IPC-CM-770D-1996 - 第140页

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

January

1996

IPC-CM-770

OETAIL

"A"

#14

gauge, black insulated

Gound

Wire

Terminal

10

Candull

1

0

808

Block

See

Oatall

"A"

1

-

To

Mat

To

firth

-

Ground

14

gauge, black insulat

nccrconnccling

Wire

rom

TOP

(0

Mal

FLOOR

MAT

(or

conductive

floo

IPC-1-00299

Figure 26-4 Installation Guidelines for Grounded Work Benches and Floor Mats

provide automatic work-in- progress (WIP) status to a cen-

tral computer. Protection of ESD sensitive components

should be provided by:

Static free conveyer belts.

Grounding of machinery.

Personnel must be grounded at all times when they touch

devicedassemblies.

26.4.2 Automatic Assembly Machines

Handler capa-

bilities and features vary according to the type of unit and

device being accommodated. Automatic units provide

throughput rates on the order of

6,000,

and greater, compo-

nents per hour. Tube adapters are designed to take most

tubes currently in use. Automatic assembly machines range

from very specific machines which repeat the same task

over and over again, to the machines which can be pro-

grammed to do several similar processes.

Consideration must be given to complexity, volume of

units per run and the product mix to be assembled. Equip-

ment should be selected for high flexibility enabling it to

handle a large product mix and short production runs, or

choose highspeed dedicated equipment which does not

allow for much product mix.

There are three basic types of equipment available.

26.4.2.1 lnline Machines

Moderate volume machines

which utilize multiple placement heads. The machine can

have each head placing a single component or a program-

mable head which can place a variety of components up to

approximately

30,000

components per hour.

6-5

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services