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  induced damage. If conductors are at different potentials,

static dissipative materials slowly equalize the charge on the conductors so that an ESD spark will not occur when the two conductors touch.

SUMMARY

• ESD is a spark.

• Rubbing insulators together and separating them results in a static charge on the two surfaces.

• Static charge induces a charge on a conductive surface.

• When two conductive surfaces of different potentials come near.or touch, a spark is generated.

• ICs can be damaged by ESD.

Antistatic devices and procedures try to slowly equalize the charge of two conductors before they meet to prevent ESD.

• Humidification

40-50% Relative Humidity is the optimum range. Getting too high can cause other problems, including corrosion.

• Antistats

Periodically sprayed or painted, they can make difficult conditions 'livable' until improvements can be made.

• Electromagnetic Wavefront Protection

What Can the Maintenance Shop do to Help Reduce ESD Component Failures? The most desirable method of winning against the effects of ESD is to prevent charges from accumulating in the first place.

Simply stated, if there are no charges, then there can be no discharges. Here are some areas to consider for reducing the accumulation of static charge:

• Floors: Rough is better than polished; if carpeted, use

anti-static varieties. If plastic mats are used to protect carpets from desk-chair wheels, use antistatic materials instead of acrylic or vinyl.

• Footwear: Leather soles are preferable, plastic and rubber are the worst.

• Clothing: Cotton is best, wool fair, synthetics poor.

GENERAL HANDLING PROCEDURES FOR ALL SEMI-CONDU.CTORS

1t is not possible to lay down a degree of electrostatic protection which would cover all types of semi-conductor.

However, there is a strong consensus of opinion that a significant reduction of dangers related to electrostatic charges can be achieved by making personnel aware of possible electrostatic generators and improved general handling techniques; such as:

(a) Not removing or replacing line replaceable units with electrical power applied.

(b) Not unnecessarily touching the connectors, leads or edge connectors, etc., of printed circuit boards containing such devices.

(c) By using conductive packaging, shorting plugs,

bands or wire when provided or prescribed in the relevant aircraft or equipment Maintenance Manual.

(d) By paying particular attention to stores procedures to ensure that protective packaging is not removed during any goods-inwards inspection.

ELECTROSTATIC-FREE WORK STATION

ƒ General: If, by the nature and volume of work, it is considered necessary to set up an

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  free work station, guidance may be obtained from

the following paragraphs which set out the various options which are open.

ƒ Humidity: A factor which needs to be considered when working with electrostatic-sensitive devices is the humidity of the working environment. The air in a very low humidity environment is dry and has-a very high resisthas-ance, such has-air will not dischhas-arge the static electricity as quickly as a moist air.

Therefore, the working environment for an

electrostatic-free work station should ideally have a relative humidity of between 30 and 50%.

ƒ Working Environment: There are two basic methods of achieving a safe working environment in which to handle electrostatic-sensitive devices.

One is dependent upon the provision of a

conductive work surface, which, together with the operator and tools in use, is bonded electrically to a common ground. The other makes use of the conductive properties of an ionised atmosphere to dissipate static electrical charges.

ƒ Conductive Work Surface Technique

(a) The work surface of a bench is covered with a sheet of conductive material, e.g. plastics, or mat which is secured to the bench to prevent it from moving. The floor area in front of the bench is also covered with conductive material and electrically bonded to the work surface by means of a bonding strap. To be effective, the bonding strap should have a resistance of approximately 2000 to 4000 ohms per linear foot, and should be as short as possible. A further bonding strap is used to link a

wrist strap, worn by the operator, to the work surface, and this should have a resistance of 200 k ohms to 1M ohms. To complete the system the work surface is connected to a suitable ground point. In addition the work seat may be covered with a conductive seat cover.

NOTE: Under no circumstances should the work surface of a static-free work station be connected to the electrical power supply ground circuit of the building.

(b) The main disadvantage of the conductive work surface is its conductivity. As each element of the system is bonded to a common ground to which the operator is connected via a wrist strap, immediately the operator is in direct contact with the work surface, which normally has a surface resistivity of approximately 3000 ohms, the wrist strap resistance is rendered ineffective.

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• Recommended Handling Procedures for ESD Components

(SHOP) The procedures and guidelines to make sure of the proper handling of ESDS components and assemblies during disassembly, repair, assembly, test setup, fault isolation, and preparation for shipment or storage are outlined in the following paragraphs. All CCAs contained in most LRUs are ESDS. Do not remove conductive dust caps unless required for disassembly, assembly, repair, or test.

For purpose of this appendix, once an ESDS subassembly has been installed into the LRU

(conductive enclosure) with external connectors capped with conductive dust caps it shall be considered

protected from electrostatic discharge.

• Dielectric strength or insulation resistance tests:

These types of tests are not recommended for equipment containing ESDS components or assemblies.

ƒ Continuity Checks

Certain metal-oxide semiconductor (MOS) devices have built-in protective diodes, which cannot sustain more than a 10-mA current. Do not attempt to perform continuity checks of these diodes.

• Clothing

Wearing apparel of personnel working on ESDS

assemblies shall not include smocks, shoes, gloves, or finger cots made of plastic, rubber, or nylon. Cotton or 2 percent stainless steel wearing apparel is preferred over any synthetic material.

• Smocks

Personnel handling ESDS sensitive components or assemblies shall wear a smock with a surface resistivity of 1012 0hms/sq foot maximum when measured according to American Society for Testing Materials (ASTM) test method 0257, or equivalent.

Personnel shall not put on or remove a smock in the area where unpackaged ESDS devices are present.

• Wrist straps and grounded work stations

ESDS devices shall be handled by personnel wearing a grounded wrist strap and working at a grounded work station. When mobility is required, conductive heel straps or conductive footwear shall be used in CCA:

CIRCUIT CARD ASSY

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  conjunction with conductive flooring or conductive floor

mats.

• Procedures

Observe the following procedures when replacing a CCA containing ESDS components:

• Power down the test setup configuration in accordance with the ATLAS procedure or automatic test.

ƒ When removing a CCA, grasp the assembly by the printed circuit board edges only, taking care not to touch any circuit paths or component leads.

• Remove the replacement CCA from the packing materials.

• When installing a CCA, grasp the CCA by the edges only, taking care not to touch any circuit paths or component leads.

• Each ESDS CCA shall be covered, wrapped, bagged, or stored in approved racks whenever it is not being tested or worked on. Material used shall have a maximum surface resistivity of

10¹²0hms/sq cm when measured according to ASTM test method 0257, 0991 or better. Wrap in a

manner that prevents any part of the CCA from puncturing the protective material.

• Packaging

When CCA containing ESDS components are shipped or stored, the CCA shall be packed so that external leads are electrically connected to each other through a

maximum resistivity of 10¹² ohms/sq cm when measured according to ASTM test method 0257, or equivalent or better.

• ESDS Labels

CCA containing ESDS components shall be shipped and stored in containers marked or tagged with a legend of an attention attracting color easily readable at a distance of 3 feet. The label will contain the following information:

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• Wrapping

CCA containing ESDS components shall be wrapped in material with a maximum surface resistivity of

10⁵0hms/sq cm if packing material has surface resistivity of greater than 10 ¹² ohms/sq cm in packing. Measurement shall be according to test method ASTM-0257-75BO-C, or equivalent.

• Brushes

Only natural bristle brushes shall be used. Acid brushes shall not be used.

ƒ Minimum Standards for Equipment

Soldering irons, soldering pots, or flow soldering

equipment shall be transformer-isolated from the power line and grounded. Resistance reading from the tip of a soldering iron to ground shall be less than 0.20 as measured on the R x 1 scale of a Simpson Model 260 meter, or equivalent. Low voltage (less than 15 V)

soldering equipment need not be grounded if the isolation transformer contains a ground electrostatic shield

between its primary and secondary windings. Other electronic equipment or tools which come into contact with ESDS components or assemblies shall be grounded (200.0 or less) when measured on the R x 10 scale of a Simpson Model 260 meter, or equivalent.

ƒ Grounding

When handling unprotected ESDS components and assemblies, personnel shall be grounded in accordance with the following:

A wrist strap is worn in direct contact with the bare skin, never over clothing. The grounding wire of the strap shall be connected to the bench top.

WARN1NG:

PERSONNEL SHALL NEVER BE ATTACHED