IPC 7711A - 第29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…

generally more economical, CCTV systems offer greater

ease of use and less operator fatigue, particularly with very

ﬁne pitch SMDs. See IPC-OI-645 for further information

on optical inspection equipment.

Component handling systems which can adequately estab-

lish and maintain X, Y, Z and THETA positioning are also

essential for successful alignment and placement during

ﬁne pitch SMD installation.

1.9.7 Selecting Optimum Process for Manual Assembly/

Rework

Other than reasons such as purchase and life

cycle costs of equipment, operator training and learning

curves as well as other economic considerations, selecting

the optimum process for manual assembly/rework depends

on a variety of factors. These include:

• Type of component

– lead (termination) type

– body composition

• Size of component

• Type of substrate (FR-4, ceramic, etc.)

• Component mounting site

– thermal mass considerations

– adjacent components

– accessibility of component or joints

• Whether the component is being installed or removed

• Whether the component being removed must be salvaged

• Applicable workmanship speciﬁcations

• EOS/ESD control requirements

Every manual assembly/rework process and its attendant

equipment has advantages and precautions in each particu-

lar component installation or removal situation. For this

reason, the Procedures contained herein recommend par-

ticular processes for each individual component

installation/removal situation.

1.9.8 BGA/CSP/Flip Chip Time Temperature Profile

(TTP)

Because the terminations and connections of BGA,

CSP and Flip Chips are under the component, the operator

does not have the ﬂexibility to modify the rework proce-

dure in process or visually inspect the end results. To

ensure acceptable results of the rework procedure, it is

critical to establish a time temperature proﬁle for the

rework process. The following steps are needed to achieve

an acceptable TTP:

A preheat temperature for both the BGA and Printed Wir-

ing Assembly (PWA) (both ceramic and plastic* BGA’s

shall be preheated, as shall all PWA’s).

NOTE: If plastic body or tape bodied components are used,

see IPC J-STD-020 (Moisture/Reﬂow Sensitivity Classiﬁca-

tion for Plastic Integrated Circuit Surface Mount Devices) for

information on moisture sensitivity classiﬁcation tests, precon-

ditioning, and attachment.

Solder paste characteristics must be identiﬁed including

viscosity, thixotropy, rheology, deposition thickness and

drying time/temperature; or if using ﬂux cored wire solder,

land preﬁll solder quantity and preﬁll coplanarity required.

Deﬁne a cleaning procedure which will meet the end item

cleanliness requirements of the customer.

Conﬁrm the destructive physical examination and/or x-ray

analysis that the process deﬁned will yield a BGA attach-

ment which meets any quality/reliability requirements

imposed.

Deﬁne, if used, an accelerated cooling system which does

not exceed thermal gradient limits of the most sensitive

component of the PWA.

CAUTION: THE TIME TEMPERATURE PROFILE IS DEPEN-

DENT, IN PART, ON AMBIENT RELATIVE HUMIDITY. RELA-

TIVE HUMIDITY VARIATIONS OF GREATER ± 15% FROM

THOSE PREVAILING WHEN THE TTP WAS ESTABLISHED

MAY REQUIRE MODIFICATION OF THE PROCEDURE

DEFINED DURING TTP.

1.9.9 Lead Free Solder

The rework of circuit boards

assembled using lead free solders are similar to common

alloys except as noted below. Proper training needs to be in

place to ensure quality and reliability of the assembly. Gen-

erally all that is needed is to understand those differences.

Those differences are:

• In most cases the newer alloys will require more time and

temperature and one must understand why

• The melting point of the solder alloys are likely to be

higher and thus may require a modiﬁed ﬂux chemistry

• Wetting times are generally extended

• Solderability indicators such as wetting angles, joint

appearance etc., will generally be different

• Higher temperatures and longer dwell times may increase

oxidation

• Component lead frames as well as circuit board ﬁnishes

must be compatible with the solder alloy

• Using alternative means of attachment for rework/repair

(such as conductive epoxies) may be advantages due to

temperature and other considerations

• For both conductive and convective assembly rework/

repair, the use of inert atmosphere (such as nitrogen)

should be considered to facilitate the process

October 2003 IPC-7711A/7721A

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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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.

Page1of4

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

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

Page2of4

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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