IPC-7721(S).pdf - 第24页

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 shield…

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

Page4of8

damage EOS sensitive components or could cause injury to

personnel. Most ESD specifications also require these poten-

tials to be electrically common. The use of ground fault inter-

rupter (GFI) electrical outlets at EOS/ESD workstations is

highly recommended.

2.1.4 Physical Handling

Care must be taken during acceptability inspections to ensure

product integrity at all times. Table 4 provides general guid-

ance.

Physical Damage

Improper handling can readily damage components and

assemblies (e.g., cracked, chipped or broken components

and connectors, bent or broken terminals, badly scratched

board surfaces and conductor lands). Physical damage of this

type can ruin the entire assembly or attached components.

Contamination

Contamination by handling with bare hands or fingers without

some form of protection causes soldering and coating prob-

lems; body salts and oils, and unauthorized hand creams are

typical contaminants. Body oils and acids reduce solderability,

promote corrosion and dendritic growth. They can also cause

poor adhesion of subsequent coatings or encapsulates.

Lotion formulated specifically for use in solder assembly areas

is available. Normal cleaning procedures will not always

IPC-610-002

Figure 2 Target condition EOS/ESD Workstation

Building Floor

Common

Ground Point

Ground

Personal

Wrist Strap

EOS Protective

Trays, Shunts, etc.

EOS Protective

Table Top

EOS Protective

Floor or Mat

1 M Ohm 10%

IPC-610-003

Figure 3 Acceptable EOS/ESD Workstation

Building Floor

Common

Ground Point

Ground

Personal

Wrist Strap

EOS Protective

Trays, Shunts, etc.

EOS Protective

Table Top

EOS Protective

Floor or Mat

1 M Ohm 10%

Table 4 General Rules for

Handling Electronic Assemblies

1. Keep work stations clean and neat. There must not be

any eating, drinking, or use of tobacco products in the

work area.

2. Minimize the handling of electronic assemblies and

components to prevent damage.

3 When gloves are used, they need to be changed as fre-

quently as necessary to prevent contamination from

dirty gloves. (See Figure 4).

4. Solderable surfaces are not to be handled with bare

hands or fingers. Body oils and salts reduce solderabil-

ity, promote corrosion and dendritic growth. They can

also cause poor adhesion of subsequent coatings or

encapsulates.

5. Do not use hand creams or lotions containing silicone

since they can cause solderability and conformal coat-

ing adhesion problems..

6. Never stack electronic assemblies or physical damage

may occur. Special racks need to be provided in

assembly areas for temporary storage.

7. Always assume the items are ESDS even if they are not

marked.

8. Personnel must be trained and follow appropriate ESD

practices and procedures.

9. Never transport ESDS devices unless proper packaging

is applied.

IPC-7721

Number: 2.1

Revision:

Date: 2/98

Subject: Handling Electronic Assemblies

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