IPC-CM-770D-1996.pdf - 第159页

IPC-CM-770

Januaty

1996

the coating material to one of higher MVTR, eliminated or

minimized the physical problem of mealing or white spots.

For contaminated boards it is not a solution, only a mask.

Mealing is caused by residues from chemical processing as

follows:

Use of copper etching solutions which are not free rins-

ing, especially those which result in insoluble precipitates

and copper complexes on dilution. Also those solutions

which do not readily etch copper foil treated surface.

Use of immersion tin plating solutions which result in

insoluble precipitates on dilution with H20.

Use of improper and inadequate post rinse operations and

cleaning and neutralizing baths.

Flux residue from reflow solder, flow solder, and solder

touchup, not removed in flux cleaning operation.

Poor quality rinse water used in processing.

Moisture or solvent not baked from board material.

Other chemical processing not covered above-solder

brighteners, protective film residues, etc.

Handling contaminations.

When curing is completed, the coating should have the fol-

lowing characteristics:

A smooth homogeneous appearance, free from bubbles,

pin holes, blistering, wrinkling, cracking and peeling.

Unless the coating is deliberately made opaque, it should

not mask the colors or identification marks on compo-

nents or the base laminate.

The coating should not discolor or damage any of the

components or other items coated.

29.5.3 Appearance

When completely cured the coating

should be free of bubbles, blisters, cracks, other defects. It

should not contain dust, hairs, or any other contaminants.

Unless the coating is deliberately made opaque, it should

not obscure the colors and markings of the components.

29.5.4 Rework

The specific coating to be removed may

have one or more of the characteristics defined in 29.2, and

consequently the removal method selected should consider

the composite characteristics. (See IPC-R-700.)

30.0 QUALITY ASSURANCE TESTING

The goal of a quality assurance strategy is to prevent non-

conformances. A high number of nonconformances means

low yields, wasted materials, labor, and capacity, high

probably of shipping nonconforming products (i.e., greater

opportunity for defect screening efforts to fail), high

inspection cost, high rework cost and higher than necessary

cost over the long run. A quality assurance strategy consists

of the following elements:

A. Nonconformance Prevention.

Nonconformances can be prevented by:

Assuring process, material, and design compatibility.

Designs should demand as little as possible from the

materials and process to achieve end item form, fit and

function requirements. Conversely, process and material

capabilities should match the demands of the end item

requirements.

Raw material control

Raw material suppliers must

understand customer requirements and use the defect

prevention systems to insure that nonconforming prod-

uct is not shipped. The customer has a responsibility to

understand the capabilities and performance of raw

material vendors.

Process control and definition

Processes must be con-

trolled and predictable. Statistical process control can be

used to monitor process performance (irregularities,

degradation). Elements of the process which require

operator or technician input must be documented

clearly. Periodic auditing is a tool to ensure that ele-

ments of the process that require ongoing attention do

not fail.

B.

Nonconformance Detection

This is a necessary nonva-

lue added cost that can be minimized by

(1)

preventing as

many defects as possible in the first place, (2) clear under-

standable definition of reject criteria,

(3)

judicious use of

statistical sampling plans, and

(4)

efficient and credible

inspection techniques.

C. Nonconformance Monitoring

Scrap and use as is parts

must be monitored and Pareto analyzed to understand

causes and trends.

D. Corrective Action

Corrective action priorities must be

based on statistical process control and nonconformance

monitoring data. Corrective action must be aimed at root

causes of problems (e.g., Is the shop process specification

correct?; Is the design as producible as possible?; Do the

raw materials conform to requirements?) Corrective actions

must be implemented through design, process, manufactur-

ing, andor vendors.

Quality assurance is an ongoing procedure which should

begin as materials and components are accepted on deliv-

ery. Consequently, certain quality assurance tests and

inspections relating to assembly processes should be com-

pleted before any components are placed or soldered.

These include solderability, surface cleanliness and coating

quality.

Following assembly, product should be inspected for

defects and other workmanship features and then verified

against the standard agreed upon between customer and

vendor. Functional and quality tests, both mechanical and

electrical, should be a part of this process. The “as pro-

duced” quality of a product will not be maintained unless

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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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Licensed by Information Handling Services

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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Licensed by Information Handling Services

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Licensed by Information Handling Services