P2 Tech 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 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
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.

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.

Indirect Methods
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.

Many other indirect measurement methods exist, including total organic carbon analysis and ion chromatography (Ref. 5).

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