IPC-7526-2007 模板和误印板清洗指南.pdf - 第19页

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 interfac…

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

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

Ideally, the heater operation should be automatically con-

trol based on the temperature measurement of a thermo-

couple. Additionally, an over temperature protection shut-

off mechanism should be incorporated. In addition, some

cleaners have air heaters designed to dry stencil and PCA

surfaces. The same precautions/recommendations also

apply to the air heating devices.

9.3.3 Filters Filters provide two main functions in spray

cleaning applications. First, an adequate ﬁltering system

can reduce solder balls redeposition onto PCA and stencil

surfaces. Secondly, proper ﬁltration can reduce clogging

inside the ﬂuid delivery pipes and pumps of spray-in-air-

systems. The ﬁlter particle size ranges from 10 µm down to

1 µm depending on the speciﬁc application. For drain sys-

tems in which the rinse water is transferred out of the

cleaning equipment, the minimum ﬁlter rating is 5 µm in

order to effectively prevent passing of solder balls.

Filters may not be necessary for many ultrasonic stencil

cleaners as the wash solution is not recirculated and the

heavy solder balls are separated naturally by gravity and

fall to the bottom of the wash tank. However, ﬁlters for

waste management may need to be considered.

9.3.4 Dryers The drying cycle is often the least impor-

tant step of the overall cleaning process as most stencils are

cleaned for inventory and can be left to dry naturally. Many

cleaning systems have a water rinse step followed by

optional drying. Generally, drying is accomplished by pass-

ing compressed air or Nitrogen through an in-line heater

and delivered through strategically located air knifes or

ﬁxed nozzles, or nozzles attached to the circulating spray

wand. Drying is also accomplished by using a fan to pass

ambient or heated air over the stencil. Adequate drying

may be essential in the case of PCAs. In many cases, the

drying cycle is the limiting step in the total cycle time and

can be eliminated to improve throughput.

9.3.5 Ion Exchange and Carbon Media The quality of

rinse water in terms of resistivity and organic content is

crucial particularly for PCA cleaning/rinsing applications.

Ionic contamination on PCA surfaces has the potential for

highly detrimental effects. Ion exchange resins absorb

anions and cations dissolved in rinse water. Carbon media

absorbs residual organic substances present in the rinse

ﬂuid. The effectiveness of the carbon and ion exchange

resins is dependent on different factors such as ﬂow rate,

volume, and loading levels. Resistivity/conductivity sen-

sors generally control the operation of the carbon and ions

exchange media. See clause on cleaning misprinted PCAs.

9.3.6 Evaporators Evaporators are generally stand-alone

systems plumbed into the cleaning equipment through

appropriate chemically resistant pipes. The main function

of the evaporators is to eliminate the waste rinse water and

aqueous wash solutions by evaporation. The successful

implementation of an evaporator requires additional facili-

ties connections such as venting and electric (or gas)

power. Evaporators are useful in cases where drains are not

available or when the wastewater contamination levels are

highly restrictive or when the user wishes to eliminate liq-

uid hazardous waste disposal and drain discharge.

While considered an accessory, the wastewater evaporator

is usually a separate machine for reducing large volumes of

wastewater down to manageable amounts for disposal. Pre-

ferred construction is stainless steel in order to prevent rust.

Since small systems are usually adequate and gas is not

always available, electrically ﬁred systems are more com-

mon. Gas ﬁred systems are available and are considered

more efficient for large volumes of water.

Evaporators can be either thermal or atmospheric. Thermal

units utilize gas or electric heat sources to vaporize (boil)

the wastewater. Atmospheric units generate a vacuum to

vaporize the wastewater. Thermal systems are more com-

mon and generally less costly. If located indoors, a water-

tight exhaust duct will be required. Ducting and venting

should always be installed and balanced by an HVAC

(Heating, Venting and Air Conditioning) professional.

Other exhaust systems in the area may be in competition

with the evaporator exhaust resulting in inadequate airﬂow.

When selecting evaporation as the method of wastewater

management, the cleaning chemistry must be evaluated for

compatibility. When using an appropriate chemistry, the

wastewater will be eliminated by sending the nonhazardous

water vapor to atmosphere leaving the hazardous solids

within the system for disposal. Dissolved lead that would

otherwise require expensive ion exchange or reverse osmo-

sis ﬁltration simply precipitates out of the process as a lead

salt. Chemistries containing hazardous ingredients, glycol

ethers, VOCs, etc., most likely will not be allowed to

evaporate to atmosphere. Always check with your local

regulating authorities for chemistry compatibility and pos-

sible permit requirements.

9.3.7 Exhaust/Venting Systems These are required in

cases where solvents are used. The minimum cfm (cubic

feet per minute) requirements depend on the speciﬁc appli-

cation. In some cases, exhaust systems are used when heat-

ing aqueous cleaning ﬂuids.

9.3.8 Cooling Coils This is a requirement in cases where

low ﬂash point ﬂuids such as 2-Propanol (IPA) is used in

the cleaning process. In addition, cooling coils are some-

times implemented for rapid heat exchange when fast cool-

ing of the wash or rinse reservoirs are required.

9.3.9 Fixture/Basket Accessories Fixtures and baskets

are generally required for safe and effective holding of

PCAs, pallets, small stencils, and tools such as squeegees.

In the case of PCAs, the board design must allow for

holding the substrates in place while preventing physical

IPC-7526 February 2007

标准分享网 www.bzfxw.com 免费下载