IPC 7711A - 第24页

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, dri…

1.7 Basic Considerations

1. Appropriate Approvals

Appropriate approvals should be obtained before pro-

ceeding with PC board modiﬁcation, rework or repair.

Such approvals should include agreements as to accep-

tance criteria and limitations.

2. Singular Procedures

Procedures in this book are presented as individual

methods. Multiple procedures may be necessary to

complete the task.

3. Quality and Reliability

All attempts to modify, rework or repair printed boards

and assemblies should seek to equal the quality and

reliability of the original, unaltered, end product.

4. Procedure Selection

The procedure selected should be on the basis of opti-

mum end product functionality. Test data should be

obtained wherever possible.

5. Patience

To achieve best results, do not rush the process. Keep

in mind that most of the cost for fabrication/assembly

has already been spent, but with care and patience,

most of this cost can be salvaged.

6. Heat Application

Incorrect heat application may cause severe damage to

board materials, conductors, components, conformal

coatings and solder connections.

7. Removal of Coatings

Coating should be removed from affected areas prior

to processing. Coatings will inhibit solder removal and

adversely affect resoldering operations.

1.8 Tools and Materials Modiﬁcation, rework and repair

of PC boards and assemblies is generally a highly labor

intensive operation relying more on individual operator

skills than automation. The use of proper tools and supplies

will often have a signiﬁcant impact on the function and

reliability of the end product. To enhance the ease of the

task at hand and to improve the potential for a successful

operation, the following equipment and supplies are recom-

mended. This list should be used as a guide only.

1. Proper Workstations

A proper workstation; ESD grounded with acceptable

lighting, outlets, and conﬁgured for comfort is prefer-

able due to the degree of concentration and dexterity

required to perform high reliability PC board modiﬁ-

cation, rework and repair.

2. High Quality Microscope

Precision work generally requires a microscope. Refer

to IPC-OI-645 Standard for Visual Optical Inspection

Aids for more information.

3. Lighting

Illuminations at the surface of work stations should be

1000 Lm/m

minimum.

4. Soldering Tools

Precision soldering is important in today’s modiﬁca-

tion, rework and repair operations. Technicians may

need an assortment of special use soldering tools

appropriate to the variety of tasks at hand. These tools

must be temperature controlled, ESD/EOS safe, ergo-

nomically designed and include a selection of tips to

suit each particular operation.

5. Component Removal and Installation

Today’s expanding variety of large and small compo-

nents require an array of special use tools and methods

for safe, efficient component removal. These tools gen-

erally use either conductive heating (by contact), con-

vective heating (by hot gas) or infrared heating (by

focused infrared lamps).

Each assembly/rework method has certain advantages

and precautions depending on the particular Surface

Mount Device (SMD) (lead/terminations design, size,

body material, etc.), component mounting site (adja-

cent components, access, substrate type, thermal mass,

etc.) and the skill level of the operator.

For example, chip components addressed in this docu-

ment have different termination styles including: Bot-

tom, three or ﬁve face terminations. Therefore some

procedures depicted in this document may not be

applicable to all termination styles.

6. Preheating (Auxiliary) Heating

Preheating printed board assemblies is sometimes rec-

ommended to avoid thermal shock to temperature sen-

sitive materials and components. Preheating also

elevates the thermal mass of the assembly to allow a

rework process to proceed in an acceptable time. Pre-

heating can be accomplished using either an oven, heat

lamp, hot plate, infrared or convective style heating

system.

7. Fume Extraction

Work environments can often expose technicians to

potentially hazardous fumes. Disposal and release of

certain materials may have a signiﬁcant environmental

impact. The use of localized fume extraction systems,

environmental control devices and other personnel

protection equipment may be necessary to comply

with MSDS requirements and applicable federal, state

and local laws.

8. Hand Held Drilling and Grinding Tool

PC Board modiﬁcation, rework and repair procedures

often require drilling, milling or grinding operations.

The best type of tool for these delicate operations is

IPC-7711A/7721A October 2003

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

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

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