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MOS (Metal Oxide Semiconductor) devices are highly sensitive devices and get damaged easily by accidental over-voltages, voltage spikes and static-electricity discharges. The human body can build up static charges that range up to 25000 volts.

These build-ups can discharge rapidly into an electrically grounded body or device, and particularly destroy certain electronic devices. The resultant high voltage pulse burn out the inputs of integrated circuit devices. This damage might not appear instantly, but it can build up over time and cause the device to fail.

The most common causes of electro-static discharge (ESD) are: moving people, low humidity (hot and dry conditions), improper grounding, unshielded cables, poor connections and moving machines. When people move, the clothes they are wearing rub together and can produce large amounts of electro-static charges in excess of 1000 volts. Motors in electrical devices, such as vacuum cleaners and refrigerators, generate high levels of ESD. Also ESD is most likely to occur during periods of low humidity, say below 50%. Any time the charge reaches around 10,000 volts, it is likely to discharge to grounded metal parts.

An important point to remember is that 10,000 to 25,000 volts of ESD are not harmful to human beings whereas 230 volts, 1 amp current produced by the mains power supply is lethal. The reason for this is the difference in current-delivering capabilities created by the voltage. The ESD voltages, though in the kilovolts range, produce currents only in the microampere range, which are not harmful for human beings. However, the level of stati c electricity on your body is high enough to destroy the inputs of CMOS (complementary metal-excide semiconductor) device if you touch its pins with your fingers.

Special care is needed in storing, handling and soldering MOS devices. The following precautions must be observed when using such devices:

(a) While storing and transporting MOS devices, use may be made of a conductive material or special IC carrier that either short circuits all leads or insulates them from external contact.

(b) The person handling MOS devices should be connected to the ground with grounding strap as shown in Fig. 5.11. These anti-static devices can be placed around the wrists or ankle to ground the technician to the system being worked on.

These straps release any static charge or the technician's body and pass it harmlessly to ground potential.

Fig. 5.11 Use of anti-static strap

Anti-static straps should never be worn while working on high voltage components, such as monitors and power supply units. Some technicians wrap a copper wire around one of their wrists or ankles and connect it to the ground side of another. This is not a safe practice because the resistive feature of a true wrist strap is missing.

The work areas should preferably include anti-static mats (Fig. 5.12) made of rubber or other anti-static materials they stand on while working on the equipment. This is particularly helpful in carpeted work areas because carpeting can be a major source of ESD build-up. Some anti-static mats have ground connections that should be connected to the safety ground of an AC power outlet.

t Fig. 5.12 Use of anti-static mat in the work area

(c) Before touching any component inside the system particularly containing MOS devices, touch an exposed part of the chassis or the power supply housing with your finger. Grounding yourself in this manner ensures that any static charge on your body is removed. This technique, however, works safely only if the power cord is attached to a grounded power outlet.

(d) Mount MOS integrated circuits on printed circuit boards after all other components have been mounted.

(e) When replacing a defective IC, use a soldering iron with a grounded tip to extract the defective IC and while soldering the new IC in place.

(f) After the MOS circuits have been mounted on the board, proper handling precautions should still be observed. To prevent static charges from being transmitted through the board wiring to the device, it is recommended that conductive clips or conductive tape be put on the circuit board terminals.

(g) To prevent permanent damage due to transient voltages, do not insert or remove

MOS devices from test sockets with the power on.

(h) Avoid voltage surges as far as possible. Beware of surges due to relays, and switching electrical equipment on and off.

(i) Signals should not be applied to the inputs while the device power supply is off.

(j) All unused input leads should be connected to either the earth or supply voltage.

(k) Personnel handling MOS devices are advised to wear anti-static clothing, synthetic fibre clothing should particularly be avoided.

(1) Do not insert PC boards into connectors that have voltages applied to them.

(m) Work stations should have non-conductive table tops, non-conductive trays, grounded soldering irons, etc.

The switching action of some controlled-output soldering implements can generate voltage spikes, which can be transmitted to and adversely affect MOS devices. Care should be taken in selecting soldering irons so that they have low voltage spikes.