IPC-7721(S).pdf - 第21页

OUTLINE Electrostatic Discharge (ESD) is the rapid discharge of electri- cal energy that was created from static sources. When the electrical energy is allowed to come in contact with or even close to a sensitive compone…

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IPC-7721 February 1998

OUTLINE

Electrostatic Discharge (ESD) is the rapid discharge of electri-

cal energy that was created from static sources. When the

electrical energy is allowed to come in contact with or even

close to a sensitive component it can cause damage to the

component. Electrostatic-Discharge Sensitive (ESDS) compo-

nents are those components that are affected by these high

energy surges. The relative sensitivity of a component to ESD

is dependent upon its construction and materials. As compo-

nents become smaller and operate faster, the sensitivity

increases.

Electrical Overstress (EOS) is the internal result of a unwanted

application of electrical energy that results in damaged com-

ponents This damage can be from many different sources,

such as electrically powered process equipment or ESD

occurring during handling or processing.

ESDS components can fail to operate or change in value as a

result of improper handling or processing. These failures can

be immediate or latent. The result of immediate failure can be

additional testing and rework or scrap. However the conse-

quences of latent failure are the most serious. Even though

the product may have passed inspection and functional test,

it may fail after it has been delivered to the customer.

It’s important to build protection for ESDS components into

circuit designs and packaging. However, in the manufacturing

and assembly areas, we often work with unprotected elec-

tronic assemblies that are attached to the ESDS components.

This section will be dedicated to safe handling of these unpro-

tected electronic assemblies.

For that purpose, the following subjects are addressed:

2.1.1 Electrical Overstress (EOS) Damage Prevention

2.1.2 Electrostatic Discharge (ESD) Damage Prevention

2.1.2.1 Warning Labels

2.1.2.2 Antistatic Materials

2.1.3 Anti-Static Work Station

2.1.4 Physical Handling

Information in this specification is intended to be general in

nature. Additional detailed information can be found in EIA-

625, Requirements for Handling Electrostatic-Discharge-

Sensitive (ESDS) Devices

2.1.1 Electrical Overstress (EOS) Damage Prevention

Electrical components can be damaged by unwanted electri-

cal energy from many different sources. This unwanted elec-

trical energy can be the result of ESD potentials or the result

of electrical spikes caused by the tools we work with, such as

soldering irons, soldering extractors, testing instruments or

other electrically operated process equipment. Some devices

are more sensitive than others. The degree of sensitivity is a

function of the design of the device. Generally speaking higher

speed and smaller devices are more susceptible than their

slower, larger predecessors. The purpose or family of the

device also plays an important part in component sensitivity.

This is because the design of the component can allow it to

react to smaller electrical sources or wider frequency ranges.

With todays products in mind, we can see that EOS is a more

serious problem than it was even a few years ago. It will be

even more critical in the future.

When considering the susceptibility of the product we must

keep in mind the susceptibility of the most sensitive compo-

nent in the assembly. Applied unwanted electrical energy can

be processed or conducted just as an applied signal would be

during circuit performance.

Before handling or processing sensitive components, tools

and equipment need to be carefully tested to ensure that they

do not generate damaging energy, including spike voltages.

Current research indicates that voltages and spikes less than

0.5 volt are acceptable. However, an increasing number of

extremely sensitive components require that soldering irons,

solder extractors, test instruments and other equipment must

never generate spikes greater than 0.3 volt.

As required by most ESD specifications including EIA-625,

periodic testing may be warranted to preclude damage as

equipment performance may degrade with use over time.

Maintenance programs are also necessary for process equip-

ment to ensure the continued ability to not cause EOS dam-

age.

EOS damage is certainly similar in nature to ESD damage,

since damage is the result of undesirable electrical energy.

2.1.2 Electrostatic Discharge (ESD) Damage Prevention

The best ESD damage prevention is a combination of prevent-

ing static charges and eliminating static charges if they do

Product Class: R, F, C, W

Skill Level: Intermediate

Level of Conformance: High

7721

Repair and Modification of

Printed Boards and Electronic Assemblies

Revision:

Date: 2/98

Handling Electronic

Assemblies

Number: 2.1

Material in this manual was voluntarily established by Technical Committees of the IPC. This material is advisory only and its

use or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of this

material. Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement.

Equipment referenced is for the convenience of the user and does not imply endorsement by the ipc.

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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

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