IPC 7711A - 第25页

Operator Friendly Pr ocess — An operator of average abil- ity can, with proper training and practice, become accept- ably proﬁcient in employing, and when required, modify- ing the process to suit any particular requirem…

preferably a lightweight, high quality, EOS/ESD con-

trolled motorized rotary tool. This tool can be used for

detailed work (i.e., solder resist and conformal coating

removal, grinding out burns or laminate defects, drill-

ing out plated holes, cutting ﬁne pitch conductors etc.).

9. Precision Drill/Mill System

Demanding projects often require the need to make

very precise holes, slots, groves etc.. Accurate depth

control and high speed may be required. A precision

drilling/milling system with ﬁxturing to hold the

printed board assembly and an attached microscope

may be advisable for those unusually demanding

projects.

10. Replacement Conductors and Lands

There are commercially available replacement conduc-

tors and lands that are normally fabricated from cop-

per foil and plated with solder or nickel and gold for

edge contact repair. These conductors and lands are

available with or without a dry ﬁlm adhesive on the

back. Adhesive backed conductors and lands are nor-

mally heat bonded to the board surface. Replacement

conductors and lands are available in hundreds of dif-

ferent shapes.

Compatible replacement conductors and features may

also be salvaged from scrap printed wiring boards, if

necessary.

11. Gold Plating System

Plating gold edge contacts or any metal surface

requires the use of materials that may have environ-

mental and safety concerns and must be handled prop-

erly. The power applied to the plating surfaces must be

controlled accurately to expect reliable results. A good

plating systems should include; a DC power supply

with voltage and current meters, plating anodes sized

for gold edge contact plating, a solution tray to collect

the solution runoff, a support for the PC board and a

tray to hold and store the various chemicals safely.

12. Epoxy and Coloring Agents

Many repair operations require the use of high

strength, high temperature epoxies. For high tempera-

ture applications two-part epoxies offer the highest

strength, thermal resistance and durability. It may also

be important to have resists or coloring agents so that

you can restore the cosmetic appearance of the board.

It is best to cure the epoxies in an oven if possible.

13. Eyelets and Eyelet Press System

Solder plated copper eyelets and an eyelet press/setting

tool to repair damaged plated through holes may be

required.

14. Cleaning Station/System

Regardless of the Class of Product serviced, a cleaning

system that is chemically matched to the ﬂux sys-

tem(s) in use will be essential to a satisfactory repair.

In organizations that perform procedures on Class 3

Products, it may also be necessary to have a cleanli-

ness test system in order to periodically evaluate the

ability of the cleaning system to meet the

requirements/expectations of the user. Interim or

in-process cleaning at the workstation should be used

pending completion of the procedure and the ﬁnal

cleaning.

15. Tools and Supplies

Also needed are a wide assortment of hand tools

including tweezers, various pliers, ﬁles, dental picks,

cutting tools and materials such as ﬂuxes, solders, and

other common items.

16. Conformal Coating Area

The cost, safety concerns and utility services (air

pressure/vacuum, power, venting, UV illuminations,

etc.) of equipment associated with both the removal

and application of conformal coating suggest to many

organizations that one central conformal coating and

encapsulant area be installed.

17. Materials

The materials listed are ‘‘generic’’ in nature. It is rec-

ommended that these materials are available or

approved by your company. The use of certain materi-

als includes some increased risk (ﬁre, personnel safety,

etc.) and such materials should not be used unless

appropriate safety precautions are enforced.

1.9 Process Goals and Guidelines In the three basic

processes of Component Removal, Land Preparation

and Component Installation/Replacement, the funda-

mental Process Goals and Guidelines are as follows:

Non-destructive Process — During any assembly or rework

process, no damage or degradation should occur to the

board (both substrate and circuit elements), adjacent com-

ponents, and the component to be installed or removed.

This damage may be either mechanical, thermo/mechanical

or purely thermal in nature and may result in either imme-

diate failure, degradation in performance over time (latent

failure) or a reduction in reliability.

EOS/ESD damage must also be avoided by employing

proper work procedures, work stations and equipment con-

trols.

Controllable, Reliable and Repeatable Process — The pro-

cess can be employed, and when necessary, modiﬁed by a

trained operator in a repetitive fashion with consistently

acceptable results.

Process Appropriate to Particular Application — The pro-

cess (or modiﬁcation thereof) employed is appropriate to

the particular application based on the relevant guidelines

described below.

October 2003 IPC-7711A/7721A

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

Operator Friendly Process — An operator of average abil-

ity can, with proper training and practice, become accept-

ably proﬁcient in employing, and when required, modify-

ing the process to suit any particular requirements of a

given task.

Effıcient Process — The process can be done repeatedly in

a production environment quickly and easily at minimal

costs with little or no down-time. Set-up and training time

must also be minimal.

1.9.1 Non-destructive Component Removal The par-

ticular process goals and guidelines for non-destructive

component removal are as follows:

Surface Mount Components

• Pre-/auxiliary heat assembly and/or component if

required

• Evenly apply heat in a rapid, controllable fashion to

achieve complete, simultaneous reﬂow (melt) of all solder

joints

• Avoid thermal and/or mechanical damage to component,

board, adjacent components and their joints

• Immediately remove component from board before any

solder joint re-solidiﬁes

• Prepare lands for replacement component

Through-hole Components

Desolder component one joint at a time using vacuum

method:

• Pre-/auxiliary heat assembly and/or component if

required

• Heat joint in a rapid, controllable fashion to achieve com-

plete solder reﬂow

• Avoid thermal and/or mechanical damage to component,

board, adjacent components and their joints

• Apply vacuum during lead movement to cool joint and

free lead

Component removal using solder fountain method:

• Reﬂow all joints in solder fountain

• Remove old component and either immediately replace

with new component, or clear through-holes for compo-

nent replacement later

1.9.2 Surface Mount Land Preparation Surface mount

land preparation should be performed prior to the

installation/replacement of a new surface mount compo-

nent. Avoidance of thermal and/or mechanical damage to

the land and substrate is critical.

• The two primary steps include:

1. Remove Old Solder — This may be performed with a

soldering iron and braided solder wicking material, or

with a continuous vacuum ‘‘Flo’’ desoldering tech-

nique employing a solder extractor and a special tip

which allows reﬂow and vacuum aspiration of the old

solder to occur continuously.

2. Clean Lands — Old ﬂux residues leftover after the

removal of old solder are cleaned in this step prior to

adding new solder.

This step is part of the Component Installation pro-

cess and is accomplished by either preﬁlling (pre-

tinning) the lands (by reﬂowing wire solder with a

soldering iron or some other heating method), or by

applying solder paste (cream) with a dispenser prior to

(or after) the component is placed on the land pattern.

The quantity of solder applied is critical to achieving

acceptable joints. For instance, J-lead solder joints

require much more solder than gull wing lead solder

joints.

1.9.3 Component Installation The particular process

goals and guidelines for component installation are as fol-

lows:

Surface Mount Components

• Preﬁll lands or apply solder paste

• Align and place component to lands (tack if necessary)

• Apply solder paste to lead/land area if not applied prior to

component placement

• Pre-/auxiliary heat assembly and/or component if

required

• Pre-dry applied solder paste

• Reﬂow solder joints (individually, in groups or all

together) with concentrated ‘‘targeted’’ heat in a rapid,

controllable manner while maintaining lead/land align-

ment. Joints should remain at target temperature (above

melting point of solder alloy) for proper time to achieve

optimal intermetallic formation.

• Avoid thermal and/or mechanical damage to component,

board, adjacent components and their joints.

• Clean and inspect

Through-Hole Components

• Insert new component into board

• Pre-/auxiliary heat assembly and/or component if

required

• Solder joints (individually, in groups or all together) with

concentrated ‘‘targeted’’ heat in a rapid, controllable man-

ner. Joints should remain at target temperature (above

melting point of solder alloy) for proper time to achieve

optimal intermetallic formation.

• Avoid thermal and/or mechanical damage to component,

board, adjacent components and their joints.

• Clean and inspect

IPC-7711A/7721A October 2003

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

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

No reproduction or networking permitted without license from IHS

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