IPC-7526-2007 模板和误印板清洗指南.pdf - 第20页

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• 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.

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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

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damage and spray ‘‘shadowing.’’ In addition, the design

must allow for easy loading and unloading of the PCAs

onto the board holder.

9.4 Compatibility Constraints with Cleaning Solutions

Care needs to be taken in the selection of cleaning equip-

ment and chemistries. The very narrow solid areas between

apertures on ﬁne pitch parts could be damaged due to

excessive pressure settings for the wash solution. Addition-

ally, if bristle brushes are used to abrade the residual mate-

rial in apertures the same situation could occur.

Excessive heat in the equipment could cause minor distor-

tion to the stencil resulting in misprints—due to the differ-

ent rates of expansion of the stainless steel foil, polyester

border, and aluminum frame. The epoxy bond could also

come under stress due to heat.

Depending on the cleaning chemistries chosen, there could

be a failure in the ﬁducials on the stencils. Fiducial ﬁllers

that are not compatible with cleaning chemistries or tem-

perature settings on equipment could release leaving no

method for alignment of the stencil to the boards.

10 ENVIRONMENTAL

Assemblers must consider the effect of the cleaning agent

on air emission, wastewater discharge, and solid waste gen-

eration. Special authorization or permits for air, wastewater

and special handling of solid waste are very often the rule

in those nations that have no precise legal deﬁnition of

permitted discharges. The following are most often a con-

cern:

Air

• VOCs (volatile organic compounds)

• Ozone depleting substances

• Global warming substances

• Odors

Wastewater

• Heavy metal cations (notably lead, tin and copper)

• Anions (notably phosphates, nitrites and sulfates)

• Complexing/chelating agents (notably EDTA salts and

ammonium/amine compounds

• PH (acid or base condition)

• COD/BOD

• Toxicity to aqueous life before and after biodegradation

• Other (temperature, solid matter, suspended matter, sur-

face tension modiﬁers, etc.)

Solid Waste

• Heavy metal cations (notably lead, tin and copper)

• All leachable toxic materials up to 100 years in autho-

rized discharge conditions

• Used solvent

10.1 Air Emissions Air emissions are a concern for some

chemical-cleaning agents used for stencil cleaning opera-

tions. Generally, there are two types of air emissions from

such a cleaning process; (1) VOCs and (2) water vapor.

VOCs are generated from cleaning with organic solvents

and aqueous cleaners that contain organic solvents. In cer-

tain nations and regions, VOCs are highly regulated

because their vapors react with nitrogen oxides (photo-

chemically) producing smog. The water vapor from an

evaporator that is used to concentrate the chemicals from a

wash tank may require an air emission discharge permit. In

any case, it is important to consult with the regulatory

agencies before a cleaning process is installed. Depending

on the particular chemical(s) emitted, the amount of the

chemical(s) emitted and the amount of other chemical

emissions from that facility, the emissions may or may not

be regulated by the State (other regional permitting author-

ity under the Clean Air Act). An air emissions permit may

be needed if the emissions thresholds for a given chemical

or a group of chemicals at that facility is exceeded. The

worldwide trend is toward increasing the stringency of

VOC regulations. However, the deﬁnition of what consti-

tutes a VOC varies much from one country or region to

another.

Odors in the workspace and/or odors exhausted to the out-

doors are often a primary concern to maintaining a good

working environment.

10.2 Wastewater The discharge of any wastewater

stream, both by total ﬂow and by chemical make-up must

conform to national, regional and local regulations in all

nations. These may vary from lax to very strict limits with

little ﬂexible conditions. Many nations, particularly in

Europe, have draconian legal requirements dictated on a

national scale, covering many aspects of wastewater qual-

ity. Others have less comprehensive regulations, covering

only the more important considerations. Local authorities

may offer derogations to national legislation where their

local treatment plant is able to handle the otherwise out-of-

tolerance waste. Derogations are frequently accorded to

small users, particularly if it may be shown that the cost of

treating a small quantity of slightly polluting waste would

be out of all proportion to the damage that could be caused.

It must be noted that because an installation is approved in

one place, it may not be elsewhere. In all cases, it is wise

to discuss an installation with the local authorities before

putting it into service, as a polluting fait accompli is never

looked at with a kindly eye.

In the USA, the discharge of the wastewater stream, both

by total ﬂow and by chemical make-up will very likely

require a permit form the local public owned treatment

works (POTW) under the Clean Water Act (CWA). In addi-

tion, such a permit may also require co-current approval

(and in rare cases, regional EPA approval). It is important

that any new or additional wastewater ﬂow from a post

February 2007 IPC-7526