IPC-CM-770D-1996 - 第137页

January 1996 IPC-CM-770 Part Six Related Subjects 26.0 HANDLING AND STORAGE 26.1 General Considerations The following general rules should be practiced when handling printed board assemblies: Work stations should be kept…

IPC-CM-770

Januaty

1996

5-34

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January 1996 IPC-CM-770

Part

Six

Related Subjects

26.0 HANDLING AND STORAGE

26.1 General Considerations

The following general

rules should be practiced when handling printed board

assemblies:

Work stations should be kept clean and neat.

There should be no eating or drinking in the work area,

to prevent contamination of the board assemblies.

Handling of the edgeboard contacts should be avoided.

Hand creams and lotions containing silicone should not

be used since they can result in solderability problems.

Lotions formulated specially for use in solder assembly

areas are available.

Stacking of board assemblies should be avoided to pre-

vent physical damage to components. Special racks are

normally provided in assembly areas create contamina-

tion; therefore, changes should be made as frequently as

necessary.

In areas where electrostatic discharge may damage sensi-

tive components, all handling and assembly should be

performed at an antistatic work station.

Caution: Certain substrates and unsealed components such

as some switches, power modules, adjustable devices, etc.,

may be damaged by cleaning agents. Care must be taken to

identify and protect these types of components through the

cleaning process.

26.2 Electrostatic Discharge (ESD) Protection

ESD is

the abbreviation for the term “Electrostatic Discharge,” a

phenomena produced by contact and separation of unlike

materials. Some common examples of activities which gen-

erate ESD are:

Walking across carpet

Combing hair

Rubbing wool against nylons

a result of these phenomena, high voltages (upward of

5,000

volts) are created which, when touching another con-

ductive surface, may create a discharge capable of damag-

ing miniature electronic components and components

mounted on printed boards.

The potentially damaging effects of ESD can be avoided

by:

Grounding personnel and components to avoid an electro-

static charge buildup.

Ionizing sources

Humidity control

Proper handling procedures

The approximate range of susceptibility to damage by

static discharge for several components is shown in the

Table

26-

Table 26-1 Susceptibility Ranges of Various Devices

Exposed to Electrostatic Discharge from a Person or

Electronic Equivalent

Range

Minimum

ESD

Device Type

Susceptibility (Volts)

VMOS

1000 to 2500 Schottky TTL

680 to 1000 SCR

500 to 1500 ECL (PDC Board Level)

380 to

7800

Bipolar Transistors

300 to 3000 Film Resistors (Thick, Thin)

300 to 2500 Schottky Diodes

250 to 3000 CMOS

190 to 2500 OP AMP

150 to 500 SAW

140 to 7000 J FET

100 PLUS EPROM

100 to 300 GaAsFET

100 to 200 MOSFET

30 to 1800

Some do’s and don’ts for proper handling are:

A. Do:

Keep paper, non-conductive plastic, plastic foams or

cardboard off the conductive benchtop. Placing a compo-

nent of printed circuit card on top of any of these mate-

rials effectively insulates the component from ground and

defeats the purpose of the conducting surface.

Keep handcreams and food away from the conductive

work surfaces. If spilled on the benchtop, these materials

will contaminate and increase the resistivity of the work

area.

Be especially careful when using soldering guns around

conductive work surfaces. Solder spills and heat from the

gun may melt and damage the conductive mat.

Check the grounding strap connections daily. Make cer-

tain they are snugly fitted before starting work with the

components and printed boards.

Touch conductive work surface before touching any

components. This drains off any static buildup from the

operator.

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IPC-CM-770

Januaty

1996

Know the ESD caution symbols.

Bolt interconnecting wires to the mats to achieve good

electrical contact.

Use a spike-free and grounded soldering iron.

Don’t:

Don’t allow anyone not grounded to touch ESD sensitive

components in the work area. To be grounded, they must

be standing on the conductive floor mat and must first

touch the conductive benchtop mat before touching the

components or printed circuit cards.

Don’t place ESD-sensitive components on ungrounded

work areas.

Don’t wear extremely thick-soled shoes (like wedgies) or

shoes with heavy plastic soles (like earthshoes). These

will act as insulators and will allow buildup of high static

charges on the operator.

Don’t touch the component by the pins or leads since the

most damage is done at these points by ESD. Handle the

components by their cap edges or body and the printed

circuit cards by their edges.

Don’t handle components or printed circuit cards during

transport from work station to work station. Components

or cards must not be directly handled by anyone not

grounded.

26.2.1 Warning Labels

Warning labels are available that

can be placed on devices, assemblies, equipment, and

packages to warn people about the possibility of inflicting

electrostatic damage to devices they are handling. There

are three different symbols. Symbol (a), specified in MIL-

STD-129, covers not only electrostatic fields, but electro-

magnetic, magnetic, and radioactive fields as well. Symbol

(b), promulgated by the Electric Industries Association

(EIA), is described in EIA Standard RS-471, Symbol (c)

seems to have no official status but is encountered quite

often. Unfortunately, a lightning bolt is also used to indi-

cate high voltage. For example, UL-1244 (and IEC-348)

requires that a lightning bolt label be placed near high

voltage terminals on test equipment. (See Figure 26-1.)

ESD sensitivity/markings indicate an assembly contains

devices that are ESD sensitive, and should be handled as

such. Note that the absence of a symbol does not necessar-

ily mean that the assembly is not ESD sensitive.

26.2.2 Protective Methods

The purpose of the antistatic

work station is to provide a path to ground for static charge

which would otherwise damage sensitive components,

the charge was allowed to discharge on a device or board

assembly. Such work stations have a conductive or anti-

static work surface which is connected to both a common

ground and the worker’s skin, commonly via a wrist strap.

The ground strap should have a minimum

500

kilohm

resistance to eliminate shock hazard. See Table 26-2. An

(a) ESD Susceptibility Symbol

(b) ESD Protective Symbol

(d)

Official Status

IPC-610-001

Figure 26-1 Warning Symbols

example of an antistatic work station is shown in Figures

26-2, 26-3, and 26-4. When necessary, an ionized air

blower may be required for more sensitive applications.

The choice of an ionized air blower versus a ceiling

mounted blower will have to be weighed depending on the

relative merits of each system, i.e., a bench mounted

blower gives localized ESD control and may be more

expensive per square foot than a ceiling mounted system.

26.2.3 Maintenance

ESD workstations require the fol-

lowing general maintenance procedures:

Daily

6-2

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