IPC-7721(S).pdf - 第19页

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…

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. 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.

6. 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.

7. 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

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.).

8. 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.

9. 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

IPC-7721 February 1998

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.

10. 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.

11. 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.

12. Eyelets and Eyelet Press System

Solder plated copper eyelets and an eyelet press/setting

tool to repair damaged plated through holes may be

required.

13. 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.

14. 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.

15. 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.

February 1998 IPC-7721

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IPC-7721 February 1998