IPC-7526-2007 模板和误印板清洗指南.pdf - 第16页

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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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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

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