- Identifying and Avoiding Electrostatic Discharge
- MOS-Handling Techniques
- Understanding Grounds
- About This Article
MOS-Handling Techniques
In general, MOS devices are sensitive to voltage spikes and static-electricity discharges. This can cause many problems when you have to replace MOS devices, especially complementary-symmetry metal-oxide semiconductor (CMOS) devices. The level of static electricity on your body is high enough to destroy the inputs of a CMOS device if you touch its pins with your fingers.
To minimize the chances of damaging MOS devices during handling, special procedures have been developed to protect them from static shock. ICs are generally shipped and stored in special conductive-plastic tubes or trays. You might want to store MOS devices in these tubes, or you could just ensure their safety by inserting the IC's leads into aluminum foil or antistatic (conductive) foam - not Styrofoam. PC boards containing static-sensitive devices are normally shipped in special antistatic bags, which are good for storing ICs and other computer components that could be damaged by ESD. They also are the best way to transport PC boards with static-sensitive components.
Professional service technicians use a number of precautionary steps when they are working on systems that might contain MOS devices, such as the grounding strap shown in Figure 1. These antistatic devices can be placed around the wrists or ankle to ground the technician to the system being worked on. These straps release any static on the technician's body and pass it harmlessly to ground potential.
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Figure 1 Typical antistatic devices.
Antistatic straps should never be worn while working on higher voltage components, such as monitors and power-supply units. Some technicians wrap a copper wire around their wrist or ankle and connect it to the ground side of an outlet. This is not a safe practice because the resistive feature of a true wrist strap is missing. As an alternative, most technician's work areas include antistatic mats made of rubber or other antistatic 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 buildup. Some antistatic mats have ground connections that should be connected to the safety ground of an AC power outlet.
Test Tip
Know when not to wear an antistatic wrist strap.
To avoid damaging static-sensitive devices, the following procedures help minimize the chances of destructive static discharges:
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Because computers and peripheral systems might contain a number of static-sensitive devices, before touching any components inside the system, touch an exposed part of the chassis or the power-supply housing with your finger (see Figure 2). Grounding yourself in this manner ensures that any static charge on your body is removed. Use this technique before handling a circuit board or component. Of course, this technique works safely only if the power cord is attached to a grounded power outlet. The ground plug on a standard power cord is the best tool for overcoming ESD problems.
Do not remove ICs from their protective tubes (or foam packages) until you are ready to use them. If you remove a circuit board or component containing static-sensitive devices from the system, place it on a conductive surface, such as a sheet of aluminum foil.
If you must replace a defective IC, use a soldering iron with a grounded tip to extract the defective IC and while soldering the new IC in place. Some of the ICs in computers and peripherals are not soldered to the printed circuit board. Instead, an IC socket is soldered to the board, and the IC is just inserted into the socket. This allows for easy replacement of these ICs.
If you have to replace a hard-soldered IC, you might want to install an IC socket along with the chip. Be aware that normal operating vibrations and temperature cycling can degrade the electrical connections between ICs and sockets over time. This gradual deterioration is referred to as "chip creep." It's a good practice to reseat any socket-mounted devices when handling a printed circuit board. Before removing the IC from its protective container, touch the container to the power supply of the unit it will be inserted in.
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Figure 2 Discharging through the power-supply unit.
Test Tip
Be aware of the effects temperature cycling can have on socket-mounted devices.
Some devices used to remove solder from circuit boards and chips can cause high static discharges that could damage the good devices on the board. These devices, called "solder-suckers," are available in antistatic versions for use with MOS devices.
Use antistatic sprays or solutions on floors, carpets, desks, and computer equipment. An antistatic spray or solution, applied with a soft cloth, is an effective deterrent to static.
Install static-free carpeting in the work area. You can also use antistatic floor mats and table mats. Install a conductive tabletop to carry static away from the work area.
Use a room humidifier to keep the humidity level above 50% in the work area.