IPC 7711A - 第27页

Convective heating devices are primarily used for surface mount component installation and removal and of fer the following potential advantages: • Can be used to ef fectively install and remove components whose solder j…

1.9.4 Primary Heating Methods Primary heating meth-

ods are those principally responsible for achieving solder

reﬂow during a component installation or removal process.

These are to be distinguished from methods used for pre-

heating and auxiliary heating which are employed in addi-

tion to primary heating methods in particular situations as

described in the Preheating and Auxiliary Heating sec-

tion.

Conductive (by contact) Heating Methods

Handheld conductive heating devices generally fall into

one of two categories: Continuously Heated Devices and

Pulse Heated Devices, each with their own potential advan-

tages and precautions.

Continuously Heated Devices

Continuously heated devices such as soldering irons, ther-

mal tweezers and thermal pick devices may be held at

selected idle tip temperatures prior to use. Continuously

heated devices generally (but not always) employ tinnable

tips to optimize heat transfer to the work.

Virtually all soldering irons and continuous vacuum solder

extractors used for through-hole component installation

and removal, respectively, are continuously heated devices.

For surface mount component installation and removal,

continuously heated devices offer the following potential

advantages:

• Effective at transferring a large amount of heat to a tar-

geted area rapidly

• Can control amount of heat delivery

• Can safely access hard-to-reach places and conﬁne heat to

limited areas with proper tip design, selection and use

• Substrate and adjacent components stay cooler during

surface mount component installation or removal

With continuously heated conductive heating devices, the

following guidelines and precautions should be observed:

• Must utilize a high-efficiency, closed-loop temperature

controlled heating handpiece that has sufficient thermal

output to keep up with thermal load of the work and duty

cycle of the application

• Tip temperature can drop below desired level during

heavy, continuous use if handpiece has insufficient ther-

mal output

• Must establish good thermal linkage between tip and

joint(s), and use appropriate tip geometry (shape) for

effective heat transfer

• Tip and work must be free of oxides and contaminates,

and tip must be tinned for effective heat transfer

• Use of external ﬂux or addition of solder sometimes nec-

essary to achieve effective heat transfer

• For surface mount component removal, must often have

precise match between tip and component geometry for

effective heat transfer to all joints

• Contact may disturb component lead-to-land alignment,

especially during SMD re-alignment operations

• May transfer heat too rapidly for use with solder paste or

sensitive components

• May obstruct view during alignment and reﬂow and inter-

fere with joint formation during solder solidiﬁcation

Pulse Heated Devices

Pulse heated devices such as lap-ﬂow type tools, resistance

tweezers and other handheld devices produce heat directly

in the tip or work with high current, low voltage power.

They are useful for cup terminal soldering and auxiliary

heating of connector pins during removal. These devices

generally employ low mass, non-tinnable tips which can

remain in contact with solder joints as they cool, thereby

facilitating proper surface mount component alignment.

Pulse heated devices offer the following potential advan-

tages:

• Effective at transferring a large amount of heat to a tar-

geted area rapidly

• Slim design tips can safely access tight places and conﬁne

heat to a limited area

• Can control amount of heat delivery with power setting

and dwell time

• Low mass tips heat up and cool down rapidly

• Non-tinnable tips can contact surface mount joint cold,

heat to reﬂow and remain in contact during solder

re-solidiﬁcation to stabilize component alignment

• More gradual heat-up works better with solder paste

• Can correct minor lead non-coplanarity during gull wing

SMD installation

With pulse heated devices, the following guidelines and

precautions should be observed:

• Less effective means to control heat delivery since hand-

held devices are generally not temperature controlled

• Must establish good thermal linkage with joints for effec-

tive heat transfer (this is more difficult since tips are gen-

erally non-tinnable)

• Improper contact may disturb component lead-to-land

alignment

• May produce unacceptable residual stress in some stiff

leads if not coplanar with lands

Convective (by gas/air ﬂow) Heating Methods

Convective heating methods are generally found in devices

such as semi-automated benchtop workstations, high pow-

ered handheld hot air guns and nozzle-focused hot air jet

handpieces.

October 2003 IPC-7711A/7721A

Provided by IHS under license with IPC

Not for Resale

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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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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

Not for Resale

No reproduction or networking permitted without license from IHS

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