IPC 7711A - 第30页

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 rel…

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

2.1.3 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

occur. All ESD protection techniques and products address

one or both of the two issues.

Product Class: R, F, C, W

Skill Level: Intermediate

Level of Conformance: High

7711A/7721A

General Information and

Common Procedures

Revision: A

Date: 5/02

Handling Electronic

Assemblies

Number: 2.1

Material in this manual, IPC-7711 Rework of Electronic Assemblies, was voluntarily established by Technical Committees of

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 IPC.

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Not for Resale

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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.

2.1.3 Physical Handling

Care must be taken during acceptability inspections to ensure

product integrity at all times. Table 3 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

remove such contaminants. The best solution is to prevent

contamination.

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-7711A/7721A

Number: 2.1

Revision: A

Date: 5/02

Subject: Handling Electronic Assemblies

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Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--``,``,-`-`,,`,,`,`,,`---

Handling Electronic Assemblies

If no ESDS markings are on an assembly, it still needs to be

handled as if it were an ESDS assembly. However, ESDS

components and electronic assemblies need to be identified

by suitable EOS/ESD labels. Many sensitive assemblies will

also be marked on the assembly itself, usually on an edge

connector. To prevent ESD and EOS damage to sensitive

components, all handling, unpacking, assembly and testing

must be performed at a static controlled work station.

Avoid contaminating solderable surfaces prior to soldering.

Whatever comes in contact with these surfaces must be

clean. When boards are removed from their protective wrap-

pings, handle them with great care. Touch only the edges

away from any edge connector tabs. Where a firm grip on the

board is required due to any mechanical assembly procedure,

gloves meeting EOS/ESD requirements need to be worn.

These principles are especially critical when no-clean pro-

cesses are employed.

Handling After Solder

After soldering and cleaning operations, the handling of elec-

tronic assemblies still requires great care. Finger prints are

extremely hard to remove and will often show up in confor-

mally coated boards after humidity or environmental testing.

Gloves or other protective handling devices need to be used

to prevent such contamination. Use mechanical racking or

baskets with full ESD protection when handling during clean-

ing operations.

Common Tools and Equipment

Work environments require tools and equipment to conduct

electronic assembly operations. The following information is

provided as guidance regarding the use of common equip-

ment. EIA-625 provides more specific information.

Table 3 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

frequently as necessary to prevent contamination from

dirty gloves.

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-7711A/7721A

Number: 2.1

Revision: A

Date: 5/02

Subject: Handling Electronic Assemblies

Page3of4

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--``,``,-`-`,,`,,`,`,,`---