IPC-CM-770D-1996 - 第139页

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.

B.

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,

if

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

(c) MIL-STD-129H Symbol

(d)

No

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:

A.

Daily

6-2

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Table 26-2 Maximum Allowable Resistances and

Discharge Times for Static Safe Operations

Maximum Maximum

Reading from

Discharge Time Resistance Operator Through:

Acceptable

Tolerable*

I I

loo0

megohms

I

Floor mat

to ground

Less than

1

sec.

I

loo0

megohms

I

Less than

1

sec.

to ground

I

Wrist

strap

I

loo

megohms

I

Less than

0.1

sec

to ground

*Minimum resistance to ground for each

is

500

kilohms.

Perform visual inspection of grounding wires and termi-

nals on floormat, benchtop, and grounding receptacle to

insure that proper electrical connections exist.

Clear workstation of static generators, plastic, foams,

cardboard, etc.

Clean benchtop mats with a soft cloth or paper towel

dampened with a mild solution of detergent and water.

B. Weekly

Damp mop floormats (according to the manufacturer’s

recommendation) to remove any accumulated dirt layer

which causes high resistivity.

26.2.4 Handling

All components and/or assemblies des-

ignated as ESD sensitive should be removed from their

protective packaging (anti-static bags, conductive contain-

ers) only at a static safe work station. The following pro-

cedure must be followed before ESD sensitive items are

removed from their protective packaging:

Clear work station of static generators, polyethylene,

vinyls, foam, notebooks, document holders, etc.

Verify grounding wires and connect wrist straps.

Discharge personal static prior to handling devices/

assemblies.

Place protective container on work surface, remove

device/assembly.

After removal of device/assembly from protective packing

and during inspection, testing or assembly, handling should

be as follows (also see Figure

26-6

for proper handling

procedures):

Devices should not be handled by the leads. Avoid touch-

ing leads, circuits, or contacts on assemblies and printed

circuit board assemblies which contain sensitive devices.

Avoid sliding movements of devices over work surface at

any time.

Do not allow devices to contact ordinary plastics or tex-

tiles. Avoid wearing nylon or synthetic type gowns; cot-

ton or a cotton blend is preferred. Short sleeves are also

preferred.

Do not probe or test static sensitive devices with a volt-

ohm meter.

When work is complete, replace device/assembly in con-

ductive bag or container and apply cautionary labels.

26.3 Storage

The preservation of solderability and the

protection of parts and assemblies from handling and ESD

damage should be a vital control aspect of the manufactur-

ing process of printed board assemblies.

Printed boards should be individually wrapped before ship-

ping or transportation to the destination where they will be

received and assembled.

Boards should not be allowed to touch or rub against one

another.

26.3.1 Bags

One of the most common forms of protec-

tion for bare boards is polyethylene bags. Heat sealed, zip-

locked or folded over and taped.

Ensure the boards are thoroughly cleaned before being

packaged and that the atmosphere in which the boards are

packed is reasonably dry. Sealing moisture or a contami-

nated environment within a bag, which is worse, can affect

the solderability and moisture content of the boards. This is

especially true during summer months with high humidity.

Laminated-welded bags (or hot sealed bags) provide the

best protection for long term storage and the boards stored

this way have better solderability than boards in other

packaging.

Printed boards, components and hardware are frequently

replaced in their original containers and put on a shelf until

they are required. Some consideration should be given at

this point to the packing material. If the manufacturer did

not provide a protective material, it is advisable to repack-

age in an approved container.

Parts should not be stored in open containers subject to the

environment, dust and air pollutants that may cause the

parts to oxidize.

Inventory control practice should carefully handle the parts

with a “first in-first out” policy.

Components and boards should not be left exposed over-

night or weekends to accumulate dust and debris from the

atmosphere.

Printed boards distributed to the manual assembly line

should either be racked with spaces between, or stacked

with interleaved paper between, particularly if they are

two-sided boards.

26.3.2 Materials

Care should be exercised to assure that

packaging materials do not contain additives or surface

treatment that will deteriorate solderability or degrade the

insulation properties of components, boards or assemblies.

26.3.3 Racks and Carriers

There are a number of racks

and carriers for assembled boards if they must be stored

6-3

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

-

Figure 26-2 Schematic

of

a Typical Grounded Work Station

Component

Function

or

ccaductive

flooring

and

Floormat

S.NOS

as

IWK

drain

br

p.-

appcosch~ng

the

work

&tion.

Drain6 away charges yeneraled

by

walking

across

floors

(or conductive

Benchtop

mat Pr0nd.s

a

auk-irao

work

ruriwx

Salic

Ch.rgos ara

dra~fmd

away from any aIec~rontc components

or

prlnled clrcull

urds

rosllng

on

th.

suriua

of

th.

mal.

Ground

cord

Prwldes

m

olutrical

connullon

be1ww1-1

lunchlop

nut

and

I(oorrne1

and

ptnr

lhose

Iwo

surfaces

lo

an sanh groutrd

bench

top)

thmm

I

MO

I.Sl*OT

500

KILOHM

RESISTOR

5

NO

TES:

1. 1/16

conductive sheet stock covering bench

top

work area.

2.

Ground strap.

3.

Wrist strap in contact with skin.

4.

Static neutralizer. (Ionized air blower directed at

work.) Primarily

for

use in areas where direct

grounding

is

impractical.

5.

If

floor

or

chair feet are non-conducting, chair must

have ground strap.

IPC-I-

Figure 26-3 Features

of

a Grounded Work Station

between component mounting and soldering. If this is the

26.4 Automation and Equipment

case, it is recommended that the racks be covered with an

dust and debris.

to the production stations is accomplished by means of the

All too frequently, problems due to improper storage arise transporter system, controlled by a central dispatch area.

in the final assembly testing operation or worse, in the form Typically the transporter consists of two levels, the upper

of field failures that can be traced "from the cradle to the one dispatching the work and the lower one for return of

grave," i.e., back to the source of the parts destined to completed work. Strategically placed photocells on the

become a printed board assembly. return conveyer read bar code labels on the tote box which

approved plastic

to

keep

them

Out Of

the

way

Of

26.4.1 Transporters

One method of providing material

6-4

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services