SC13 Handling - Philips Semiconductors / NXP Semiconductors

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 kΩ 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.