IPC-7526-2007 模板和误印板清洗指南.pdf - 第17页

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be included in the automated program. While not necessar-

ily more effective, an automated system can be more con-

sistent than a semiautomatic system because of reduced

user interface.

Automation itself may be more effective due to the consis-

tent nature of automation. Semiautomated stencil cleaners

rely on an operator’s selective judgments to determine the

degree of rinsing required to effectively displace the wash

solution and remove residual solderpaste or adhesive resi-

dues from the stencil. Additionally, the operator must deter-

mine how long to subject the stencil to a drying process.

Automation allows all stencils to be subjected to speciﬁed

cycle parameters, therefore providing results that are more

consistent.

The wash tank features multiple side-mounted ultrasonic

transducers for thorough paste/adhesive removal. Depend-

ing on the chemistry used, automatic wash-solution ﬁltra-

tion may be required and can be achieved via a built-in ﬁl-

tration pump. Particle waste is captured in a solder paste

tray for fast and easy removal. Some ultrasonic stencil

cleaners use just a solder paste trap while others utilize

only a pump-driven ﬁltration system.

Rinse tank predrained ﬁltration systems are needed to auto-

matically ﬁlter rinse water prior to treatment and discharge.

A summary of rinse water treatment: (1) ﬁltered for reuse,

(2) potentially drain disposed, (3) evaporated to atmo-

sphere in standard wastewater evaporation equipment.

Certain cleaning agent characteristics can simplify the

cleaning operation. A ‘‘no rinse’’ chemistry eliminates the

need for a rinse cycle/chamber. Some cleaning chemistries

displace the residue with the soil while not dissolving in

the wash bath. Light residues that ﬂoat can be removed

from the ultrasonic chamber surface via an overﬂowing

cascade stream. Heavy residues that sink can be collected

via a trough shaped ﬂoor and entrained into a drainage

stream. These streams are continuously cleared of the resi-

due content by ﬁltration or routine maintenance and the

residue free cleaning media is returned to the cleaning

chamber. A no-rinse process should be qualiﬁed to assure

that chemical agents remaining on the stencil cause no

effect on future printing or on stencil life (delamination of

the elastomer frame) before use in production.

6.7 Rotating Wand/Fixed Nozzle Spray-In-Air Cleaning

These systems consist of a rotating spray wand, which cre-

ates zones of constantly changing force for improved

cleaning performance. The systems are fully automatic and

do not require transferring the stencil between wash, rinse,

and dry. The systems are PLC control with a wide range of

options.

The systems are designed to use a wide range of engi-

neered cleaning ﬂuids. Aqueous, semiaqueous and solvent

technologies can be selected to remove nonreﬂowed solder

paste, uncured adhesive, and reﬂowed ﬂux residues from

stencils and pallets. Solvent systems require ﬂame suppres-

sion systems (or intrinsically safe design). Managing rinse

water and soil load are critical process items that need to

be conﬁgured to meet the overall-cleaning requirement.

6.8 Automated Ultrasonic Sponge Stencil Cleaning The

stencil is ﬁxtured into a cleaning cabinet. A stainless steel

tray that holds a sponge is placed over the stencil apertures.

The sponge is wetted with IPA (isopropyl alcohol) or other

compatible solvent. Ultrasonic transducers vibrate the

uncured paste or adhesive into the sponge material. The

waste from the process is accumulated into the sponge. The

sponges are dual hazardous waste due to heavy metal and

solvent content. Fugitive solder balls are commonly rede-

posited onto the stencil surface and require wiping. Wiping

the stencil surface caused solder balls to be redeposited

into the stencil apertures.

6.9 Multifunctional Batch System for Stencils, Boards,

and Maintenance Cleaning

Given the proper chemis-

try(s), many stencil cleaner styles provide multi-

functionality for cleaning stencils, misprints and production

circuit assemblies. In the age of no clean, many assemblers

have eliminated the cleaning process. Multifunctional sys-

tem designs provide a footprint for stencil cleaning, deﬂux-

ing of populated circuit cards, and cleaning of maintenance

items such as pallets. Many spray or ultrasonic systems

have process variables that are PLC controlled with pro-

grammable process settings. These systems are usually

designed for aqueous engineered cleaning ﬂuids. Filtration

and water management options need to be conﬁgured to

meet the process requirement.

7 CLEANING CHEMISTRY OPTIONS

7.1 Cleaning Chemistry Selection

Removal of solder

paste deposits requires a cleaning chemistry that wets, dis-

solves, saponiﬁes, or displaces the ﬂux vehicle. Cleaning

process development hinges on the cleaning chemistry.

Wetting occurs by reducing surface and interfacial tension

by using low surface tension materials that allow the

cleaner to penetrate and undercut the soil-substrate bond.

Dissolution of the ﬂux vehicle allows the metallic spheres

to separate and drop from the aperture. Saponiﬁcation is

the reaction of free alkalinity that reduces the ﬂux resin

while forming a water-soluble soap. Displacement occurs

by bombarding the contaminant with mechanical force that

facilitates the removal of soil from the tiny apertures. The

cleaning equipment greatly facilitates the cleaning process

and impacts reproducibility. Process variables inﬂuencing

cleanliness include cleaning chemistry, concentration, bath

temperature, cleaning time, and mechanical action

imparted by the equipment used.

7.2 Chemistry Choices and the Cleaning Process Sten-

cil cleaning products fall into three broad categories:

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1. DI-water only, 2. Aqueous with additives, and 3. Organic

solvents. No one product or classes of products are likely

to satisfy all cleaning requirements. The cleaning agent

must be matched to the soil, the substrate, the cleaning

requirements, drying requirements, and other performance

and environmental constraints. Inorganic soils are often

referred to as hydrophilic; they dissolve effectively in

water. Organic-based soils, often referred to as hydropho-

bic, tend to dissolve more effectively in organic solvents.

Table 7-1 is a guide for choosing the right cleaning chem-

istry for stencil and/or misprint cleaning applications.

7.3 Solvent Cleaning A hydrocarbon solvent represents

an organic compound containing the elements carbon and

hydrogen. These compounds are primarily from petro-

leum’s, coal tar and plant sources. The principle types of

organic solvents include alcohols, aliphatic petroleum’s,

aromatic hydrocarbons, oxygenated and halogenated sol-

vents, esters and terpenes. A general rule of solvent clean-

ing is ‘‘like dissolves like,’’ which means that usually non-

polar contaminants are best removed by nonpolar solvents,

while polar or ionic contaminants are best removed by

polar solvents.

7.4 Aqueous Cleaning Aqueous cleaning refers to those

processes in which a substrate is ﬁrst washed with a water

based cleaning agent, generally followed by a water rinse

(DI or facility water). Water based cleaning agents can be

deﬁned as a combination of water and an additive, usually

organic. These agents are generally combined with some

form of mechanical agitation, such as spray in air, spray

under immersion or ultrasonic that promotes the removal of

the contamination such as solder paste and SMT adhesives.

When cleaning stencils and misprinted electronic assem-

blies, the objective is remove contaminates such as solder

paste, ﬂux residue (one side misprinted the other reﬂowed

or wave soldered) from the surface of stencil or electronic

assembly. Once the wash step is completed, the parts are

rinsed with water, either DI or facility water, to remove the

cleaning agent and contamination. The parts are then dried,

through either heated air or ambient air in the case of sten-

cils. The quality of the rinse water needed in a cleaning

application depends on the substrate; for example, facilities

water can be sufficient for stencils but is usually not suffi-

cient for electronic assemblies.

Table 7-1 Cleaning Chemistry Selection Guide

Process Nonreﬂowed Solder Paste/Flux Uncured Adhesive

Manual Stencil Cleaning

Solvent Wipe Solvent Wipe

Aqueous Wipe Aqueous/Solvent Wipe

Stencil Printer Understencil Wipe

Solvent Wipe Solvent Wipe

Aqueous Wipe Aqueous/Solvent Wipe

Aqueous/DI-Water Rinse

Spray Under Immersion

Aqueous

Aqueous/Solvent Mix

Aqueous Surfactant

Aqueous/Solvent Mix

Ultrasonic Agitation

Aqueous

Aqueous/Solvent Mix

Aqueous Surfactant

Aqueous/Solvent Mix

Spray in Air

Aqueous

Aqueous/Solvent MixAqueous Surfactant

Aqueous/Solvent Mix

Aqueous/No Rinse

Spray Under Immersion

Aqueous Surfactant

Aqueous/Solvent Mix

Ultrasonic Agitation

Aqueous Surfactant

Aqueous/Solvent Mix

Spray In Air

Aqueous Surfactant

Aqueous/Solvent Mix

Aqueous/Solvent Emulsion

Solvent Cleaning

Spray Under Immersion Solvent Solvent

Ultrasonic Solvent Solvent

Spray in Air Solvent Solvent

February 2007 IPC-7526

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There is a broad array of aqueous cleaning agents. Each

uses inorganic and/or organic mixtures. Most are a combi-

nation of low vapor pressure solvents, surfactants or

saponiﬁers, and if required, inhibitors. They are designed to

remove polar and nonpolar contamination.

The physical properties do however vary in a number of

ways, the most important being, pH and vapor pressure.

Saponiﬁer/surfactant mixtures usually have a pH around 11

to 13. Certain new formulations have a neutral pH. The

advantage of a pH below 12.5 is user safety and material

compatibility, especially in connection with the stencil. The

advantage of higher pH is the improved effectiveness for

removing ﬂux residues. The other signiﬁcant differences

for readers to consider are the volatile organic content of

cleaning agent. An inorganic containing cleaning agent has

a low volatile organic content and may therefore qualify

for Clean Air Solvent Certiﬁcation from districts such as

South Coast Air Quality Management District. However,

some inorganic formulations may not be not sufficient for

adhesive removal, and even in some cases solder paste and

ﬂux residue removal. Furthermore, some inorganic formu-

lations may leave behind white inorganic residues after the

cleaning stage, which may not be as easily removed as an

organic based cleaning agent.

Another issue to consider is bath life. Saponiﬁer formula-

tions have a limited bath life since the components are used

up in the cleaning process. Formulations that are more

modern have a different contamination removal mechanism

and allow the contamination to be largely ﬁltered out.

7.5 Semiaqueous This group of cleaners includes blends

or hybrids with water and organic solvents. Aliphatic, oxy-

genated, ester or terpene solvents are blended with surfac-

tants, builders, stabilizers and inhibitors. Sometimes they

contain water and sometimes they are applied as received

without water, but then are water rinsed. These emulsion

cleaners are used in manual, immersion or coarse spray

applications to saponify, solubilize, emulsify or disperse

soils including grease, oils, wax, adhesives, ﬂux and mis-

print solder pastes where water can be tolerated.

Semiaqueous cleaning refers to a process whereby the sub-

strate is washed in a solvent followed by a rinse with water.

The solvents used in these cleaning agents possess a vari-

ety of characteristics. Many semiaqueous cleaning agents

are formulated to clean a wide range of soils, including

solder pastes and SMD adhesives. They are either soluble

in water or insoluble in water, and all commercially avail-

able semiaqueous cleaning agents are formulated so that

they can be rinsed with water. The cleaning media, along

with agitation, will remove the soils from the stencil sur-

face, and the rinse with water will remove any polar or

ionic soils, as well as residual solvent and undissolved soils

that remain on the surface. Waste management can be com-

plicated because of the creation of both solvent and aque-

ous waste streams.

8 CLEANING PROCESS CONSIDERATIONS

Physics centered on Test identify the chemical aspects of

cleaning; the purely physical mechanisms also play a role.

Generally, the higher the thermal and mechanical energy

applied, the better the cleaning will become. When clean-

ing stencils, thermal energy can affect the adhesive bond

that holds the stencil to the frame.

8.1 Common Rules that Center on Aqueous Cleaning

8.1.1 Temperature

Cleaning effectiveness and speed

improve as temperature increases.

• Temperature generally above 110°F (to be consistent with

clause 3.2) may delaminate adhesive holding stencil to

frame.

• Temperature is typically proportional to cleaning time.

• Reﬂowed ﬂux resin softens at 140 - 176°F, but can be

cleaned with cleaning agents at ambient up to 120°F.

• A rise in temperature typically reduces the cleaning time.

8.1.2 Energy Higher mechanical action (cavitation or

impingement) improves cleaning.

• Represents one of the greatest variations amongst clean-

ing chemistries as some require heat and some do not. If

the chemistry will clean without heat, additional heating

options may not be required on the cleaning machine

which may lower the equipment cost and save energy.

• High spray pressure, low ultrasonic frequencies and high

ultrasonic power densities could cause stencil and/or mis-

printed PCA damage.

• There needs to be a balance to address compatibility con-

straints.

8.1.3 Solvency/Concentration Higher cleaning agent

concentrations may improve cleaning. However, high con-

centrations may decrease ultrasonic cavitation and reduce

the overall scrubbing action.

• Cleaning ﬂuid should be selected ﬁrst based on soil com-

patibility.

• Poor solvency for the soil cannot be overcome by using

mechanical force.

• Machine should be integrated with the cleaning ﬂuid.

8.1.4 Time Exposure time to the cleaning agent is

critical.

• Most elastic of the four variables.

• Increase in temperature, energy, and solvency allows a

decrease in time.

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