IPC-7526-2007 模板和误印板清洗指南.pdf - 第18页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ 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, inhibitor…

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

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

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

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.

IPC-7526 February 2007

标准分享网 www.bzfxw.com 免费下载

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

• Decrease in temperature, energy, and solvency creates a

need for more time.

• Additional cleaning time may cause moisture absorption

to the stencil adhesive, which could weaken the adhesive

bond interface to the frame and metal etched foil.

8.1.5 Cleanliness Final rinse cleanliness is critical to

part cleanliness.

8.1.5.1 Water Quality and the Cleaning Process When-

ever water is used during the cleaning process, whether for

diluting the chemistry or as rinsewater, DI-water should be

considered to ensure optimal cleanliness.

8.1.5.2 Rinses in the Cleaning Process Although rins-

ing is not always needed, it can be an important step in the

aqueous or semiaqueous cleaning process. Often deionized

or softened water is used, especially in a ﬁnal rinse stage

to minimize spotting.

Rinsing can be accomplished in many ways. Water spray or

mist can be incorporated directly over a wash tank. This

minimizes product carryover and replaces volume lost due

to evaporation, but may dilute the chemistry concentration

in the tank.

Other types of rinses may include spray-in-air or agitation,

dipping or overﬂow rinses accomplished in immersion

tanks. Immersion rinses are more prone to contamination

from carryover; therefore, it is not unusual for more than

one rinse stage to be used. In precision cleaning applica-

tions, deionized or reverse osmosis (RO) water is used in

the ﬁnal rinse where water quality is rigidly maintained

through continuous monitoring of conductivity and/or

refractive index.

8.2 Drying Essentially, there are three common types,

natural, mechanical and evaporative, but many machines

use a combination of them. A third, but less frequently

used, drying method is with use of organic solvents via

displacement or adsorption modes.

When off the printer, stencils are normally cleaned for

inventory and can therefore be allowed to dry naturally.

This conserves energy and frees the stencil cleaner for

additional wash cycles.

There are two ways commonly used for mechanical drying,

centrifugal and high velocity air (air knife). Mechanical

drying, if it is possible, offers the following advantages:

• Low energy requirements

• Rapid and effective

• Removes any residual contaminants in the rinse media

Possible disadvantages of mechanical drying include:

• Heated air used for drying can damage the stencil adhe-

sive in the same manner as heated wash solutions.

• Heated air can cause irregular expansion and contraction

of the stainless steel etched foil, causing distortion of the

etched image.

9 EQUIPMENT ACCESSORIES

9.1 Equipment Accessories

In general, stencil-cleaning

machines include a series of accessories or options that will

enhance the performance and proper operation of the

equipment. These accessories are classiﬁed according to

different categories depending on the end use and function:

9.2 Process Control Accessories Depending on the

level of automation and sophistication of the cleaning

equipment, these accessories provide the necessary controls

to achieve proper operation of the cleaning equipment. In

many cases, these accessories include a series of measuring

devices and sensors that are electrically connected to a

central PLC (programmable logic controller) or to a micro-

processor. Functions can be programmed to execute (or

abort) different steps according to the feedback received

from the measuring devices. Typically, the measuring ele-

ments include:

• Temperature sensors.

• Pressure sensors for both air and ﬂuids.

• Resistively/conductivity monitors for tracking ionic con-

tamination and water quality.

• Refractometers to measure the refractive index of aque-

ous solutions to determine the volumetric concentration.

• Timers can be either incorporated within PLC software or

installed as separate devices. Timers provide control of

the different cycle steps such as wash, idle (drag), rinse,

and dry times.

9.3 Operation Accessories These devices ensure proper

and efficient equipment operations. Proper rating and speci-

ﬁcation of these must take place in order to guarantee a

safe and effective equipment operation. In general, opera-

tion accessories can be classiﬁed as follows (in no particu-

lar order):

9.3.1 Pumps The pumping mechanism is critical to

maintain the desired ﬂow rates, impingement pressures,

and efficient ﬁltration. Pumps are used for wash and rinse

ﬂuid delivery, ﬂuid transfer (i.e., drain), metering proper

concentrations, etc. In general, the performance of the

cleaning operations is directly related to the impingement

pressure delivered by the wash pump onto stencil and PCA

surfaces.

9.3.2 Heaters The heating elements, placed inside the

wash reservoir, heat the ﬂuid to the desired temperature.

The power rating of the heaters should take into account

factors such as the ﬂuid volume, operating temperatures,

heat exchange losses during operation, start-up time, etc.

February 2007 IPC-7526