Component Handling Precautions

Most constructors know that CMOS ICs need careful handling but some, especially beginners, may not know just what may cause damage and what precautions to take. Obviously one should take reasonable care in handling all components, especially nowadays when so many are of small size, but certain components can be damaged by high voltage static charges. It is easy for these to occur without evidence of their presence, because they are generated by friction between insulating materials, and because so many different plastic materials with very low conductivity are in common everyday use. For instance, if I walk across the nylon carpet from the door of my study to the desk, I can accumulate a static charge of several hundred volts. It has been said in relation to humans that it is the current that kills, and you may have seen demonstrations in which sparks can be drawn from a person who has been charged from an electrostatic generator. But it is the voltage which is lethal to electronic devices.

CMOS devices can be damaged by voltages as low as 250V, so the first precaution to observe is to ensure that the handler is not carrying a static charge. An operative handling such devices in the course of their daily work may have an earthed mat on the bench and an earthed wrist strap to allow any charge to leak away, but for the amateur constructor such extreme precautions are really unnecessary. It is however wise to earth one's body momentarily before handling a CMOS IC and this can be done by touching the metal casing of any earthed electrical appliance which is plugged in but not switched on e.g. a cold soldering iron.

CMOS ICs are supplied in some form of conductive packing, which may be simply aluminium foil, or black (carbon filled) plastic foam. Shielding bags are sometimes used, but the plastic tubes marked ANTISTATIC are not very effective and ICs in these should be handled with extra care. A sensible way to ensure that devices are not damaged is to use IC holders and to insert the devices only when construction is complete and the layout has been checked. Plastic insertion and removal tools are not expensive and are preferable to fingers for manipulating the devices. No device should be inserted while power is applied to the Circuits and obviously one should make sure that the device is inserted the right way round.

Brian Marshall has provided a list of components and the voltages which can damage them, and also some details about protecting such devices:

Device Type             Minimum damaging voltage
VMOS                      30
MOSFET                   100
EPROM                    100
JFET                     140
Op-amp                   190
CMOS                     250
Schottky diodes          300
Film resistors           300
Bipolar transistors      380
SCRs                     680
Schottky TTL            1000

Most CMOS ICs are equipped with internal protective diodes which limit voltages at the inputs to a safe level, but care is still advisable in handling them. If they have to be soldered in directly, the V_DD pin should be connected before any earth connection is made.

Some beginners are anxious about the possibility of component damage during soldering operations. It is true that some components are affected by heat, for instance, resistors change their value when heated, and extreme heat can damage Semiconductors such as diodes and transistors. The best way to avoid such damage is to acquire sound soldering technique. If the surfaces being soldered are clean, the soldering iron need only be applied to the joint for a short time, and the temperature rise in the body of the component will be too small to cause any damage. One can buy soldering stunts to clip on transistor or other leads during soldering operations, but they are really unnecessary - a pair of flat nosed pliers can be applied where one needs to avoid heat being conducted to a sensitive device.