IPC 7711A - 第28页

generally more economical, CCTV systems of fer 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 handl…

Convective heating devices are primarily used for surface

mount component installation and removal and offer the

following potential advantages:

• Can be used to effectively install and remove components

whose solder joints are not accessible, e.g., BGAs (Ball

Grid Arrays) and chip components with bottom only ter-

minations.

• Non-contact process which, if used correctly, won’t dis-

turb joints or obstruct view

• Can often be used to re-align slightly skewed (mis-

aligned) surface mount components without having to

remove ﬁrst

• External ﬂux or tinning generally not necessary to aid

thermal transfer

• Leaves less residue than conductive methods for surface

mount component removal

• For surface mount component removal, match between

nozzle and component geometry less critical

• Works well with solder paste under most conditions

• Can control amount of heat delivery with:

– Gas/Air temperature

– Gas/air ﬂow rate

– Distance of nozzle from work

– Dwell time

• Well designed, powerful convective heating devices pro-

vide continuous output of heated gas/air at a desired set

temperature irrespective of the thermal load of the work

and duty cycle of the application

With convective heating devices, the following guidelines

and precautions should be observed:

• Must properly focus and control heated gas/air ﬂow to

minimize errant heating of substrate, adjacent compo-

nents and their joints

• Must adequately control exit gas/air velocity (via pressure

or ﬂow rate) to avoid:

– displacement of applied solder paste

– disturbing the lead/land alignment of surface mount

components during installation, and to

– minimize errant heating

• Heated air ﬂow inefficient means of primary heat delivery

when compared to conductive heating methods

1.9.5 Preheating and Auxiliary Heating Methods There

are two principal reasons for preheating and auxiliary heat-

ing during component installation and removal:

First, preheating is required when there is a risk of thermal

shock in the substrate, components or both. The goal is to

‘‘ramp up’’ the assembly and/or component at an accept-

ably safe rate until it reaches a target temperature. The

assembly (or component) is then ‘‘thermally soaked.’’ This

eliminates dangerous temperature gradients which could

produce immediate damage, degradation over time or

reduction of reliability.

For avoidance of thermal shock, the rate of ‘‘ramp up’’ can

be critical. For example many ceramic chip capacitor

manufacturers have traditionally recommended that pre-

heating occur at a rate of no greater than 2-4 degrees C/sec.

until a given minimum temperature is reached.

Second, preheating/auxiliary heating is required when the

primary heating method cannot bring all of the solder joints

completely up to proper reﬂow temperature at all or in an

acceptable period of time. This may be due to heat sinking

by nearby portions of the substrate, circuit elements and

adjacent components. The goal is to bring the assembly (or

a portion thereof) up to a sufficient (yet safe) temperature

at which the rate of heat sinking is low enough that the

primary heating device can effect proper solder reﬂow in

an acceptable period of time.

For example, bottom side preheating is often used to speed

up a BGA installation and removal process. The primary

heat source typically delivers heat (usually convective)

only through the top of the component body and it would

otherwise take too long before enough heat passes through

to the joints causing reﬂow.

For through-hole desoldering on heavy multilayer boards

with internal ground planes, auxiliary heating is often used.

This is typically done by positioning a soldering iron tip on

the component side of the lead since the tip of the solder

extractor may not be able to deliver enough heat to com-

pletely reﬂow the joint prior to activating the vacuum.

Preheating is typically accomplished from the bottom side

of the circuit assembly by either a temperature controlled

conductive heating plate, a controlled convective heating

device, or a system which combines both conductive and

convective heating. Controlling both the rate of tempera-

ture ‘‘ramp up’’ as well as the ‘‘soak’’ temperature at which

the assembly is held during the primary reﬂow process is

critical to avoiding damage and optimizing the component

installation or removal process.

1.9.6 Vision Systems and Surface Mount Component

Placement

As high lead count, ﬁne pitch SMDs become

commonplace, the task of properly aligning and placing

these devices during manual SMT rework becomes more

challenging.

Appropriate vision systems with sufficient magniﬁcation,

resolution, ﬁeld of view and working distance are critical

for viewing alignment of component leads to land and

monitoring joint reﬂow during SMD installation.

Vision systems come in various forms including large

lenses, stereo microscopes, trinocular microscopes and

CCTV (video) systems. While microscopes and lenses are

IPC-7711A/7721A October 2003

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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

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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

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No reproduction or networking permitted without license from IHS

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