IPC-CM-770D-1996 - 第160页

January

1996

IPC-CM-770

adequate handling procedures are operative. Care should be

taken to ensure that tests and testing reflect the desired end

product quality and that subsequent to acceptance nothing

is done to degrade this quality.

30.1 Preassembly Assurance

Both materials and com-

ponents should be of the agreed composition and standards.

Materials

Solders

Should be of the agreed upon composition and

purity. Purity specifications for electronic solders are pro-

vided in J-STD-001.

Flux

The flux, as either liquid, solder paste flux, cored

wire flux or preform flux, should suit the electronic sol-

dering operations. The type of flux affects cleanliness

requirements on the printed board assembly. J-STD-004

specifies a flux designation system and the corresponding

activity levels.

Cleaning Agent

The cleaning agent used to remove

grease, oil, wax, dirt, flux and other debris should also

remove flux residue, ionic, ionizable nonpolar and par-

ticulate contaminants. The cleaning agent should not

degrade the materials and parts being cleaned.

Adhesives

Adhesives used for securing components in

place during soldering should not degrade the solderabil-

ity of nearby interconnection sites or lead to corrosion or

unacceptable insulation resistance properties of the

assembly. Insulation testing of adhesives is described in

J-STD-004.

Components

Components selected for assembly to

printed boards shall be compatible with all solders, pro-

cess chemicals, and temperatures used to manufacture the

assembly. Components which do not meet these require-

ments must be given special handling.

Printed Boards

Printed boards shall be in accordance

with the requirements of the applicable design and perfor-

mance specification.

Sensitive-Component Handling

To prevent damage by

static electricity, persons coming into contact with or han-

dling electrosensitive components, such as some semicon-

ductors, should be grounded prior to touching or install-

ing the component. Certain electrosensitive devices may

require additional precautions and should be handled in

accordance with manufacturers recommendations. (See

Section

26.)

Plating

Plated coatings should be uniform and of the

specified thickness. They should be thoroughly cleaned of

all residues, capable of adequate storage life times, and

impart solderability to the lead or termination which they

cover.

Silver plated terminals, component leads, or printed

boards shall not be used for Class

3

electronic equip-

ments, and may be detrimental in Class

1

or

2

electronic

equipments.

Visual Examination and Testing

Prior to soldering, each

printed board structure and its components should be

examined to ascertain that no damage has been incurred

during transit or handling, and that component mounting

is in accordance with the appropriate requirements.

30.2 Solderability

In order to successfully deal with sol-

derability problems, one must be able to test for it. In the

last ten years, many tests have been devised for both com-

ponent leads, terminals and printed boards.

The following specifications list the current requirements

and test methods to be used in accordance with the custom-

ers needs.

Leads in Terminals

MIL-STD-202, Method 208

J-STD-002

Solderability Tests for Component Leads, Ter-

minations, Lugs, Terminals and Wires

Printed Boards

J-STD-003

Solderability Testing of Printed Boards

Through the development of the various test methods and

their use in evaluation of solderable surfaces, much has

been done to improve the solderability of component leads

and printed boards. There are generally three methods of

obtaining good solderability in a product. The best results

are obtained by combining three methods.

(1):

One must use inherently solderable base materials.

(2):

One must control the manufacturing process by which

a part is produced to maintain build-in solderability.

(3):

There must be proper surface preparation.

30.3 Quality Assurance Provisions

30.3.1 Visual Joint Inspection

All finished assemblies

should be examined. The solder joint should indicate evi-

dence of wetting and adherence where the solder blends to

the soldered surface, forming a small contact angle; this

indicates the presence of a metallurgical bond and metallic

continuity from solder to surface.

Smooth clean voids or unevenness on the surface of the

solder fillet or coating are generally acceptable. A smooth

transition from land to connection surface or component

terminal should be evident. The solder fillet should appear

to be concave. The solder joint connecting the component

should be examined for evidence of the following charac-

teristics:

A. Bridging

Solder shorting, or the spanning by solder, of

section(s) that should be open between two or more con-

ductors and/or component terminals.

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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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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

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Licensed by Information Handling Services