IPC-CM-770D-1996.pdf - 第154页

January

1996

IPC-CM-770

This process uses a screen (usually stainless steel) held taut

in a rigid frame. A master artwork stencil is then laid on

top of the screen and a photographic emulsion is printed

onto the screen using the artwork stencil. The emulsion is

then exposed and developed to obtain a hard finished pat-

tern. The artwork is removed, and then solder paste placed

across the entire screen. The solder paste can only pass

through the screen where there is no hardened photo-

graphic emulsion. The quality and accuracy of screen print-

ing depends upon the size of the screen mesh, the solder

paste properties (particle size and shape, rheology, compo-

sition), the screen frame tooling, and the printing process

itself (squeegee pressure and angle, etc.)

The screen material is usually 304 stainless steel, and

frame sizes very considerably. Generally, fine mesh sizes or

small mesh openings are used when screening very thin

deposits, (e.g., for up to

0.002

inches use

250

to

150

mesh,

for 0.003 inches to

0.005

inches use

150

to

105

mesh, and

for greater solder screened thicknesses use

80

mesh

screens). A

0.005

inches increase in both the

X

and

Y

directions of the stencil pattern openings may be necessary

to compensate for the stainless steel mesh. Additional

allowances may be necessary for very thick solder paste

deposits.

To facilitate passage of solder paste through the screen, the

mean particle size of the solder paste should be about one-

third the mesh size. For example, for a mean mesh opening

of 0.0065 inches, the mean particle size of the solder paste

should be about

0.0020

inches.

The screen should be positioned approximately 0.013

inches to 0.063 inches above and parallel to the printed

board structure. The structure should be located in exact

registration with the screen image and held in place with a

reliable tooling pin setup, double-sided adhesive tape, or by

some other reliable means.

27.5.4.1.2 Stencil Printing

Pattern metal-foil stencils

avoid the registration and allowance problems of a screen

mesh, while increasing the solder paste volume deposited

and improving the uniformity of deposition. Stencils may

be used on conventional screen printing equipment, pro-

vided the equipment is adjusted for contact printing. Use

sufficient squeegee pressure to wipe the metal foil without

“dishing-out’’ paste from the etched pattern. Stencil print-

ing can deposit solder paste up to

0.015

inches thick with

center-to-center land spacings as small as

0.007

inches.

The fabrication of metal-foil stencils by chemical etching

becomes increasingly more difficult for opening/aspect

ratios (stencil opening widthhtencil thickness) less than

2.

In addition, for aspect ratios less than

2,

paste begins to

cling to the side walls of the stencil and irregular paste

deposits may result. The stickiness of the flux vehicle can

also cause this.

The openings in the stencil should be at least as large as the

outlines for the surface attachment lands. Since melted sol-

der paste tends to pull towards wettable printed board

structure lands, the stencil openings may be larger than the

lands in order to provide additional solder volume. How-

ever, leave sufficient space between deposits to prevent sol-

der bridges.

27.5.4.1.3 Dispensing

In the dispensing process paste

from a supply cartridge is pneumatically or mechanically

forced through small-diameter tubes or orifices to cause

small discrete deposits of paste to be placed at interconnec-

tion sites. Solder paste dispensing is best suited to repair,

production volume applications, model fabrication, and in

mixed technology, where surface mounted chip carriers are

added after the wave-soldered through-hole mounted com-

ponents. Multipoint dispensers should be used for the local

deposition of paste at a complete chip carrier site, whereas

single site dispensers are best suited to the local repair of

individual joints.

27.5.5 Quality Assurance

Prior to soldering, unsoldered

assemblies should be examined to ascertain that no damage

has occurred during transit or handling, and that compo-

nent mounting is in accordance with the appropriate

requirements.

When, for any reason, a component lead is terminated

so

that the lead is allowed to stress the soldered joint, the

working of the joint has been observed to result in the gen-

eration of cracks which tend to relieve the stresses. In its

ultimate condition such a connection, with extended cracks

and discontinuities, might result in a fractured joint.

These connections are called “disturbed joints.

“

In the past

they were called “cold solder joints.

“

The consensus is that

cracking is usually caused by mechanical stress on the joint

which may be induced either mechanically or thermally.

Mechanical or thermal conditions causing cracking can be

created by component mounting impacts such as design

established mismatches or component lead forming.

Some examples are:

A component (such as a power transistor or module)

mounted flat on the surface of a board with non-plated

holes. In this case, the differential in coefficient of expan-

sion between the board material and the lead material acts

directly on the solder joint.

Unsupported holes much larger than the component leads

causing the solder fillet to thin-out on one side of the hole

to

0.1

mm or less. This situation is particularly noticeable

when clinched leads are used.

The use of eyelets in interfacial holes causing a thermal

coefficient mismatch between the board material and

eyelet.

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COPYRIGHT Association Connecting Electronics Industries

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IPC-CM-770

Januaty

1996

Care should be taken in clinching leads to ensure that the

stress-relief bends are not reduced by straightening.

Parts which generate heat, if mounted directly on the

board, can cause locally severe thermal coefficient mis-

matches and result in cracking.

28.0 CLEANING-RELATED CONSIDERATIONS

In the case where printed board assemblies are to be con-

formally coated, the assembly should be free of flux resi-

dues and other contaminants prior to the application of

conformal coating.

The cleaning agent(s) used for the removal of grease, oil,

wax, dirt, flux and other debris, should be selected for its

ability to remove flux residue, ionic, ionizable, nonpolar

and particulate contaminates. The cleaning agent should

not degrade the material and parts being cleaned. (See IPC-

SC-60 and IPC-AC-62.)

28.1 General Considerations

28.1.1 Electromigration

Metallic growth or conductive

contaminants connecting any conductors causes problems

ranging from micro-amp current leakage to electrical dead

shorts. It is difficult to detect and in many cases impossible

to see this contaminant. Metallic growth usually forms

after assembly and/or coating, since growth is fostered with

either time, moisture, applied voltage, or all three.

A more elaborate cleaning procedure may be desired for

extremely high reliability assemblies exposed to severe

environmental conditions in order to reduce the risk of

electromigration. A thorough cleaning is important, there-

fore, for this purpose.

28.1.2 Surface Mounting

Surface mount component

assemblies create unique characteristics, which should be

taken into consideration for cleaning. Vapor phase solder-

ing, with its low temperatures and high speeds, greatly

reduces polymerization and charing of flux rosins, and

immediate transfer into a vapor degreaser may be

sufficient.

If

flux cannot be removed from beneath surface mounted

components, it may pose a potential threat to reliability.

Some solder paste flux systems have been found to leave

residues that are hard to clean. It is important that the

cleaning process used be capable of removing all the flux

residues from the solder paste used in the assembly

process.

28.2 Pre-Cleaning

Printed boards and components leads

to be soldered may need to be cleaned prior to assembly

and/or soldering to improve solderability.

If

required, sur-

faces to be soldered should be cleaned as follows:

Grease, oil, and other foreign matter should be removed

from conductors and terminals by using suitable cleaning

solution. Cleaners should not remove markings or dam-

age the part in any way.

Oxides and varnishes should be removed by methods

which do not damage leads or parts, and which do not

cause contamination or hinder solder wetting.

Sand blasting should not be used.

Dust or other loose matter should be removed.

28.3 Post-Soldering Cleaning

(See IPC-SC-60 and IPC-

AC-62) When required, flux residue should be removed as

soon as possible, but not later than one hour after soldering

by applying cleaning agents. Some fluxes may require

more immediate action to facilitate adequate removal. Flux

used in the process of soldering is divided into three basic

types. The type characterization is related to factors based

on the corrosive or conductive properties of the flux or flux

residue. The three basic types are as follows:

Low or no flux/flux residue activity.

Moderate flux/flux residue activity.

High flux/flux residue activity.

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.

Ultrasonic cleaning may damage certain parts. Therefore,

tests should be conducted to determine the applicability of

the process.

The post soldering cleaning procedure should be as defined

in

J-STD-001,

depending upon end product requirements

and fluxes used as follows:

A rough cleaning step for the removal of most flux resi-

dues (ionic and non-ionic).

A rough cleaning step for the removal of most flux resi-

dues (ionic and non-ionic) followed by a fine cleaning

step for the removal of the remaining flux residues (ionic

and nonionic).

A rough cleaning step for the removal of most flux resi-

dues (ionic and non-ionic) followed by a fine cleaning

step for the removal of remaining flux residues (ionic and

non-ionic), then followed by a final cleaning step that

includes a solvent or solution removal operation for the

removal of final traces of contamination.

Because of generally smaller spacing between leads,

smaller clearances between the substrate and the compo-

nent body and large area beneath the devices, chip carriers

present a more difficult cleaning situation than through-

hole mounted devices. Clearance under the package should

be adequate to facilitate effective cleaning operation.

28.4 Quality Assurance

Proper storage and handling

will greatly reduce the probability of problems. To main-

tain cleanliness, assemblies should be stored in air tight

packages in a clean, moderate environment.

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

The cleanliness tests of

J-STD-001

should be used to vali-

date the cleaning process chosen and should be performed

at a frequency that ensures compliance.

29.0 CONFORMAL COATING-RELATED

CONSIDERATIONS

A conformal coating is a thin layer of insulating material

which is applied to a printed board assembly. This material

follows closely the contours of the board and components.

It “conforms” to the shape of the assembly, and ideally

will produce a film of consistent thickness over the entire

assembled printed board. Assembled printed boards are fre-

quently given a conformal coating to assist them in func-

tioning under certain environmental conditions.

29.1 General Considerations

Correctly chosen, and

carefully applied, conformal coating will help to protect the

assembly from the following hazards:

Humidity

Dust and dirt

Airborne contaminants-.g., smoke, chemical vapors

Conducting particles-.g., metal chip, filing

Accidental short circuit by dropped tools, fasteners, etc.

Abrasion damage

Vibration and shock (to a certain extent)

Conformal coating is not a substitute for good design, or

the selection of adequate components and materials. It

does, however, assist the designer in producing equipment

which will live under hostile conditions. The conformal

coating should be compatible with any soldermask used on

the assembly. Conformal coatings protect the electrical

characteristics of the assembly by doing the following:

Preventing contamination of the dielectric surface by field

soil, which in humid environments can cause electrical

leakage.

Inhibiting the growth of fungus, thereby protecting the

electrical characteristics. Even non-nutrient surfaces can

support fungus growth when contaminated with field soils

such as oil vapor.

Suppressing electrical flashover between conductors at

high altitudes.

The secondary function of conformal coating is to help

support the components

so

that the entire mass of the com-

ponent is not carried by the solder joints.

Properties to be considered in quality test and/or

evaluations:

Appearance

Thickness

Fungus resistance

Adhesion

Shelf life

Pot life

Abrasion resistance

Solvent resistance

Flammability

Dielectric withstanding voltage

Moisture resistance

Thermal shock resistance

Thermal humidity aging

Fluorescence

Resonance

For additional information on conformal coating see IPC-

CC-830.

29.1.1 Selection Criteria

Conformal coating resins are

selected to fulfill the above requirements listed for adhe-

sives and protective coatings plus several other minor ones

such as transparency (to permit reading component values

after coating) and flexibility (to prevent damage to compo-

nents in temperature cycling). However, certain limitations

are inherent in conformal coatings:

Since they are permeable to water vapor and are not for-

mulated with corrosion inhibitors such as chromates, they

will not prevent corrosion caused by active electrolytic

salts on the part being coated or salts trapped under the

coating on the surface of the part.

Since they are permeable to water, their insulation resis-

tance decreases as the thickness of the film increases,

particularly in a fillet of resin around a component (such

as in integrated circuit).

Since coatings are organic and fill the voids between con-

ductors, they cause a marked change in interlead capaci-

tance.

Coatings have a high coefficient of thermal expansion,

so

they can exert a lifting force certain components, causing

solder joint on failure.

Coatings do not exhibit exceptional adhesion to metals,

particularly solder.

Paraxylylene coatings excepted, most conformal coating

resins are similar to organic finishes and will exhibit pin-

holing and thin spots on sharp points, edges of parts and

conductor edges.

The above limitations can be overcome through careful

design of the assemblies and care in the application

processes.

29.2 Materials

The conformal coating should be suffi-

ciently flexible to permit bending without cracking or craz-

ing at design temperatures.

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COPYRIGHT Association Connecting Electronics Industries

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services