IPC-7526-2007 模板和误印板清洗指南.pdf - 第13页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ 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’ ’ …

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This issue changes the scope of the cleaning process and

cleaning chemistry selection. In manufacturing operations

that have this requirement, it may be necessary to develop

a new cleaning process that optimizes the static and

dynamic cleaning forces.

4.2 Adhesives Cleaning uncured SMT adhesive from

stencils and boards can also be accomplished in an auto-

mated cleaning process. Many aqueous cleaning chemis-

tries are not designed for removing SMT adhesives. When

removal of SMT adhesives is required, selection of a clean-

ing chemistry will be necessary to meet this challenge.

4.3 PCB Cleaning Process Considerations Misprint

Board cleaning requires a number of process consider-

ations. The following list comprises factors for consider-

ation when engineering the process for cleaning misprinted

boards.

• When using a spray-in air cleaning system, both sides of

the board need to be cleaned simultaneously.

• Spray-in-air systems should be equipped with a ﬁlter in

the recirculation wash to remove dislodged particles from

the cleaning ﬂuid.

• A clean water rinse prevents wash contamination from

redepositing on the surface of the board.

• Rinse pressure and duration are important process consid-

erations when removing the cleaning chemistry from

under components.

• The drying cycle when cleaning a misprint from the sec-

ond side of a double-sided populated assembly may need

to be lengthened.

• Positioning of the PCB is important when cleaning a mis-

print board with high impingement air spray. An adjust-

able board holder that secures the board in place and

maintains the correct impingement angle for particle

removal facilitates this process.

• Cleaning systems that use ultrasonic technology should

have the PCBs oriented with unreﬂowed solder paste side

down to allow gravity to carry the solder paste away.

• A separate ultrasonic DI water rinse may be necessary

when cleaning boards.

• Studies by the Electronics Manufacturing Productivity

Facility (www.empf.org) suggest an ultrasonic frequency

of 40 kHz or higher is effective and should alternate or

‘‘sweep’’ ± 3 kHz.

• Sweep technology eliminates hot spots or focused ultra-

sonic energy in the wash bath.

• The lower the ultrasonic frequency, the more aggressive

the scrubbing action (cavitation).

• The power density is the amount of electrical energy

delivered to the bath via the ultrasonic generators. This

energy is measured in watts per liter of wash solution.

For example, if the wash bath contained 100 liters and

the generator is rated at 1000 watts, the power density

would be 10 watts/liter. The same studies indicate that

the power density should be around 10 watts per liter or

less.

• Table 4-1 lists process recommendations when cleaning

different types of contaminants from misprinted boards.

Table 4-1 Process Recommendations for Misprinted PCBs with Different Types of Contaminants

Contaminant Source Nature of Contaminants Cleaning Process Steps

Adhesive Dispense Misprinted, Uncured Adhesive Do not scrape the adhesive - Remove all compo-

nents carefully if populated and not reﬂowed

Make sure no adhesive residues are present on

lands or in the via holes

Send the board through the cleaning system with

appropriate cleaning chemistry

Adhesive Curing Populated with components and cured The board is not cleanable

Paste Printing Wet or dry misprinted solder paste - no compo-

nents placed

Wet or dry misprinted solder paste - components

placed

Wet or dry solder paste + uncured adhesive with

populated or unpopulated boards

Do not scrape wet solder paste or placed compo-

nents from the boards. If parts have been placed,

pick off carefully.

Brush with appropriate cleaning chemistry to dis-

solve the adhesive and/or solder paste and send

the board through the cleaning system

Clean and Inspect per IPC J-STD-001

Printing, Placement

and Reﬂow Soldering

Excessive SMT no clean solder paste ﬂux residue

>20 solder balls per panel or all over the laminate

(>5 mils diameter or 5 solder balls/square inch)

<20 Solder balls at a ﬁne pitch SMT component

Micro solder balls (<6 solder balls to a side of a

land) or >20 solder balls in via holes

Clean in the Aqueous/Semiaqueous Wash System

with appropriate Wash Chemistry.

IPC-7526 February 2007

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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

February 2007 IPC-7526

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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.

IPC-7526 February 2007

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