IPC-7721(S).pdf - 第23页

S8.1-1992 as well as the Electronic Industries Association (EIA) in EIA standard RS-471. Note that the absence of a symbol does not necessarily mean that the assembly is not ESD sensitive. When doubt exists about the sen…

occur. All ESD protection techniques and products address

one or both of the two issues.

ESD damage is the result of electrical energy that was gener-

ated from static sources either being applied or in close prox-

imity to ESDS devices. Static sources are all around us. The

degree of static generated is relative to the characteristics of

the source. To generate energy relative motion is required.

This could be contacting, separation, or rubbing of the mate-

rial.

Most of the serious offenders are insulators since they con-

centrate energy where it was generated or applied rather than

allowing it to spread across the surface of the material. Com-

mon materials such as plastic bags or Styrofoam containers

are serious static generators and as such are not to be

allowed in processing areas especially static safe areas. Peel-

ing adhesive tape from a roll can generate 20,000 volts. Even

compressed air nozzles which move air over insulating sur-

faces generate charges.

Destructive static charges are often induced on nearby con-

ductors, such as human skin, and discharged into conduc-

tors. This can happen when a printed board assembly is

touched by a person having a static charge potential. The

electronic assembly can be damaged as the discharge

passes through the conductive pattern to a static sensitive

component. Static discharges may be too low to be felt by

humans (less than 3500 volts), and still damage ESDS

components. Typical static voltage generation is included in

Table 2

2.1.2.1 Warning Labels

Warning labels are available for posting in facilities and place-

ment on devices, assemblies, equipment and packages to

alert people to the possibility of inflicting electrostatic or

electrical overstress damage to the devices they are handling.

An example of frequently encountered labels are shown in

Figure 1.

Symbol (a) ESD susceptibility symbol. A triangle with a reach-

ing hand and a slash across it. This is used to indicate that an

electrical or electronic device or assembly is susceptible to

damage from an ESD event.

Symbol (b) ESD protective symbol. This differs from the ESD

susceptibility symbol in that it has an arc around the outside

of the triangle and no slash across the hand. This is used to

identify items that are specifically designed to provide ESD

protection for ESD sensitive assemblies and devices.

Symbols (a) and (b) identify devices or an assembly as con-

taining devices that are ESD sensitive, and that they must be

handled accordingly. These symbols are promoted by the

ESD association and are described in EOS/ESD standard

Table 1 Typical Static Charge Sources

Work surfaces Waxed, painted or varnished surfaces

Untreated vinyl and plastics

Glass

Floors Sealed concrete

Waxed or finished wood

Floor tile and carpeting

Clothes and

personnel

Non-ESD smocks

Synthetic materials

Non-ESD Shoes

Hair

Chairs Finished wood

Vinyl

Fiberglass

Non-conductive wheels

Packaging and

handling materials

Plastic bags, wraps, envelopes

Bubble wrap, foam

Styrofoam

Non-ESD totes, trays, boxes, parts

bins

Assembly tools and

materials

Pressure sprays

Compressed air

Synthetic brushes

Heat guns, blowers

Copiers, printers

Table 2 Typical Static Voltage Generation

Source 10-20% humidity 65-90% humidity

Walking on carpet 35,000 volts 1,500 volts

Walking on vinyl

flooring

12,000 volts 250 volts

Worker at a bench 6,000 volts 100 volts

Vinyl envelopes

(Work Instructions)

7,000 volts 600 volts

Plastic bag picked

up from the bench

20,000 volts 1,200 volts

Work chair with

foam pad

18,000 volts 1,500 volts

IPC-7721

Number: 2.1

Revision:

Date: 2/98

Subject: Handling Electronic Assemblies

Page2of8

S8.1-1992 as well as the Electronic Industries Association

(EIA) in EIA standard RS-471.

Note that the absence of a symbol does not necessarily mean

that the assembly is not ESD sensitive. When doubt exists

about the sensitivity of an assembly, it must be handled

as a sensitive device until it is determined otherwise.

Recommend removing the reference symbols c and d since

they were removed from the MIL specs over 10 years ago.

Symbol (c) covers not only electrostatic fields, but electromag-

netic and magnetic fields as well. The use of this symbol to

designate ESD is discouraged since it does not specifically

identify EOS/ESD sensitivity.

Symbol (d) has been used widely as an ESD symbol. The

standard meaning for the lightning bolt symbol is high voltage

in the context of electrical utility power, and typically damag-

ing to personnel. For example, UL-1244 (and IEC-348)

requires that a lightening bolt label be placed near high volt-

age terminals on test equipment. Unfortunately, this symbol is

encountered quite often to designate static sensitive devices.

The use of a lightning bolt to designate ESD is discour-

aged to avoid possible dangerous confusion.

2.1.2.2 ESD Protective Materials

ESDS components and assemblies must be protected from

static sources when not being worked on in static safe envi-

ronments or work stations. This protection could be conduc-

tive static-shielding boxes, bags or wraps.

ESDS items must be removed from their protective enclo-

sures only at static safe work stations.

It is important to understand the difference between the three

types of protective enclosure material: (1) static shielding (or

barrier packaging), (2) antistatic, and (3) static dissipative

materials.

Static shielding packaging will prevent an electrostatic dis-

charge from passing through the package and into the

assembly causing damage.

Antistatic packaging materials are used to provide inex-

pensive cushioning and intermediate packaging for ESDS

items. Antistatic materials do not generate charges when

motion is applied. However, if an electrostatic discharge

occurs, it could pass through the packaging and into the part

or assembly, causing EOS/ESD damage to ESDS compo-

nents.

IPC-610-001

Figure 1 Frequently Encountered EOS/ESD Warning

Labels

▼

CAUTION



SENSITIVE ELECTRONIC DEVICES



DO NOT SHIP OR STORE NEAR STRONG

ELECTROSTATIC, ELECTROMAGNETIC,

MAGNETIC OR RADIOACTIVE FIELDS

CAUTION

STATIC

ELECTRICITY CAN DAMAGE

CONTENTS

OPEN ONLY

AT STATIC-FREE STATION

(b) ESD Protective Symbol

(d) No Official Status

(a) ESD Susceptibility Symbol

IPC-7721

Number: 2.1

Revision:

Date: 2/98

Subject: Handling Electronic Assemblies

Page3of8

Static dissipative materials have enough conductivity to

allow applied charges to dissipate over the surface relieving

hot spots of energy. Parts leaving an EOS/ESD protected

work area must be overpacked in static shielding materials,

which normally also have static dissipative and antistatic

materials inside.

Do not be misled by the ‘‘color’’ of packaging materials. It is

widely assumed that ‘‘black’’ packaging is static shielding or

conductive and that ‘‘pink’’ packaging is antistatic in nature.

While that may be generally true, it can be misleading. In addi-

tion, there are many new clear materials now on the market

that may be antistatic and even static shielding. At one time,

it could be assumed that clear packing materials introduced

into the manufacturing operation would represent an EOS/

ESD hazard. This is not necessarily the case now.

Caution: Some static shielding and antistatic materials and

some topical antistatic solutions may affect the solderability of

assemblies, components, and materials in process. Select

only non-contaminating packaging and handling materials for

in-process assemblies and use them with regard for the ven-

dors instructions. Solvent cleaning of static dissipative or anti-

static surfaces can degrade their ESD performance. Follow

the manufacturers recommendations for cleaning.

2.1.3 Anti-Static Work Station

An EOS/ESD safe work station prevents damage to sensitive

components from spikes and static discharges while opera-

tions are being performed. Safe work stations should include

EOS damage prevention by avoiding spike generating repair,

manufacturing or testing equipment. Soldering irons, solder

extractors and testing instruments can generate energy of suf-

ficient levels to destroy extremely sensitive components and

seriously degrade others.

For ESD protection, a path-to-ground must be provided to

neutralize static charges that might otherwise discharge to a

device or assembly. ESD safe work stations also have static

dissipative or antistatic work surfaces which are connected to

a common ground. Provisions are also made for grounding

the worker’s skin, preferably via a wrist strap to eliminate

charges generated on the skin or clothing.

Provision must be made in the grounding system to protect

the worker from live circuitry as the result of carelessness or

equipment failure. This is commonly accomplished through

resistance in line with the ground path, which also slows the

charge decay time to prevent sparks or surges of energy from

ESD sources. Additionally, a survey must be performed of the

available voltage sources that could be encountered at the

work station to provide adequate protection from personnel

electrical hazards.

For maximum allowable resistance and discharge times for

static safe operations, see Table 3.

Note: the selection of resistance values are to be based on

the available voltages at the station to ensure personnel safety

as well as to provide adequate decay or discharge time for

ESD potentials.

Examples of static safe work stations are shown in Figures 2

and 3. When necessary, air ionizers may be required for more

sensitive applications. The selection, location, and use proce-

dures for ionizers must be followed to ensure their effective-

ness.

Keep static dissipative or antistatic work station(s) free of

static generating materials such as styrofoam, plastic solder

removers, sheet protectors, plastic or paper notebook folders,

and employees personal items.

Periodically check EOS/ESD work stations to make sure they

work. EOS/ESD assembly and personnel hazards can be

caused by improper grounding methods or by an oxide

build-up on grounding connectors. Tools and equipment must

be periodically checked and maintained to ensure proper

operation.

Particular care must be given to ‘‘third wire’’ ground termina-

tions. Frequently, instead of being at workbench or earth

potential, the third wire ground may have a ‘‘floating’’ poten-

tial of 80 to 100 volts. This 80 to 100 volt potential between

an electronic assembly on a properly grounded EOS/ESD

work station and a third wire grounded electrical tool may

Table 3 Maximum Allowable Resistance and

Discharge Times for Static Safe Operations

Reading from

Operator

Through

Maximum

Tolerable

Resistance

Maximum

Acceptable

Discharge Time

Floor mat to

ground

1000 megohms Less than 1 sec.

Table mat to

ground

1000 megohms Less than 1 sec.

Wrist strap to

ground

100 megohms Less than 0.1 sec.

IPC-7721

Number: 2.1

Revision:

Date: 2/98

Subject: Handling Electronic Assemblies

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