1995 Mar 21 37
Philips Semiconductors
General Handling MOS devices
ELECTROSTATIC CHARGES
Electrostatic charges can exist in many things; for
example, man-made-fibre clothing, moving machinery,
objects with air blowing across them, plastic storage bins,
sheets of paper stored in plastic envelopes, paper from
electrostatic copying machines, and people. The charges
are caused by friction between two surfaces, at least one
of which is non-conductive. The magnitude and polarity of
the charges depend on the different affinities for electrons
of the two materials rubbing together, the friction force and
the humidity of the surrounding air.
Electrostatic discharge is the transfer of an electrostatic
charge between bodies at different potentials and occurs
with direct contact or when induced by an electrostatic
field. All of our MOS devices are internally protected
against electrostatic discharge but they can be damaged
if the following precautions are not taken.
WORK STATION
Figure 5 shows a working area suitable for safely handling
electrostatic sensitive devices. It has a work bench, the
surface of which is conductive or covered by an antistatic
sheet. Typical resistivity for the bench surface is between
1 and 500 k per cm2. The floor should also be covered
with antistatic material. The following precautions should
be observed:
Persons at a work bench should be earthed via a wrist
strap and a resistor.
All mains-powered electrical equipment should be
connected via an earth leakage switch.
Equipment cases should be earthed.
Relative humidity should be maintained between 50 and
65%.
An ionizer should be used to neutralize objects with
immobile static charges.
RECEIPT AND STORAGE
MOS devices are packed for dispatch in
antistatic/conductive containers, usually boxes, tubes or
blister tape. The fact that the contents are sensitive to
electrostatic discharge is shown by warning labels on both
primary and secondary packing.
The devices should be kept in their original packing whilst
in storage. If a bulk container is partially unpacked, the
unpacking should be performed at a protected work
station. Any MOS devices that are stored temporarily
should be packed in conductive or antistatic packing or
carriers.
ASSEMBLY
MOS devices must be removed from their protective
packing with earthed component pincers or short-circuit
clips. Short-circuit clips must remain in place during
mounting, soldering and cleansing/drying processes. Do
not remove more devices from the storage packing than
are needed at any one time. Production/assembly
documents should state that the product contains
electrostatic sensitive devices and that special precautions
need to be taken.
During assembly, ensure that the MOS devices are the last
of the components to be mounted and that this is done at
a protected work station.
All tools used during assembly, including soldering tools
and solder baths, must be earthed. All hand tools should
be of conductive or antistatic material and, where possible,
should not be insulated.
Measuring and testing of completed circuit boards must be
done at a protected work station. Place the soldered side
of the circuit board on conductive or antistatic foam and
remove the short-circuit clips. Remove the circuit board
from the foam, holding the board only at the edges. Make
sure the circuit board does not touch the conductive
surface of the work bench. After testing, replace the circuit
board on the conductive foam to await packing.
Assembled circuit boards containing MOS devices should
be handled in the same way as unmounted MOS devices.
They should also carry warning labels and be packed in
conductive or antistatic packing.
1995 Mar 21 38
Philips Semiconductors
General Handling MOS devices
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Fig.5 Protected work station.
(1) Earthing rail.
(2) Resistor (500 kΩ± 10%, 0.5 W).
(3) Ionizer.
(4) Work bench.
(5) Chair.
(6) Wrist strap.
(7) Electrical equipment.
(8) Conductive surface/antistatic sheet.
(9) Antistatic floor.