wearing anti-static wrist straps are
recommended ways of preventing this problem. The dissipative mats must be connected to
ground via a resistance and the wrist strap to the same ground point. Anti-static bags must be
used when storing or transporting static sensitive components.
Conductive mats are no longer regarded as being suitable, as a PCB or component could be
electrostatically charged and it would be rapidly discharged by placing it on the conductive
mat. This sudden discharge would be as damaging as placing a charged object in contact
with the PCB or device.
All static sensitive devices or assemblies must be placed within an anti-static bag or container
whenever being moved away from the specialised handling area.
For further information on static precautions and soldering equipment refer to Appendix A.
Activity
10-2
20% Relative Humidity
Walking across carpet
35,000 Volts
Walking across vinyl floor
12,000 Volts
Working at bench
6,000 Volts
Plastic folder
7,000 Volts
Poly bag lifted from bench
20,000 Volts
Foam padded work chair
18,000 Volts