Manual cleaning often involves the use of a swab or wiper and a cleaning solvent to directly clean the surface of a part. In lieu of a swab, alternate processes may involve a spray from a pressurized can, or use of a wiper. Cleaning with a swab involves immersing the swab into cleaning fluid, then wiping the part’s surface. Alternatively, the cleaning agent can be applied directly to the surface prior to application of the swab.

Swabs are generally used in specific spots, wipers can cover a larger specific area, and sprays are used over a more general area. Manual cleaning can be performed with either aqueous or solvent-based cleaning fluids.

Swabs are designed with a variety of materials. The applicator head is, generally, made of cotton. The head can be round, pointed, flat, or spiral. The shaft can be composed of plastic, wood, or paper, depending on application requirements.

Wiper cleaning can be performed with a variety of media, typically a woven or nonwoven fiber consisting of cotton, an inert polymer, or a mixture of the two. Choice again depends on requirements for softness, particle count in the cloth, and disposability.

Solvent choice is a major consideration. There is no one solvent of choice for all applications. The solvent used depends on the application and customer requirements. Cleanliness requirements may necessitate the use of certain solvents. It is therefore important to understand and consider selection criteria before making a decision on which solvent to use.

Selection criteria
Manual cleaning is often the preferred method of cleaning when:

• The part is too large to fit within a cleaning tank
• The part is immobile
• Only portions of a larger assembly, such as electric contacts, require cleaning
• Some parts of the assembly, such as polycarbonate or ABS plastic components, may be incompatible with certain solvents in the primary cleaning operation

Aerosols utilize a unique technology that allows cleaning agents to be stored in a hermetically sealed container and used in specific quantities as needed. They also have the benefit of being applied directly to an area that is contaminated without coverage on the rest of the part. For this reason, they are mostly used for hand wiping/cleaning.

Aerosol cleaners are supplied as cleaning liquids that are pressurized in cans by a variable quantity of a compressible gas (i.e., a fluorocarbon or, in some cases, nitrogen). This liquid/compressed gas mixture is held in the can via a patented valve system, which only allows flow out of the can, thus keeping the contents sterile at all times. Many products need to be shaken before use to allow the liquid and compressed gas mixture to exit the valve in the proper proportion; failure to do this usually results in the compressed gas exiting first and leaving the user with no way to extract the remaining cleaning solution.

Currently, most aerosol cleaners in the cleaning industry are organic solvents, as these materials do not usually require a rinsing step and are easier to package and stabilize in aerosol form. Most cleaners in this category have been specifically formulated for applications such as electronics, optics, and oil/grease removal; because the soils being removed are quite varied, the "one size fits all" approach is virtually impossible.

Many aerosol cleaners are considered "contact cleaners," a term that has acquired a dual meaning in the marketplace: 1. an agent that cleans on contact, and 2. an agent that cleans "live contacts" like those found on a circuit board. In order to be functional on contacts with current in them, the cleaner must be nonconductive and have no flashpoint, to prevent ignition and fire. There are many nonflammable solvents like HCFCs, HFCs, PFCs, and HFEs that can be useful in these applications, but careful selection must occur to ensure optimum performance.

The following are some factors to consider when choosing aerosol cleaners:

• Static discharge or other spark sources are not compatible with flammable formulations.
• Flashpoints over 140° F are not considered flammable for many shipping purposes.
• The rate of evaporation can impact cleaning performance.
• Some residues may require rinsing or wiping.
• Some plastics are soluble in certain solvents.
• Selection may be affected by local emissions regulations.
• The formulation should match its application.

Plasma can be defined as an ionized gas with a roughly equal number of positively and negatively charged particles. The properties of plasma are similar to both gas and liquid.

Plasma cleaning systems utilize excited gas-plasma to chemically crack and dislodge contaminants from the part's surface. Cleaning plasma is created by application of a radio frequency or microwave frequency radiation to gas in a hermetically sealed, low-pressure reaction chamber.

The "dirty" parts are placed within the reaction chamber. Ions and electrons are accelerated in opposite directions. Gas molecules are accelerated to an excited electronic state, releasing active chemical free radicals and ultraviolet energy. The free radicals activate a chemical reaction at the part's surface.

Commonly used gases include hydrogen, helium, argon, carbon tetrafluoride, or sulfur hexafluoride. Oxygen is frequently used for removal of organic contamination.

Plasma cleaning leaves no residue on the surface, so there is no need for a rinsing step. However, aqueous or semi-aqueous cleaning is often used as a precleaning step prior to plasma treatment, as plasma cleaning is not effective with inorganic materials.

Ionization of gas is accomplished at low frequency (40-100 kHz), radio frequency (13.56 MHz), or microwave frequency (2.45 GHz). Radio frequencies produce the most homogenous plasma.

Plasma is ideal for removing thin-filmed organic contaminants in precision applications. Plasma can easily penetrate small porous networks within the surface, areas not readily accessible with traditional cleaning solvents. This behavior gives plasma its utility in the manufacture of semiconductors.

Plasma can act on the surface either chemically or mechanically. Chemical reactions involve interaction of the plasma with the surface contaminants. Oxidation converts organic contaminants into carbon dioxide, carbon monoxide, and water. Noble gases, like argon, are used to mechanically dislodge surface contaminants, which are then evacuated into a vacuum stream to avoid redisposition onto the surface.

SELECTION CRITERIA
Plasma cleaners are especially appropriate where:

• complex geometries and surface tension properties reduce the effectiveness of liquid cleaning.
• monolayer removal of organic contamination is desired.

References
Nickerson R. Plasma: Will it do Everything? In: Precision Cleaning '97 Proceedings; April 15-17; Cincinnati, Ohio.
Kohli R. Nonaqueous processes provide critical cleaning alternatives. Precision Cleaning. November 1997.
Schmid H. Super-fine cleaning with liquid phase pre-cleaning and subsequent plasma treatment. Precision Cleaning. October 1995.

Laser systems can be used in conjunction with plasma cleaning methods for thin-film contamination. They can also be used for the ablation (removal) of particulates, photoresist, adhesives, and residues, as well as epoxy, urethane, or oxide layers from surfaces. Laser systems have the potential to be a replacement technology for many traditional cleaning methods.

Laser systems are considered a noncontact technology. Unlike other precision cleaning systems, there is no need for chemicals, and no secondary wastestream is produced. Laser treatment breaks hydrocarbon-based surface coatings into carbon dioxide, water vapor, and trace quantities of other gases. For epoxy-coated and lead-based paint coated surfaces, the wastestream volume is approximately 25 percent of the original coating.

Function and Design
The laser emits energy in the form of photons (light). This energy causes surface contaminant electrons to become “excited,” whereby the electrons are energized to higher energy levels. “Relaxation” of these electrons then occurs, and additional photons are released. Contaminant removal occurs as a result of the localized energy transferred to the surface from the relaxation process. Little heat is generated from the short, high intensity photon pulses. As a result, a cool and dry surface can result, showing no thermal or mechanical strain.

A typical laser includes a light source (lamp), an optical train to direct the laser beam to the surface, a stage to hold the substrate in place, and a gas source. The laser system can be adapted to clean virtually any portion of a substrate.

Gases are contained within the lamp, which serves as the incident photon source. Common gases used include mercury/xenon (Hg/Xe), argon fluoride (ArF), and krypton fluoride (KrF). The wavelength (and frequency) of the relaxation energy is characteristic of the atomic structure of the gas. Pulsed, single-wavelength lasers are commonly used for contaminant removal.

Advantages

• Highly selective over a defined area
• No introduction of foreign contamination
• No thermal diffusion
• Varied rate of contaminant removal (which is determined by the pulse rate)
• Potentially low operating and waste disposal costs

Ultraviolet (UV) light and ozone are commonly used together in wastewater treatment to perform oxidation of organic materials, including microbial organisms. The use of UV/ozone can be extended to removing organic contamination on a substrate surface. Major applications of UV/ozone cleaning include removal of certain photoresists and carbonaceous removal from semiconductors.

The energy put forth by UV light will catalyze the decomposition of ozone or hydrogen peroxide. Ozone (O3) occurs as a gas, as does its sister, oxygen gas (O2). In the cleaning process, ozone is dissolved in the solution being treated. Since ozone is about 12.5 times more soluble than normal oxygen gas, a reasonable level of efficiency can be expected. Lower temperatures are generally recommended because the solubility of ozone decreases as the temperature increases.

Cleaning occurs as a result of the free radical formation that accompanies ozone and hydrogen peroxide decomposition. Ozone (O3) breaks down into oxygen gas (O2) and a free oxygen molecule (O), while hydrogen peroxide (H2O2) breaks down into water (H2O) and a free oxygen molecule (O). These free oxygen molecules are very reactive. They will not re-form hydrogen peroxide and ozone, as these are not preferred states. Therefore, they can do two things: they can bond together to form oxygen gas (which accounts for some of the reaction), or they can react with something else. In the cleaning process, that "something else" is organic contamination on parts. The oxygen radical will react with organic compounds and cause them to cleave and break down. These reactions will continue in the presence of oxygen radicals until the organics can break down no further.

Many organics and break-down products, such as carboxylic acids (eg, RMA and RA fluxes), will not oxidize, so knowledge about the soil being removed is paramount to ensure complete and efficient cleaning. Also, since this process would be hindered by time in the case of thick layers of soils, a precleaning step may be recommended and may need to be coupled with other processes to remove inorganics and other ionics.

NOTES

• Higher pHs generally act to catalyze the reaction, as they serve to speed decomposition and provide a more favorable situation for the production of hydroxyl radicals.
• Photocatalytic oxidation hydrocarbons have been found to occur in the absence of oxygen; this suggests that UV/ozone cleaning may occur under ultra-high vacuum.
• Upon comparison with 'wet" hydrogen peroxide methods, UV/ozone results in a lesser potential for metallic contamination.
• Used with silicon wafers, UV/ozone was found to convert the surface from being "hydrophobic" to "hydrophillic."
• UV/ozone cleaning works best at UV wavelengths below 300 nm.

Wet parts are generally unacceptable in manufacturing processes. Drying is usually the last step in any liquid cleaning process, whether it is aqueous, solvent, or semi-aqueous. Drying is especially critical in applications where the finished product is moisture sensitive (eg, in cases where flash rusting is possible).

The extent to which a part has been dried is a crucial element in some processes. Insufficient drying may be cause for rejection of a cleaned part; a part might also be damaged or corroded during transit due to lingering moisture. Circuit boards are extremely sensitive to aqueous moisture because they are hindered by conductive contaminants. Besides causing surface coatings to fail, aqueous and solvent residue can degrade labels and outer packaging.

For critical cleaning applications, drying must be achieved without recontamination of the surface. Unacceptable contaminants for many precision applications include dust, dirt, and other particles that might recontaminate the parts during drying. High-efficiency particle air (HEPA) filters are commonly used to assure the purity of the drying air; heated nitrogen is also used in some critical applications. However, using just heat in some applications may result in baking of the contaminants onto the surface.

Commonly used drying systems include blow-offs and engineered air knives, single-pass and recirculated hot air, centrifuging of the parts, nitrogen-based systems, slow-pull capillary systems, vacuum drying, water-displacing oil, and solvent drying.

Mechanical drying systems typically fall into one of the following categories:
• Low-velocity air (<10,000 CFM)
• High-velocity air (10,000-35,000 CFM)
• Centrifugal dryers
• Superheated vapor

References
Leech CS. Rinsing and drying issues and answers. Precision Cleaning. January 1994.
Vanderpyl DJ, McGlothlan K. Precision drying completes precision cleaning. Precision Cleaning. March 1995.
Green T. Superheated vapor drying systems. Precision Cleaning. January 1996.
Seelig SS. Appropriate drying aids aqueous cleaning. Precision Cleaning. October 1997.

One of the most important topics in the industry today is the filtration and recycling of wash, rinse, and wastewater. It is a key step in saving money and saving the environment. With proper filtration and recycling techniques, manufacturers can increase wash bath life; maintain cleaner, more effective rinsewater; and reduce surcharges on waste effluent.

There are numerous types of available chemistries and a wide a range of soils being cleaned. Consequently, there are many types of filtration/recycling systems that can be employed and an even greater number of combinations. Making the right choice depends on the specific cleaning equipment, chemistry, and soil.

There are three main categories into which a filtration/recycling system might fall:
* Closed-Loop Recycling-Treating and recycling all wash and rinse baths for reuse in the cleaning system.
* Zero Discharge-Treating or recycling all, part, or none of the wash and rinse baths, with the remainder being collected and hauled to another location for treatment and/or disposal.
* End-of-Pipe Wastewater Treatment/Recycling-A system that treats all or part of the effluent from wash and rinse baths such that the final product can be discharged directly to local sewer lines or streams. The remainder of solution is either reused or hauled away for treatment.

Assessing Contaminants
There are a variety of contaminants that can be separated from cleaning solutions, and the nature of these contaminants dictates how they can best be removed:
* Tramp soils are those that immediately float to the surface of a cleaning solution.
* Mechanically dispersed soils refer to oils, greases, and other organic soils that have been dispersed through agitation; they will eventually either float to the top of the cleaning solution or fall to the bottom within a given period of time.
* Chemically dispersed soils refer to contaminants that are naturally soluble in water or have been chemically dispersed by surfactants to form a stable emulsion.
* Particulate soils include carbon, rust, iron filings, etc.

Types of Filtration/Recycling Components
Conventional filtration, or macrofiltration, is employed for cleaning applications that involve a particle size range of approximately 1 micron and larger. Filtration is commonly accomplished via bag filters and depth filters; however, sand filters and strainers are also used. These types of filters are generally used as a first step to remove any large particles or debris that could be harmful to the remainder of the system.

Decanting is generally only applicable when there are large amounts of tramp oil present in a solution. The oil is essentially overflowed from the surface of the solution into a holding tank. The main limitations of this method are that it does not account for any chemically dispersed soils, is unable to filter out much of the mechanically dispersed soils (unless the solution is allowed to stand and break), and results frequently in the contamination of tramp oils with cleaning solution that also has overflowed.

Skimming works on the premise that a lipophilic surface (wheel, rope, or belt) passes in and out of the cleaning solution. As oils adhere to the surface, they are removed from the solution and deposited elsewhere. Skimming has its benefits in that large amounts of cleaning solution are typically not removed. Its downfall is that it also has the strong potential to remove surfactants and some corrosion inhibitors in conjunction with the emulsified soils.

Coalescing is accomplished by passing mechanically emulsified oil across plates or media, which will selectively adsorb the oil. As more and more oil accumulates, it will coalesce and float to the surface, where it can be decanted or skimmed. This method also can remove surfactants and some corrosion inhibitors.

Absorption technologies function on the premise of passing a contaminated solution over a media (generally woven or granular), which is lipophilic and hydrophobic. In this manner, only organic material is absorbed, and all aqueous entities are retained in solution. Again, this method suffers from the potential to remove surfactants and some corrosion inhibitors.

A current method of oil-water separation uses Bernoulli's principle. The wastewater is split into two laminar flows. Oil is continuously collected and concentrated in a second chamber, which is separated by a baffle from the primary chamber and a reduced-pressure area below. The reduced pressure directs the flow of water down, away from the second chamber.

The oil is recovered from the top of the concentrated layer upon reaching a design thickness. Higher quality oil is recovered this way and then likely reused or burned as fuel.

Membrane technologies, like macrofiltration, serve the purpose of separating contamination from the water. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis are all membrane technologies, which are listed here by relatively descending pore sizes (see Figure). These filtration methods are tailored to be most effective within certain contaminant size ranges. This is done via engineering specific pore sizes in the membrane material.

All of these methods work on the principle of passing a stream of contaminated water over a membrane. As this is done, two streams are ejected. One stream is termed the “permeate” stream and includes particles that were small enough to pass through the membrane. The other stream is termed the “retentate” (or “concentrate”) stream and contains all of the particles that did not make it through the membrane. This retentate stream can be either rejected or passed across the membrane again for further purification.

Most membrane technologies are limited by their tendency to remove emulsified soils along with the surfactants that encapsulate them. It is therefore important to understand that the type of cleaning agent being used will influence the membrane system. Membranes also have the inherent potential for clogging, so they may need to be cleaned and/or replaced periodically. These systems are exceptionally well suited for rinsewater recycling, assuming that contamination is minimized in the rinsewater.

Ion exchange is primarily used for the purification of incoming water and the treatment and recycling of rinsewater. This is because rinsewater will ideally have a very low level of ionic contamination and thus the lifetime of the ion exchange media will not be unnecessarily short.

Ion exchange media is generally supplied in the form of resins contained in enclosed “resin beds.” The resin material generally consists of functional groups attached to naturally occurring or synthetic polymer backbones. These functional groups carry either a positive or a negative ionic charge and serve to retain anions and cations, respectively. Ion exchange media can be purchased in a variety of forms depending on the application. Naturally, this type of system has the potential to remove any ionic constituents from aqueous cleaners, such as saponifiers, builders, corrosion inhibitors, etc.

Activated carbon can be used to remove organic materials and residual chlorine in many instances where organic contamination of baths is fairly low. Carbon is generally placed downstream from skimming and filtration processes to minimize unnecessary contamination. It is generally used only for recycling rinsewater due to the presence of organic surfactants that will be removed from wash baths. Activated carbon works by selectively adsorbing certain molecules (mostly organic ones) to the inner surface of the carbon structure.

Ultraviolet (UV) radiation, or catalytic oxidation, can be used in lieu of using carbon to remove organic residues. This technique is widely used in the water-purifying arena for its biological sterilization abilities. In this process, water is treated with hydrogen peroxide and/or ozone gas. The solution is then pumped through a UV light tube that catalyzes the decomposition of ozone and peroxide into oxygen gas, water, and free radicals. Free radicals are exceptionally strong oxidizers and serve to react with and break down organic molecules.

Water Cleanliness
The cleanliness of incoming/recycled water is affected most significantly by the arrangement of the carbon, ion-exchange, and/or filtration systems. The quality of the water produced can be measured by its resistivity. Contaminated influent water, with much dissolved and suspended matter, has a high electrical conductivity and thus a low resistivity (measured in megohms [MW]):

• Medium-quality water (1-2 MW)
• High-quality water (8+ MW)
• Superior-quality water (up to 18 MW)

Water treatment is necessary to reduce the level of contamination present in effluent water; therefore, it is a consideration when selecting and operating many cleaning systems.

Contaminants of concern may include insoluble oils, emulsified oils, other dissolved organics, suspended solids, and dissolved solids (eg, chlorides, nitrates, phosphates, and metals). Consult the Publicly Owned Treatment Works (POTW) regarding acceptable criteria for pH, biological oxygen demand (BOD), and chemical oxygen demand (COD). The requirements for discharge to the local POTW vary from state to state, municipality to municipality. Specific discharge criteria will be noted on your permit.

Pretreatment
Pretreatment may consist of a variety of unit operations, all performed to reduce the volume of solids and wastewater.

Filtration/screening is commonly used to remove any large particulate matter that should not enter the main treatment system.

Evaporation is used to reduce the volume of water needed for further treatment. The water is evaporated by heating it to its boiling point, or under vacuum. The contaminants then become concentrated within the remaining solution. The water is transferred to a holding tank, where it is allowed to cool to room temperature. Then, it is either discharged to the POTW (with the necessary permits and approvals in place), or moved along for further treatment.

Treatment Methods
pH
Wastewater operations can use pH control in order to promote solubility or flocculation of contaminants at various stages of treatment.

Solids
Acid conditions are commonly used to enhance break-up of the oil-water emulsion. Acids also make anions (eg, phosphates) react more readily with materials such as lime and ferrous oxide in order to make them separate. Once the "lock-up" reactions occur, the solution enters a "floccing" stage.

A cationic flocculant polymer is added at this stage. This aids both in the formation of filterable particulate and in the adsorbtion of oils and other organics to solids. An anionic coagulant can then be added to enhance particulate agglomeration, thus making it easier to filter.

The wastewater is pumped to a clarifier, where contaminated solids settle to the bottom of the chamber as sludge. The sludge is then dried via a filter chamber in preparation for disposal. The supernatant liquid is pumped to a process tank for pH adjustment and release back to the POTW.

Oils
Traditional mechanical separation of oil from wastewater involves the use of skimmers, tank-overflow, and decanting methods. These end-of-pipe (EOP) methods can be inefficient, increase maintenance downtime, create disposal problems, and increase costs.

Gravity separation of non-emulsified oils within wastewater can occur in the clarification tank. Optimized influent and effluent flow rates allow efficient separation of the lighter oil layer from the water. An inclined plate will direct the flow of the oil layer away from the wastewater. The oil droplets coalesce into larger globules and rise on the plate underside, while the sludge particles flow over the plate separator to the tank bottom.

A current method of oil-water separation uses Bernoulli's principle. The wastewater is split into two laminar flows. Oil is continuously collected and concentrated in a second chamber, which is separated by a baffle from the primary chamber and a reduced-pressure area below. The reduced pressure directs the flow of water down, away from the second chamber.

The oil is recovered from the top of the concentrated layer upon reaching a design thickness. Higher quality oil is recovered this way and then likely reused or burned as fuel.

Activated carbon
Activated carbon filters can remove organic constituents from the wastestream. An activated carbon filter is capable of absorbing material many times its own weight. The organic matter is absorbed within the extensive pore network of the carbon.

UV treatment
Ultraviolet (UV) light is an effective means to destroy organic chemicals and biological organisms. A UV-oxidation system can be employed, as necessary, to reduce the BOD of the wastewater.

Ion-exchange treatment
Ion exchange involves cationic and/or anionic species being adsorbed onto a resin. The ions will attach to the resin until the resin is saturated. The resin can then be regenerated by the manufacturer.

Selection Criteria and Compatibility Concerns
Each of the above unit operations may or may not be applicable to all water treatment systems. The requirements of a wastewater system are governed by the extent of contaminant reduction necessitated by POTW permit requirements. Below are criteria to be considered prior to installation of a wastewater system:

• Wastewater treatment options should be studied by an engineer to assess process alternatives. The system should be optimized for proper flow rates, filter capacities, throughputs, etc., for the contaminant reduction required by the discharge permit.
• Capital and operating costs may justify recycling of wastewater in a closed-loop system. Capital costs of a closed-loop system vary depending on the system capacity and complexity. However, long-term profits may outweigh short-term capital expenses.
• Process chemistries must be thoroughly reviewed to determine impact on treatment process.
• Design, maintenance, and operations should have contingencies for downtime.
• Equipment configuration should be considered (wash tank, rinse tank, motorized equipment).
• Annual operating costs should be considered (utilities, materials, labor, maintenance, disposal costs).

The level of cleaning obtained by a given process is determined at the substrate surface, upon completion of all cleaning/rinsing/drying procedures. For critical cleaning applications, this is generally done via instrumental methods. The complexity of this verification system is determined by the quality objectives for the method and the finished product.

A traditional, noncritical method of verification is visual inspection and includes such assessments as the "white glove" and "water break" tests. For noncritical applications, this approach may be all that is required, as long as the limitations are understood. For applications that relate to semiconductors, printed circuit boards, military specifications, pharmaceuticals, and any other instances where high-precision cleaning is required, instrumental methods may be appropriate.

Particle counting methods
Particle counting (PC) methods are based on light absorption and/or light scattering from a given sample that represents the surface in question. The most quantitative method of accomplishing this is by extracting any potential contaminants from the cleaning fluid liquid or the substrate surface. A photodetector diode is able to detect the degree of light scattering (small particles scatter less light). Problems associated with PC include both the nonspherical nature of most particles and the presence of bubbles, which may alter the scattered-light characteristics of the sample. Light scattering techniques are more commonly used for small or light-colored particles.

Contact angle measurement
Measurement of the contact angle of a droplet on a surface is used to determine the wettability of the surface. Liquids that wet a surface (or spread) have a low contact angle; liquids that do not wet, but rather form a bubble over the surface, have a high contact angle. This is the basis for the "water break test." In the water break test, if water "beads," the surface is considered to be contaminated with a hydrophobic substance (oil/grease). If the water "breaks" or sheets off, the surface is considered clean.

This method is very subjective, and what constitutes "breaking" may be different among observers. Limitations of this test include the fact that very light or scattered contamination may not be discernable. Also, some contaminants are water soluble, especially aqueous-based cleaner residues. Such contaminants are rarely obvious in a water break test, so it is important to be aware of them.

A goniometer is an instrument that can be used to measure contact angles to determine the extent of surface cleaning. It is important, however, to understand the relationship of the critical surface tension of the surface being tested to the liquid being used in the test. For example, very few things will wet certain plastics (eg, PTFE), so they may appear contaminated when they are actually clean. Essentially, contact angle measurement is best only when the cleanliness of the surface is not absolutely critical. It makes a great initial (pass/fail) test to determine if a part should be recleaned or sent on for more critical analysis.

Ultraviolet photoelectric emission
In many cases ultraviolet (UV) light can be used to determine the surface cleanliness. In these cases, the contaminants being sought will "fluoresce" in the presence of UV light. Fluorescence occurs when the energy from UV light is absorbed by a contaminant-that is, "excites" the electrons in that contaminant-and raises those electrons to a higher "electronic state."

The electrons are not stable in this state of higher energy and subsequently will go through a "relaxation" process. During this relaxation, the absorbed energy is released in the form of a photon (the "glow" seen in fluorescence). This cycle of excite-relax happens thousands of times per second, so the glow does not flicker but appears uniform.

Fluorescence can provide a visual indication of where contamination remains on a surface. The intensity of the radiation can also be measured via a registered signal on an instrument, which dictates the degree of contamination on a surface. This form of analysis is useful for locating contamination, but will not enable identification. Other forms of analysis (eg, electron spectroscopy for chemical analysis [ESCA], otherwise known as X-ray photoelectron spectroscopy [XPS]), may be used for identification purposes.

Optically stimulated electron emission
Optically stimulated electron emission (OSEE) works according to another theory based on UV light. Here, when high energy UV light hits a surface, electrons will be emitted, and that reflected "current" can be measured. A clean surface will give the highest return current, so any drop in current will represent contamination.

This method is good for seeing low levels of contamination (both ionic and nonionic), but suffers the same shortcoming as UV photoelectric emission: it can detect contamination, but not identify different species.

Scanning electron microscopy
Scanning electron microscopy (SEM) utilizes a beam of electrons that is passed over a very small area of a surface. This beam scatters when it strikes the surface, outlining the topography of the surface. The "back-scattering" carried in by the return beam of electrons is measured by the microscope. The result is a finely detailed, 3-D image of the surface being scanned.

Scanning microscopes are capable of magnifying an image to more than 100,000 times its original size. This method is well suited for identifying particulate and potentially nonuniform or thick films of contaminant. It would not work as well in uncovering very thin, uniform films. It also cannot "look at" a very large area.

Microscopy methods might be used to directly count particles of various sizes. This can be very time consuming, however, so technicians often measure the amount of particles in a small area and then multiply the results according to the dimensions of the entire part.

Secondary ion mass spectroscopy
Secondary ion mass spectroscopy (SIMS) is used for the composition analysis of surfaces. An energized primary ion beam (Ar+, Cs+, N2+, or O₂+) is directed at the surface, resulting in the ejection of surface atoms as secondary ions. This process is known as "sputtering."

This spectroscopy technique does a great job of identifying elements, but does not identify bonding characteristics as would ESCA. SIMS can detect both positive and negative ions, establishing the nature of the charge of the contamination, if any. The typical sampling depth ranges from 2 to 6 angstroms.

Auger electron spectroscopy
Auger (oh-jay) electron spectroscopy (AES) is used for compositional analysis, determining which atoms are present on a surface. Electrons are directed toward the surface, ionizing surface atoms by causing the removal of an electron from the atom's inner shell. The atom now becomes "excited" and must release energy to "relax" and return to its original state. This is done by transferring the extra energy to an electron that can leave the atom. That exiting electron is known as the auger electron.

The AES method of analysis measures the energy of the auger electron, which is unique to each particular atom. AES is a destructive analysis but is useful in looking for concentrated areas of contamination. It can also be used quantitatively. The typical sampling depth for AES is 20 to 50 angstroms.

Electron spectroscopy for chemical analysis
ESCA is a spectrophotometric technique where X-ray bombardment of the surface results in emission of an electron from a given atom. Knowing the energy of the X ray and measuring the energy of the emitted electron can determine the binding energy of the electron. Peaks represent the various oxidation states that are associated with each energy level.

Due to the penetrating nature of X rays, the substrate surface is commonly analyzed by grazing the X-ray beam across the surface. ESCA methods are more commonly used than AES methods. They have the benefit of revealing chemical structure, bonding, and oxidation state. ESCA is considered nondestructive and has the potential to be very useful in identifying organic compounds.

Measurement and Evaluation of Surfaces by Evaporative Rate Analysis
Measurement and evaluation of surfaces by evaporative rate analysis (MESERAN) utilizes a radioactive decay to quantify organic contamination on a surface. This may involve direct surface analysis or indirect methods, with initial utilization of a solvent extraction. The extent of radioactivity depends on the extent of surface contamination present. A slope method can be used to achieve detection limits at 2 ng/cm² (versus 70 ng/cm² using the total count method).

Nonvolatile residue analysis
Nonvolatile residue analysis (NVR) involves the extraction of contaminants into a solvent, with subsequent evaporation of solvent. The quantity of nonvolatile residue is then determined by weighing. The extraction generally takes place in a container of known weight. Once the extracting solvent has been driven off, the total weight minus that of the container yields the weight of contaminant. This method may suffer in the case of very thin films, where an extremely large surface area must be extracted to obtain accurate data.

Thermogravimetric methods
Thermogravimetric methods are used to monitor the controlled desorption or decomposition of chemically adsorbed contaminants from a substrate surface. A thermobalance detector is used to determine contaminant weight loss as the temperature is increased. In most cases, mass versus temperature is plotted. This curve reveals data such as moisture content and polymer type and quantity.

Total organic carbon analysis A total organic carbon (TOC) analyzer can be used to quantify the extent of organic materials on a substrate surface. TOC analysis is also useful for process control of water within a closed-loop system. In-line sampling of recycled water can be analyzed for TOC to gauge the extent of organic contamination from what should be pure water when it goes back into the rinse tank.

TOC analysis measures the amount of carbon in an organic substance; however, it cannot quantify or identify any particular species. It can be used very effectively to determine the presence of organic contamination.

Phase imaging
Phase imaging is a surface-mapping technique that utilizes an oscillating probe brought into soft contact with a surface. The amplitude of the oscillations (signal) varies with changes in the surface topography. Surfaces are illustrated as light and dark areas. This technique can locate areas of surface contamination, which will stand out as topographically distinct.

Gas chromatography/mass spectrophotometry
Gas chromatography/mass spectrophotometry (GC/MS) is used to identify surface contamination by extracting contaminants into solvent and then analyzing them. Organic compounds are separated via GC and are then identified, based on molecular weight, by MS.

Fourier transform infra-red spectroscopy
Fourier transform infrared (FTIR) spectroscopy is an infrared technique that, like conventional infrared spectroscopy, is able to identify functional groups present among surface contaminants. The FTIR technique makes use of all applicable wavelengths simultaneously due to mirror arrangement, whereas standard infrared spectroscopy involves the scanning of wavelengths. Qualitative or quantitative data can be obtained.

SELECTION CRITERIA
The choice of instrumentation used to measure cleanliness depends on the degree of precision needed. In many cases, to obtain the most complete analysis possible, multiple forms of analysis should be used. For example, a water break test can indicate gross cleanliness and make a determination as to whether to reclean or analyze further. From there, SEM, TOC, and/or ESCA can be performed to further quantify the level of cleanliness.

Another consideration is the extent of quality control required. It may be necessary to analyze standards, controls, and duplicate samples. In most cases, every part need not be analyzed, just a representative sample at preset intervals to track cleaning performance over time. Data accuracy and precision should be established to the level dictated by data quality objectives. All results should be accurately documented.

 

In a recent Industry Practices Study, 4000 respondents were asked to respond to whether or not they clean to a specified standard. Results were as follows:
Most of the respondents who answered "no" were found in the industrial cleaning segment of the industry. However, a number of these respondents stated that they utilize a water break test to determine gross cleanliness.	Most respondents who replied "yes" were found in the precision cleaning segment.

References:
Benkovich MG, Anderson JL. Measurement of organic residues on surfaces to a low fraction of a monolayer. Precision Cleaning. May 1996.
Chernoff D. Phase imaging: what, how, and why. Precision Cleaning. December 1996.
Adamson AW. Physical Chemistry of Surfaces, 4th ed. New York: John Wiley & Sons, 1982.
Cala FR, Winston AE. Handbook of Aqueous Cleaning Technology for Electronic Assemblies. British Isles: Electrochemical Publications, 1996.
Skoog, Leary. Principles of Instrumental Analysis, 4th ed. Fort Worth: Saunders College Publishing, 1992.