Types of Cleanliness

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Types of Cleanliness

Measurement Methods

The wide range of measurement methods available can be differentiated in many ways. One

simple way to divide measurement methods is by whether they are direct or indirect. Direct

methods actually measure cleanliness on the part of interest by analyzing the surface of the partdirectly. Indirect methods typically use a solvent of some type to extract the contaminants of

interest from the part and the solvent then is analyzed for contamination. Selected direct andindirect methods are presented below.

Direct Methods

Direct methods of cleanliness measurement work directory with the part being cleaned and,therefore, avoid many of the problems inherent in collecting contaminants off the part to beanalyzed indirectly. However, since the part is being analyzed directly, there is a limitation

parts analyzed directly often must be quite small to fit into the measurement equipment.

y Magnified Visual Inspection. Visual inspection using a magnifying glass or low-powermicroscope can be used to look at a part made of any material directly and observe any

gross contamination that may not be visible with the naked eye but is still larger than themicron range. The method requires the part be taken to the inspection area and out of

production to be inspected. It is a pass/fail measurement method that may be used as across-check in precision cleaning applications that also use a more precise measurement

method, or as a primary method in non-critical cleaning applications where only grosscontamination need be removed. Magnified visual inspection is only effective from a

practicality standpoint with smaller parts that can be handled by an operator andinspected in several short scans. It has the advantage of requiring minimum equipment.

However, an area separate from production, such as a small laboratory, is almost arequirement, and inspectors must be well-trained and thorough.

y Black Light. This test requires a dark room and black light source for direct visualinspection of parts. This method is a pass/fail test that will work on any material with acontaminant that fluoresces under black light, provided the part itself does not fluoresce.

The operator simply places the part under the black light and inspects the part. Thismethod has most of the same application issues that magnified visual inspection does,

except since the contaminants fluoresce if present they are even easier to notice. Onceagain, this method is only practical for testing smaller parts.

y Water Break Test. This unsophisticated method takes advantage of the fact that manycontaminants of interest are hydrophobic. In this pass/fail test, which is typically used for

metal surfaces, water is flowed over the part. If it sheets off the surface evenly, the part is"clean." If the water channels or beads on certain areas the part is rejected or sent for


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contaminant combination to be tested. OSEE does not work well with contaminants thatfluoresce (Ref. 1).

y Direct Oxidation Carbon Coulmetry (DOCC). DOCC uses oxygen gas in acombustion chamber at a set temperature to combust carbon-based contaminants into

carbon dioxide that is then detected by CO2 coulometric detection. Coulometric detectionuses electricity to electrochemically measure the weight of carbon combusted in thecombustion chamber. The method is very sensitive and can detect as little as one

microgram of carbon. DOCC works on a variety of materials and is surface-geometryindependent. The method works only on small parts or pieces of larger parts. Due to the

high temperature in the combustion chamber (more than 750r F) the method is notsuitable to parts sensitive to high temperature. The method is expensive, but not as much

as such as Optically Stimulated Electron Emission or X-Ray Photoelectron Spectroscopy(discussed below). In addition, DOCC only detects carbon-based contaminants, although

this is generally not an issue since the majority of contaminants encountered in amanufacturing environment are carbon based. DOCC can be used in a laboratory but is

adaptable to production environments (Ref. 2).

y X-Ray Photoelectron Spectroscopy (XPS). This highly sophisticated and expensivemeasurement method uses special equipment to bombard the surface of interest with x-rays under vacuum conditions, causing electrons to be released from the surface. Since

each type of element (i.e. carbon, oxygen, etc.) releases a unique amount of electronsunder these conditions the actual elemental composition of the surface can be quantified.

The test requires a very small, flat surface and is not only expensive but lengthy. Itsapplication is limited to mostly research and development, but it can be used to calibrate

and evaluate other, less sophisticated measurement methods. XPS equipment is typicallyonly present in specialized university laboratories and larger industrial research

laboratories (Ref. 1).

Many other highly sophisticated direct measurement methods exist, including the scanningelectron microscope, auger electron spectroscopy, secondary ion mass spectrometry, and fourier

transform infrared spectroscopy. Like OSEE and XPS these methods are very costly and requirevery skilled operators. Recent research has even lead to the development of in-process

measurement methods that can report on surface cleanliness in real time, allowing immediateadjustments to be made on the production floor based on the results (Ref. 3). When newer

technologies such as this are fully commercialized efficiencies of cleaning in manufacturing willincrease further.

Indirect MethodsMost indirect methods of cleanliness measurement depend on a solvent of some type to dissolve

any contaminants left on the part so that they can then be analyzed using the method. Thisrequires that the solvent used be stronger than the solvent that was originally used in the cleaning

to remove any residual the actual cleaning solvent was not able to remove. Historically, thesemethods used solvents that are the type many manufacturers are trying to eliminate from their

cleaning processes. Recently, more environmentally benign alternatives have begun to beevaluated for this class of measurement methods.


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In addition, indirect methods that use solvents to extract contamination are usually only practicalfor small parts due to the large volume of extraction solvent that would be needed for larger

parts. Still, this method can analyze larger parts than direct methods such as contact angle wherevery small parts must actually be able to fit in the equipment. Also, when extraction is used none

of the geometric limitations exist as they do for contact angle and some other direct methods.

y Gravimetric Analysis. This method uses the same equipment as the direct gravimetricmethod discussed in the previous section and has many of the same issues. However,

instead of weighing the part directly the contaminants are flushed off the part using asolvent, which is then passed through a fine filter patch. The weighing is done on the

filter patch.

y Ultraviolet (UV) Spectroscopy. This method has been used to measure flux residue lefton printed circuit boards in the electronics industry, and has also been adapted to detect

oils and greases on metal parts. This method requires the use of extraction equipment andan UV spectrometer, which are moderately expensive. In addition, the method requires

that the contaminant to be analyzed has a unique absorption wavelength that can beidentified in the ultraviolet spectrum. A calibration curve then is created by measuring

samples of the solvent containing known concentrations of the contaminant at the uniquewavelength. The method is only usable in the concentration ranges where the calibration

curve is straight. Parts that are to be analyzed are extracted in a known amount of solventto remove any of the contaminant. Typically, agitation or sonication is required during

the extraction, which must be done in the same manner for each sample for the results tobe meaningful. The solvent extract is then analyzed in the UV spectrometer at the unique

wavelength. The absorbance then is compared to the calibration curve to find theconcentration of the contaminants in the extraction solvent. Based on the total volume of

solvent and this concentration, the actual amount of contamination is derived. Thismethod must be conducted in a laboratory and requires a skilled operator(Ref. 1).

y Optical Particle Counter (OPC). An OPC gives both a count and size of particles in thesolution measured, and can therefore be used to find out very specific information about

the nature of the contaminants on the part. The method is typically useful when particlesize is of interest, because, for example, particles below a certain size are acceptable as

residual contamination while those above the designated size are not. OPC requiresextraction equipment to prepare a sample for analysis, and the OPC equipment itself.

Two major techniques are available for particle counting light extinction and lightscattering. Light extinction (also called light blocking) use a light source to provide a

beam of light through a flow channel. Particles that pass through the beam block some ofthe light. The blockage of light creates an electrical pulse that is proportional to the

particle size. A microprocessor counts and sorts the pulses according to size. Lightextinction can measure particles as small as one micron. Light scattering is a more

sensitive method that measures the light "scattered" by a particle as it passes through alight beam. Light scattering detects particles as small as 0.1 micron or even smaller but

does not work for particles bigger than about 25 microns. The sensitivity of OPC comesat a price. OPC is very expensive and requires skilled operators (Ref. 4).


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Many other indirect measurement methods exist, including total organic carbon analysis and ion

chromatography (Ref. 5).

Types of Cleanliness

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