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 part directly. 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 and
indirect methods are presented below.
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 be analyzed 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.
- Magnified Visual Inspection. Visual inspection using a magnifying glass or low-power microscope 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 the micron 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 a cross-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 gross contamination need be removed. Magnified visual inspection is only effective from a practicality standpoint with smaller parts that can be handled by an operator and inspected 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 a requirement, and inspectors must be well-trained and thorough.
- Black Light. This test requires a dark room and black light source for direct visual inspection of parts. This method is a pass/fail test that will work on any material with a contaminant 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. This method 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. Once again, this method is only practical for testing smaller parts.
- Water Break Test. This unsophisticated method takes advantage of the fact that many contaminants 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 additional cleaning. This test can be done in production areas or as a batch test and is also usable on very large parts, such as airplane wings. To be effective, the water used in the test must be free of surfactants or others contamination that would cause the water to flow evenly even in the presence of contamination, and the parts must be of a geometry that allows water to flow across the surface of interest.
- Contact Angle. This method can be thought of as a more sophisticated water break test, as it also takes advantage of the fact that most contaminants cause water to bead up due to their hydrophobic nature. The test requires a small laboratory area and a highly skilled operator. To use this method a contact angle goniometer is required, and the part to be analyzed must be flat and of small size (about 3-inches x 3-inches, or less). In addition, distilled water must be used, and other parameters must be carefully controlled, such as static electricity and humidity. The test is done by applying a distilled water droplet of reproducible size to the test surface. After waiting a couple of minutes for the drop to equilibrate, the operator examines the droplet using the goniometer and records the angle of contact the drop has with the surface. An idealized, perfectly clean metal surface would have a contact angle of 0° , which is impossible to obtain in laboratory air. A contaminated metal part would have a high contact angle, such as 90° or more. Some parts, such as plastics, have positive contact angles even when "clean" so the method is not typically used for cleanliness analysis for these materials. While a number is obtained from this test, the contact angle, the test still is non-quantitative in terms of the contaminants on the part (Ref. 1).
- Gravimetric Measurement. This method requires a highly sensitive scale that can weigh parts to an accuracy of plus or minus one milligram, or better. This method is used on small parts of any material to detect gross contamination, but does not require any other sophisticated equipment besides the scale. Parts must be weighed prior to being contaminated and then after contamination. Then after cleaning, the part is oven dried and weighed again. The difference between the initial weight and post-cleaning weight is attributed to any residual contamination left on the part. If there is no difference in these two weights, the part is considered clean. This is a good gross measurement method when extremely high purity is not required. A small laboratory is needed to conduct these tests, and large parts cannot be tested in this way. An indirect method of gravimetric analysis is also used and is discussed in the next section.
- Optically Stimulated Electron Emission (OSEE). This test, for use on metal surfaces, can be used in the production line. A probe illuminates the surface to be tested with ultraviolet light of a particular wavelength. Illumination stimulates the emission of electrons from the metal surface. These electrons are collected and measured as current by the instrument. Contamination reduces the electron emissions and, therefore, the current measured. The equipment may be connected to a computer that color-codes results for each particular piece tested. This allows before and after comparisons of a single cleaner or side-by-side comparison of two pieces cleaned in alternative cleaners. OSEE is simple to operate, fast, and relatively inexpensive. In addition, it is quantitative, non-destructive, and non-contact. However, it requires a calibration for each part-contaminant combination to be tested. OSEE does not work well with contaminants that fluoresce (Ref. 1).
- Direct Oxidation Carbon Coulmetry (DOCC). DOCC uses oxygen gas in a combustion chamber at a set temperature to combust carbon-based contaminants into carbon dioxide that is then detected by CO2 coulometric detection. Coulometric detection uses electricity to electrochemically measure the weight of carbon combusted in the combustion 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-geometry independent. The method works only on small parts or pieces of larger parts. Due to the high temperature in the combustion chamber (more than 750° F) the method is not suitable 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 a manufacturing environment are carbon based. DOCC can be used in a laboratory but is adaptable to production environments (Ref. 2).
Many other highly sophisticated direct measurement methods exist, including the scanning electron microscope, auger electron spectroscopy, secondary ion mass spectrometry, and fourier transform infrared spectroscopy. Like OSEE and XPS these methods are very costly and require very 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 immediate adjustments 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 will increase further.
- X-Ray Photoelectron Spectroscopy (XPS). This highly sophisticated and expensive measurement 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 electrons under 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. Its application is limited to mostly research and development, but it can be used to calibrate and evaluate other, less sophisticated measurement methods. XPS equipment is typically only present in specialized university laboratories and larger industrial research laboratories (Ref. 1).
Most 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. This requires 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, these methods used solvents that are the type many manufacturers are trying to eliminate from their cleaning processes. Recently, more environmentally benign alternatives have begun to be evaluated for this class of measurement methods.
In addition, indirect methods that use solvents to extract contamination are usually only practical for 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 where very 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.
- Gravimetric Analysis. This method uses the same equipment as the direct gravimetric method 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 a solvent, which is then passed through a fine filter patch. The weighing is done on the filter patch.
- Ultraviolet (UV) Spectroscopy. This method has been used to measure flux residue left on 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 and an UV spectrometer, which are moderately expensive. In addition, the method requires that the contaminant to be analyzed has a unique absorption wavelength that can be identified in the ultraviolet spectrum. A calibration curve then is created by measuring samples of the solvent containing known concentrations of the contaminant at the unique wavelength. 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 solvent to 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 to be 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 the concentration of the contaminants in the extraction solvent. Based on the total volume of solvent and this concentration, the actual amount of contamination is derived. This method must be conducted in a laboratory and requires a skilled operator (Ref. 1).
Many other indirect measurement methods exist, including total organic carbon analysis and ion chromatography (Ref. 5).
- Optical Particle Counter (OPC). An OPC gives both a count and size of particles in the solution 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 particle size 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 requires extraction equipment to prepare a sample for analysis, and the OPC equipment itself. Two major techniques are available for particle counting light extinction and light scattering. 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 of the 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. Light extinction 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 a light 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 comes at a price. OPC is very expensive and requires skilled operators (Ref. 4).
Continue on to the bibliography page of the Cleanliness Measurement Technology Review.
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