IPC-7526-2007 模板和误印板清洗指南.pdf - 第14页

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5 CONTAMINANT TYPES AND REMOVAL CHARACTER-

ISTICS

5.1 Polar Residues

Materials that form ions when dis-

solved in water are termed ionizable or polar. For example,

when a typical polar residue, ‘‘salt’’ (sodium chloride) in a

ﬁngerprint deposit, dissolves in water, the sodium chloride

molecule dissociates in water into positive sodium ions and

negative chloride ions: (NaCl

=Na

+Cl

). In its ionized

form, sodium chloride will increase the electrical conduc-

tivity of water. The more ions formed the higher the elec-

trical conductivity of the water. Ionic residues cause signal

changes in electrical circuitry and may initiate electromi-

gration and corrosion. Highly ionic contamination over the

board surface may reduce coating adhesion.

As polar residues are soluble in water, they can be removed

by a plain water wash. However, polar residues are often

embedded in water-insoluble deposits. A good example is a

ﬁngerprint residue, where salt and perhaps some water-

soluble amino acids are covered by skin oil. As water can-

not wet the composite residue and solubilize the oil ﬁlm,

additives must be used to remove the oil ﬁlm and allow

dissolution of the embedded polar materials.

Typical Polar Residues:

• Plating and etching materials

• Chemicals from the substrate or component fabrication

process

• Water soluble soldermask constituents

• Deposits from manual handling

• Water soluble solder ﬂux constituents

• Activators from rosin or SA type solder ﬂux

5.2 Nonpolar Water Soluble Residues Organic materi-

als that are water soluble but do not ionize in water are

capable of interfering with wetting and bonding of confor-

mal coatings when cleaning misprinted circuit boards. If

the nonpolar materials are hygroscopic, formation of sur-

face water ﬁlms can be expected with a resulting decrease

in surface resistivity and, under favorable conditions, elec-

tromigration may occur.

Polyglycols are water-soluble but nonionizable. They are

widely used in water-soluble ﬂux formulations and in wave

oils. The degree of solubility of polyglycols varies with

compound type used.

5.3 Nonpolar Water Insoluble Residues Rosin, no-clean

resins, and SMT adhesives are common examples of water

insoluble residues. The presence of these residues can

interfere with wetting, bonding and coating operations, as

both wetting of surfaces and bond development will be

adversely affected when cleaning misprinted circuit boards.

Typical Nonionic Water Insoluble Residues

• Rosin

• Synthetic resin

• Organic compounds from low residue/no-clean ﬂux

formulations

• Plasticizers from core ﬂux

• Greases and oils

• Finger print oils

• Release agents on components

• Insoluble inorganic compounds (oxidation products)

• Rheological additives to solder pastes

• Improperly cleaned ﬂux residue

5.4 Nonreflowed Solder Paste Fluxes used for solder

paste comprise resins, activators, solvents, and rheological

additives. For special systems, additives such as tackiﬁers,

surfactants, and corrosion inhibitors may also be used. Res-

ins are organic materials compounded with medium and

high molecular weight rosin, synthetic materials, and poly-

mers. Activators, being acidic, boost ﬂuxing activity and

are easily removed by the cleaning agent. Oxygenated sol-

vents are used to give the solder paste a maneuverable

homogeneous ﬂuid form. Rheological additives increase

wetting, spreading and tack life of the solder paste. Fluxes

are categorized as water-soluble, rosin, no-clean and syn-

thetic. The cleaning chemistry must dissolve the ﬂux com-

position to allow the solder balls to break up and remove

from the stencil aperture.

5.5 Reflowed Flux Residue Reﬂowed ﬂux residues are

more difficult to clean than nonreﬂowed solder paste.

Water-soluble ﬂux residue is the easiest residue to clean

and is easily removed with most cleaning chemistries used

in the stencil cleaning process. Rosin and no-clean

reﬂowed ﬂux residues require engineered cleaning chemis-

tries speciﬁcally designed to remove baked-on ﬂux residue.

SMT assemblers who have this requirement must select a

cleaning chemistry with this application in mind.

5.6 Uncured (Wet) SMT Adhesive SMT adhesives are

engineered with polymers, a thermosetting component to

adhere surface mount components onto SMT boards. SMT

adhesives cure during the reﬂow process. Stencil printed

SMT adhesives must be cleaned from the stencil before the

adhesive cures. This requires a cleaning process that dis-

solves or displaces the adhesive. Since SMT adhesives are

insoluble in water; cleaning agents must be engineered

with ingredients that couple the adhesive with the water-

based cleaning bath. When diluting the cleaning chemistry,

the solvent phase will dissolve or disperse SMT adhesive

and remove the uncured adhesive from the stencil aperture.

The adhesive will separate from the aqueous cleaning solu-

tion and ﬂoat to the surface. Filtration systems are needed

to remove the adhesive from the wash bath.

5.7 Insoluble Residues Removal of residues that are

insoluble in water and organic solvents, and cannot be

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solubilized by reactive and nonreactive additives, requires

strong mechanical action. Dynamic forces require direct

spray impingement, brushing or ultrasonic cavitation

effects. Insoluble residue disperses within the wash media.

As one portion of the dispersion dissolves, the insoluble

portion ‘‘ﬂoats’’ on the wash surface and is carried away by

the wash action. Ultrasonic agitation is the most effective

method to remove this residue type.

Typical insoluble residues are:

• Siliceous material from dust and dirt

• Hydrolyzed or oxidized rosin

• Aged (oxidized) chemical process compounds from

board/component fabrication

• Silicone greases/oils

• Glass ﬁber from the laminate

• Silica and clay type ﬁllers of solder masks (permanent

and water soluble masks)

6 STENCIL CLEANING PROCESSES

6.1 Under Stencil Wiping Process

Reliable, high-yield

paste depositions for ﬁne and ultra-ﬁne pitch devices

demand stringent automated control over all sub-processes

involving material and equipment. Fine pitch stencils

require in-process cleaning to ensure accurate solder paste

deposition in height and volume. Under stencil-wipe auto-

mated systems provide hands-off programmable cleaning

of the stencil’s bottom side, using a cleaning solvent and

lint-free paper, to remove paste bleed-out. A vacuum sys-

tem removes unreleased solder paste from the apertures.

The process reduces maintenance by ﬁltering particulate

within the paper, while a porous solvent bar wets the paper

through osmosis, aiding in loosening tacky ﬂux and reduc-

ing residual cleaning solvent. A programmable vacuum

system removes solder paste from stencil apertures, elimi-

nating opens on ﬁnal assembly.

There are several contributing factors in determining when

underside stencil cleaning is required and how often. The

frequency of the wipe is generally determined by a combi-

nation of variables; stencil type, solder paste, PCA/

substrate co-planarity, printer set up, and pitch of ﬁnest

device.

In the event that the screen printer is not equipped with an

automated understencil cleaning system, two options are

available. First, the stencil printer manufacturer may be

able to retroﬁt existing equipment with an automated

understencil cleaning system. Secondly, manufacturing

sites can also use a hand wipe process. In general, a pre-

saturated, lint free wipe material is used to manually wipe

the stencil surfaces. This method is operator dependent,

and vacuum removal of particulate from apertures is not

possible.

6.2 Manual Stencil Cleaning Manual cleaning of sten-

cils is widely used by process technicians. However, the

inherent limitations and hazards associated with manual

cleaning usually far outweigh the beneﬁts. Stencil cleaning

has been identiﬁed as the most hazardous process with the

highest potential environmental impact of any process

associated with SMT assembly. Heavy metal exposure,

noxious, ﬂammable or caustic chemistries and vapors all

pose hazards to the operator. Cleaning utensils, uneven

manual pressure and general handling contribute to stencil

damage. Manual cleaning baths, chemical wipes and

human error often lead to poor waste management,

whereas a stencil cleaning process will normally safeguard

against many or all of these hazards, and provide more

consistent and predictable cleaning results.

Manual stencil cleaning is usually accomplished at the

expense of stencil aperture cleanliness. When solder paste

is wiped from the metal etched foil, it is likely to deposit

fugitive solder balls back into the apertures. Effective aper-

ture cleaning using manual techniques is not effective and

can lead to additional stencil damage. Compressed air used

to ‘‘blow-out’’ the apertures will bend the delicate land

mass areas between ﬁne-pitch apertures much the same

way as high-pressure water sprays can cause bending. The

compressed air can also broadcast the solder paste onto

other surfaces or personnel. Solder paste that dries in the

apertures will be hard as cement and very difficult to

remove. Dry solder paste is a leading cause of aperture

blockage and insufficient solder paste deposition leading to

production downtime and trouble shooting.

Manual stencil precleaning may be necessary for certain

stencil cleaning machines. This step can be performed

while the stencil is still on the printer, or immediately

before placing the stencil in an automated cleaning

machine. Usually, just removing the excess solder paste by

use of a blade or spatula is adequate. Caution should be

taken not to preclean using a solvent or other chemistry

different than that used in the stencil cleaning machine as

these chemistries could have adverse reactions. Precleaning

chemistries or wipes containing different chemistries may

change the chemical composition of the solder paste. This

condition may render a more difficult to clean residue or

produce unwanted and difficult-to-remove white residues.

If a wiping material is to be used, select a stencil wiping

material, which does not leave lint, ﬁbers, or adhesive on

the stencil. These contaminants can degrade subsequent

print runs. If the application requires a precleaning chem-

istry, select a solvent that dries relatively slow, nonﬂam-

mable, low in toxicity to skin contact, dries free of residue,

low odor, and exhibits good ability to dissolve the solder

paste or SMD adhesive.

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6.3 Semimanual Stencil Cleaning A number of designs

are available for semiautomatic wiping. Common designs

used within industry employ ultrasonic agitation.

Design #1: A large sponge is placed into a prefabed stain-

less steel tray. Solvent cleaners are added to the tray. The

sponge absorbs the solder paste or adhesive dislodged from

the stencil during the cleaning process. The cleaning sol-

vent will saturate the sponge such that solvent leaks from

the sponge upon touch. The stencil is placed on top of the

sponge. Solvent is applied to the top of the stencil. The

stencil must remain wet during the cleaning process. An

ultrasonic handheld cleaning head is applied to the stencil

in a motion similar to ‘‘ironing clothes.’’ Minimal pressure

is applied to the cleaning head during the cleaning process.

After cleaning, the stencil is removed from the tray and

wiped on the top and bottom of the stencil with a clean,

wet SMT wipe to remove any remaining solder balls or

adhesive. After wiping, the stencil is examined using a

magnifying glass to inspect for solder paste in the aper-

tures, especially ﬁne pitch. A note of caution, ‘‘fugitive’’

solder balls may be pushed back into stencil apertures.

Design #2: Semiautomatic ultrasonic stencil cleaners

require the operator to move the stencil or substrate from

the wash tank, to the rinse tank while drying the stencil

manually. The actual cleaning is accomplished automati-

cally by the chemistry and ultrasonic cavitation.

Handheld sprays or spray nozzles that are usually operated

by a foot switch may accomplish rinsing. Drying is accom-

plished by hand-held low-pressure dry compressed air or

by natural ambient drying.

Consistent cleaning is accomplished by programmed wash

cycles and is less dependent on the operator interface. Cau-

tion should be taken not to allow a stencil to ‘‘soak’’ for

long periods because of the potential for moisture absorp-

tion to the stencil adhesive, which could weaken the bond

interface to the frame.

6.4 Single Chamber Equipment Most stencil cleaners

operate with a single chamber utilized for both washing

and rinsing operations. Although segregated wash and rinse

tanks are utilized, single chambers have a tendency to cre-

ate extensive chemical drag-out (wash solution entering the

rinse tank). This is an issue when the process chamber is

large and when common plumbing between the wash and

rinse is utilized. When a single process chamber is used,

the customer should expect increased drag-out due to the

surface area of the chamber. This increased drag out makes

close looping the stencil cleaner economically difficult.

6.5 Ultrasonic Agitation Ultrasonic Agitation is created

by the generation of high frequency sound waves (above

20 kHz) vibrating through a liquid cleaning medium. This

action, known as cavitation, consists of the formation and

instantaneous collapse of millions of microscopic vapor

pockets, or bubbles in the liquid. These vapor pockets

occur throughout the liquid even in recesses and tight tol-

erance areas such as ﬁne-pitch apertures and substrate vias.

40 kHz is most commonly used for stencil cleaning. Lower

frequencies create higher mechanical agitation, which

could potentially damage the stencil or misprinted PCA.

While higher frequencies could technically clean solder

paste from stencils, the cycle time would normally need to

increase to compensate for the more gentle agitation. The

use of higher frequencies is generally more costly and,

because the adhesives used to construct stencils are hygro-

scopic, longer exposure to moisture could weaken the

adhesive bond. ‘‘Sweep’’ or ‘‘Multiple’’ frequency technol-

ogy is standard throughout the industry and is used to dis-

tribute the ultrasonic energy evenly to eliminate ‘‘hot

spots’’ in the cleaning bath.

An ultrasonic cleaning system consists of four fundamental

components: Generator, Transducer, Cleaning Chemistry

and Tank. Performance and reliability of the system

depends upon the design and construction of the transduc-

ers and generators. The number of transducers and genera-

tor size is predicated on the tank size, and efficiency of the

cleaning chemistry used. Process effectiveness of the clean-

ing is dependent on the cleaning chemistry. The use of

ultrasonics without the proper cleaning chemistry is

equivalent to cleaning rosin ﬂux with water only. The tank

should be stainless steel and not plastic. Plastic will absorb

the sound waves and reduce efficiency. The size of the tank

is dictated by the size of the stencil, loading and unloading

ergonomics and determines a large portion of the waste-

water generated - the remainder being determined by the

rinse cycle.

Cavitation is produced by the alternating patterns of com-

pression and rarefaction generated by the rapidly expand-

ing and contracting transducers during sound wave trans-

mission. As the liquid is stretched beyond its tensile

strength during rareiﬁcation, these bubbles grow from

microscopic nuclei and then upon compression, they

implode violently. This phenomenon occurs at the rate pro-

portional to the ultrasonic frequency generated. Individu-

ally, these minute vapor pockets release only an extremely

small amount of energy. However, their cumulative effect

can be intense resulting is a very effective mechanical

scrubbing action which literally ‘‘pulls’’ the contaminant

away from the substrate after the chemistry loosens it.

6.6 Automated Ultrasonic Stencil Cleaning Automatic

stencil cleaners either transport the stencil or misprinted

substrate automatically from wash tank to rinse tank, or

transport the wash and rinse solutions into and out of a

single process tank. Usually PLC controlled; the operator

need only load the substrate into the carrier mechanism and

press a start button. Ultrasonic wash, rinse, and drying can

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