IPC-CM-770D-1996 - 第161页

IPC-CM-770

Januaty

1996

B. Webbing

A

continuous film or curtain of solder paral-

lel to, but not necessarily adhering to, a surface or between

separate sections of circuitry, that should be free of solder.

C. Ball

Small spheres of solder adhering to laminate,

mask and/or conductor surfaces.

D.

Sticking

Tiny balls, flecks or specks of solder adhering

to laminate or mask.

E.

Slivers

Portions of tin-lead (solder) plating overhang

on conductor edges partially or completely detached.

F.

Whiskers

Slender acicular (needle-shaped) metallic

growth between conductors and/or lands.

30.3.2 Verification Inspection

Where required, verifica-

tion inspection should consist of the following:

A.

Determine that the manufacturer has an acceptable

documented quality control system.

B.

Surveillance of the operations to determine that prac-

tices, methods, procedures and inspection plans are being

properly applied.

C. Deviations from the prescribed procedures, or instances

of poor practice which might affect the product quality,

should be noted and failure to promptly correct the defi-

ciencies may be cause for suspension of acceptance until

correction has been made or until conformance of the prod-

uct to prescribed criteria has been demonstrated.

30.3.3 Workmanship

Workmanship inspection criteria

should be fully established and agreed to, both interdepart-

mentally and between customer and vendor. Photographic

or sketch arbiter examples are almost essential. See

J-STD-001

for the complete and current requirements.

30.4 General ModificationlRepair Procedures

The fol-

lowing general modification and repair procedures focus on

the preparation of printed board and printed board assem-

bly surfaces for printed board substrate of conductive pat-

tem modifications and repairs.

30.4.1 Cleaning

General Cleaning for the printed board

(or assembly) prepares the board for modification or repairs

by reducing surface contaminations.

A

general rule of thumb is “like dissolves like.” Usually

organichonpolar contaminants are best removed by nonpo-

lar solvents and inorganic polar contaminants are best

removed by polar solvents.

A

desirable cleaning solvent

should possess excellent wetting ability, dissolve and

remove soluble and particulate contaminants, be compat-

ible with the printed board, printed board assembly, equip-

ment and work area, be stable during use, and not endan-

ger employees.

Process residue should be removed by applying appropriate

solvents or cleaning solutions and drying. Mechanical

means such as agitation, spraying, brushing, etc., or vapor

degreasing and other methods of application may be used

in conjunction with the cleaning medium. The cleaning

solvents and methods used shall have no deleterious effect

on the parts, connections, and materials being cleaned.

Ultrasonic cleaning can damage certain parts and should

generally be avoided.

A

regular cleaning schedule is required and should include

the following:

A

rough-cleaning step to remove most residues (ionic

and non-ionic).

A

fine-cleaning step to remove the remaining residues

(ionic and non-ionic).

A

final-cleaning step that includes a drying operation, for

the removal of the final traces of ionic contamination.

This step should meet the cleanliness requirements as

determined by an ionic extract cleanliness testing.

Under some circumstances, the fine-cleaning step and

final-cleaning step may be deleted, depending on the clean-

liness level requirements (if any) that must be maintained.

30.4.2 Identification of Coatings

One problem faced by

modification and repair technicians is to determine which

coating removal methods should be used for a specific

coating. Generic and commercial identification of a spe-

cific coating is usually available in a factory; i.e. acrylic

lacquer, varnish, silicone elastomers, polyurethane, epoxy,

RTVs, etc. Consequently, the coating removal methods

used in the factory can usually be specified because the

coating is known.

However, when identification data is not available, the fol-

lowing procedure of simple observation and testing will

help identify the coating characteristics

so

that the techni-

cian can select the proper removal procedure. (Note: The

generic or commercial identification of the coating is not

necessary to accomplish coating removal.)

A.

Hardness:

Penetration test in a noncritical area to

determine relative hardness. The harder the coating, the

more suitable to pure abrasive techniques. The softer and

gummier the coatings, the more suitable to the brushing

removal method.

B. Transparency:

Obviously transparent coatings are usu-

ally more suitable for removal than the opaque type.

Removal methods used with opaque coatings must be far

more controllable and less sensitive to damaging the cov-

ered components and printed board surfaces, and are usu-

ally slower.

C. Solubility:

Test the coating for solubility characteris-

tics in a noncritical area with solvents of low toxicity and

mild activity.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

D.

Thermal Parting:

Use thermal parting device with con-

trolled heating and without a cutting edge to determine

whether the coating can be thermally parted at tempera-

tures at least

50%

below the melting point of solder.

If

the

coating flows or gums up, you are too hot or the coating is

not suitable for thermal parting. Caution40 not exceed

the maximum component storage temperature or other

limitation.

E.

Stripability:

Carefully slit the coating with a sharp

blade in a noncritical area and try to peel back from the

surface to determine if this method is feasible. Due to the

adhesion required of coating materials, strippable tech-

niques without chemical aids is usually very limited.

F.

Thickness:

Determine if the coating is thick or thin by

visual means. Thin coatings show sharp component out-

lines and no fillets while thick coatings reduce sharp com-

ponent outlines and show generous fillets at points of com-

ponent or lead intersection with the printed board. Thick

coatings usually require two step removal methods to pre-

vent surface damage to the board. First reduce the thick

coating down to a thin one and then use pure abrasion

methods to reach the surface of the board. The specific

coating to be removed may have one or more of these

characteristics and consequently the removal method

selected should consider the composite characteristics.

30.4.3 Coating Removal

The surface of printed boards

and printed board assemblies needs to be prepared and

conditioned to provide a stable base for the next step of

modification and repair procedures.

Give consideration to procedures for reducing moisture

absorption and surface preparation to promote the adhesive

characteristics of the board surfaces.

Adhesive promotion procedures should be prepared by

cleaning and coating removal as mentioned in

30.5.1.

Conditioning for moisture absorption (baking) should be

performed prior to any major resoldering operation to pre-

vent blistering, measling or other laminate degradation.

The baking (drying out) procedures must be carefully

selected to insure that temperature and time cycles

employed do not degrade the product in work. Environ-

mental conditions must also be carefully considered to

insure that vapors, gases, etc. generated during the heating

process do not contaminate the product’s surfaces.

30.4.4 Legends and Markings

Modifications and repairs

may be needed on printed board structures (and assem-

blies) for legenddmarkings. Legenddmarkings can be

added, changed or replaced in any one of several ways.

The modification/repair methods include ink stamping,

handtemplate lettering, inkhpray stenciling or stick-on

labels, etc.

In all cases, though, several considerations should be given

to the new legendmarking:

The color should be selected for maximum contrast and

legibility.

The location of the new labeling/marking should be at

least spot cleaned to remove surface contaminants on the

printed board structure (and assembly) surface.

The location of the new legendmarking should be spot

abraded to improve adhesion of the legendmarking to the

printed board structure (and assembly) surface.

The labeldmarking inks, paints, or stick on label material

should be electrically nonconductive, otherwise consider-

ation should be given for electrical conductive pattern

spacing.

Stick-on labels should not be located over multiple con-

ductive patterns, unless precautions are used to eliminate

moisture traps between conductive patterns and the label.

30.5 Printed Board Structure ModificationlRepair.

There are several modification and repair procedures in

IPC-R-700 that can be used with printed board structure

base materials, conductive patterns, printed contacts and

printed boards and used with printed board assemblies.

Prior to performing these modifications or repairs, the

printed board structure (or printed board assembly) should

be prepared for these procedures by cleaning to remove

surface contaminants and removing coatings.

30.6 Printed Board Assembly ModificationlRepair Meth-

ods

The general modification and repair procedures for

printed board assembly focus on the removal and replace-

ment of through board or surface mounted components,

and the addition of jumper (hay) wires on the printed board

structure. Prior to performing these methods, printed board

assemblies may need to be cleaned, conformal coatings

may need to be removed, and bolt-on hardware or compo-

nents may need to be removed prior to component

removal/replacement or the addition of jumper wires. After

the components are removedreplaced (or jumper wires are

added), the discrete wiring or printed board assembly may

need to be cleaned, and the conformal coating should be

replaced (if required).

30.6.1 Removal and Replacement of Components

30.6.1.1 General Requirements

The following are the

general requirements for the removal and replacement

(R&R) of through-hole mount conventional (components,

dual-inline backs, and pin-grid arrays) parts and surface

mount (leadless and leaded) components. A controlled pro-

cess is essential for reworking and repairing modem-day

electronic assemblies. The process should allow for the

R&R of an individual component within the defined ther-

mal, mechanical and electrical requirements to assure sus-

taining the quality of the original assembly.

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

While no known universal component R&R equipment (for

all types of printed boards exist, structures and all types of

components) the general R&R controlled process equip-

ment requirements include the following:

A. Controlled application of heat to melt the solder joints

of the component in question without causing thermal dam-

age to the base material, or to any component, or the remelt

of any adjacent solder joints.

B.

Controlled removal of the component after sensing sol-

der melt, to prevent delamination of the lands or destruc-

tion of the plated through holes on the printed board struc-

ture.

C. Preparation of the printed board lands, removal of

residual solder in plated through holes, and pre-tinning of

the component leads or lead surfaces in preparation of

component replacement and resoldering.

D. Controlled positioning of the replacement component

on the printed board structure land or through hole pattern.

E. Reflow soldering of the surface-mount solder joints or

soldering the through-hole mounted component with con-

trolled heating.

These generalized component R&R requirements are avail-

able in equipment that has been developed for a specific

component mounting technology.

30.6.1.2 Component Removal Methods

This section

discusses various methods for removing solder, through-

hole mounted components, and surface mounted devices

(from printed board assemblies). Printed board assemblies

require refined component and solder removal/- replace-

ment methods. A controlled process is essential for the

removal and replacement of both through-hole and surface

mounted components. The component/solder removal/

replacement process should operate within defined thermal,

mechanical and electrical limits. There are several factors

that need to be considered in selecting an appropriate

componentholder removal method:

A. Analyze the task to be performed.

B.

Determine any constraints on removing the compo-

nent(s).

Can you save the component and sacrifice the printed

board assembly? Or, do you need to save both the compo-

nent and the printed board assembly?

Can you sacrifice the component and save the printed

board assembly? Or, do you need to save both the compo-

nent and the printed board assembly?

C. Determine if the printed board assembly needs to have

surface contaminations and conformal coatings removed

prior to solder/component removal.

D. Examine all of the solder joints to determine if they

have the same thermal mass and surface configurations.

E. Determine the relative proximity of the solder/

component to be removed to other solder joints and com-

ponents in order to eliminate the following:

possible thermal damage or stress to the other solder

joints and components, or

solder reflow of the other solder joints.

F.

Visually determine if the component to be removed is

adhesive bonded to the printed board assemblies surface by

residual conformal coatings or staking adhesives.

The goal for successful solder joint and component

removal is to get in and get out as rapidly as possible (try

to be in and out in less than

3-5

seconds). Do not push,

apply pressure or impart any mechanical movement to the

printed board structure’s conductive patterns with any heat

source.

30.6.2 Heat Factors

The next step, after analyzing the

task and preparing the printed board structure’s surfaces for

removing the solder or the component, should be to deter-

mine the thermal characteristics of the solder joint’s heat

factors.

The removal of solder or components from a printed board

structure depends on remelting solder in solder joints. The

most critical aspects of solder or component removal from

printed board assemblies are the application and control of

the applied heat, the selection of a suitable method, skilled

and qualified personnel, and good control of the machine/

tool process.

Excessive heat can damage the component or the printed

board structure. Excessive heat can destroy the adhesive

bond of the resin system to the conductive patterns which

delamination consists of the lands, conductors and plated

through holes in the base material.

Insufficient heat will not adequately or rapidly melt the

solder, thereby affecting solder or component removal.

Conductive patterns (lands, conductors and plated-through

holes) can be pulled off the printed board structure with the

component by not adequately melting the solder in all of

the solder joint(s).

Several heat considerations affect the selection of an opti-

mum solder or component removal method. The amount of

heat needed to remelt a solder joint (in order to remove the

solder from a solder joint or to remove a component from

a printed board assembly) is critical to the solder/

component removal process. There are several heat factors

that determine the amount of heat coupled to the solder

joint:

The mass of the solder joint.

The mass and thermal characteristic of the heat source.

The thermal coupling between the solder joint and the

heat source.

Thermal mass and other thermal characteristics of the sol-

der joint determine the amount of heat needed to remelt the

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services