Static electricity: Protections
2013-11-05
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Sensitive Materials to the static and static guards
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STATIC ELECTRICITY

The term static electricity refers to the accumulation of an excess of electric charge in an area with low electrical conductivity, insulation, so the accumulation of charge persists. The effects of static electricity are familiar to most people because you can see, notice, and even feel the sparks of discharges that occur when the overloading of the loaded object is placed near a good electrical conductor (such as a driver connected to a ground) or other object with an excess of load, but with the opposite polarity.

Static electricity is something with which all are familiar in its most impressive manifestation: the rays.
Another example of static electricity that can be easily experienced is the shock that you receive when you leave a car. The synthetic materials used in dresses as well as inside the vehicles are capable of producing large amounts of static load that is only released when the driver or the passenger put their feet on the ground.
When two different materials not originally downloaded drivers are rubbed together, friction causes to transfer a load to another and as a result the electrical potential between them rises.


DEVICES SENSITIVE TO STATIC

All modern microelectronic components are prone to damage from scattered electric charges, but some are more than others. Devices that are more prone to damage tend to be those that are more based on field effect technology in the union bipolar technology. They include logic CMOS devices (such as logic implementations and logic MSI ("medium scale integration"), (such as transistors) MOSFET devices, VLSI NMOS and PMOS circuits (used in devices from dynamic memory, microprocessors, etc.). Microwave transistors and diodes (due to their small sizes and small areas of union) are also particularly sensitive to static as well as some devices are optoelectronic and screens. In case of doubt, the basic principle is to treat any semiconductor device with great care and always avoid situations where static charges could come into contact with the device.

Since the CMOS have an input resistance extremely large they should never be left disconnected, all must be connected to a fixed voltage level, this is the CMOS are, like the MOS very susceptible to electrostatic charges and noise that could damage the devices.


LOGOS OF ESD

Sensitive components to static (including printed circuit boards, modules, circuits and devices of connection cards) are invariably labeled with warnings. These notices are normally printed with text in black on yellow background, as shown in the image.



HANDLING AND TRANSPORT OF THE ESD

Special precautions should be taken when handled, transported, installed, or an ESD is removed. These precautions include the following:

1 Special bracelets should be used when handling the ESD. These bracelets ("wrist strap") consist of conducting bands that are connected to an effective Earth via a short cable Fig 5.12.6. Do cable also has built-in resistance 1 M? helps minimize any potential hazard for which carries it (resistance in series serves to limit the current that passes through the user in the event that is put in contact with a conductor in tension).
2. Use of bangles ankles that function similarly to wrist bangles.
3. Use of soils and static dissipative mats.
4 Avoid very dry environments (or at least to take special precautions when the relative humidity is low).
5 Availability of connections to Earth
6. Use of test equipment connected to Earth.
7 Use of low voltage welding equipment and static (welding electrodes for low voltage with grounded corresponding ends).
8 Using tools anti-static for integrated circuits insertion and disconnection.
9 Avoid nearby high-voltage sources (e.g. fluorescent light units).
10. Application of antistatic packaging materials (static-sensitive components and printed circuit boards should be stored in their antistatic packaging until the moment of being used).

TRIBOELECTRICAS SERIES

Triboelectricas series classified the different materials according to how create static electricity when they rub against other material. The series are arranged on a scale of increasingly positive and increasingly negative materials.

Materials transferred electrons and become positive when loaded to rub against other (positive) materials:

• Air (more positive).
• Human skin.
• Leather.
• Rabbit fur.
• Glass.
• Human hair.
• Nylon.
• Wool.
• Lead.
• Cat skin.
• Silk.
• Aluminum.
• Paper (less positive).


Materials that do not easily tend to attract or lose when put in contact or rub them with other materials (neutral):

• Cotton.
• Steel.

Materials that tend to attract electrons when rubbed them against other materials and spend to stay loaded negatively (negative):

• Wood (less negative).
• Amber.
• Vulcanized rubber.
• Nickel, copper, brass and silver.
• Polyester.
• Polystyrene.
• Chloride polivinidelo PVDC (saran).
• Polyurethane.
• Polyethylene.
• Polypropylene.
• Polyvinyl chloride (PVC).
• Silicon.
• Teflon (more negative).


DEVICE ESD PROTECTIVE MATERIALS

It must be taken into account that there are three main classes of materials used to protect static sensitive devices. These are:

Conductive materials (such as metal and plastic impregnated carbon lamellas)
Dissipating static (a cheaper form of conductive material) materials
Antistatic materials (are neutral materials in the scale triboelectric as wood, cotton and cardboard).

Of them all, the conductive materials offer greater protection while antistatic materials offer less protection.
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