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How clean is clean? Clean specification test methods for parts cleaning

Best Technology often asks “How clean do your parts need to be?” but many times our customers just know they need to be “clean”.  Since clean is relative for every application, it is important to understand part cleanliness and various clean test specifications.  The important aspect in parts cleaning systems is understanding the post cleaning process which the part will endure after cleaning: passivation, welding, assembly, medical device implant, IC circuit board final assembly, etc.

Gravimetric / Millipore patch test is method looks for the particulates that remain on the part after cleaning. A membrane filter disk of pore diameter (usually 0.45 μm or 0.80 μm) is weighed before the test. A solvent wash of the “clean part” collects the particulates on the filter disc which filtered and weighed. The difference in weight characterizes the amount of particulate left on the part or gravimetric weight (typically in milligrams – mg)

Also called a solvent extraction test, the particles are flushed off the part and collect on the filter membrane paper can be analyzed microscopically using automated scanning particle counters such as a Leica to characterize the size and amount of particulate collected.  ISO 1632-10 is one such spec which determines part cleanliness based on particulate size and quantity.

FTIR test (or Fourier transform infrared spectroscopy) scans the IR infrared spectrum absorption of a substrate sample to determine the constituents.  This test method is commonly used for testing organic cleaniness of a part before and after parts cleaning.  A part is typically solvent rinsed onto a test substrate which is then scanned via FTIR.

Total Organic Carbon (TOC) test characterizes the carbon loading of soils.  Such tests can be run before and after part cleaning to determine part cleanliness improvement or also after cleaning to determine if any alkaline chemistry dragout remains on the part.  A typical analysis measures Total Carbon (TC) – Organic carbon (OC) and Inorganic Carbon (IC)- content of dissolved carbon dioxide and carbonic acid salts. Subtracting the inorganic carbon from the total carbon yield the Total Organic Carbon.  This test method is can also be effective in determining rinse water quality.

Water break free test or water break test is commonly the simplest and gross tests of part cleanliness.  The water break test is a simple and quick to test for containment, oil and other hydrophobic (water fearing) films. The test is most frequently applied on freshly cleaned stainless steel, titanium, steel, aluminum, brass and other metals which are hydrophilic (water attracted/loving)  in a totally clean state are hydrophilic.  The test simply immerses the part in fresh, clean rinse water at a vertical or angle to look for complete “sheeting” or shedding of water.  Any droplets that form rather than sheet can indicate oils or other residues.

Dyne testing allows for determining the surface energy of various films and often time plastics.  Dyne test inks and fluids (also referred to as corona test fluids) provide a easy measurement of surface energy or wetting tension based on the known surface tension of the fluid. This “wetting” is essential for coating and coverting processes; if the part surface energy does not greatly exceed the surface tension of the fluid/coating/film which is to cover it, wetting will be difficult and a poor bond may result.

For part clean testing of metals, since the surface energy of metals is much higher than that of surface residue/contaminants; the higher the dyne level, the cleaner the part is. ASTM D2578 specifies the fluids used in DYNE test marker pens.

Conductivity rinse testing simply in-process tests the water (typically DI water) conductivity / resistivity before, during and after rinsing.  The change and stabilization of rinse water conductivity can indicate the rinsing effectiveness

Resistivity of Solvent Extract (ROSE) test method is used to detect the presence of bulk ionics. The process involves using a solvent solution to draw ions present on PCB or other substrates off and into the solvent solution.  The solution resistivity can then be analyzed before and after to determine total ionic contamination of a PCB, typically in units of per square inch.

Ion Chromatography Testing (IC Test) takes the ROSE test above a step further and measures the total ionic constituents extractable from PCBs (printed circuit boards) and other substrates or parts.  Thermal extraction is used to extract ionics into solution; after extraction the solution is tested using various standards in an ion chromatograph. The results indicate the individual ionic species present and the level of each ion species per square inch. The conductivity of the extracted solution is measured and expressed as sodium chloride equivalence per unit area of a circuit board. IPC-TM-650, method 2.3.28, Ionic Analysis of Circuit Boards, Ion Chromatography Method describes the procedure in detail.

White glove or white wipe test is a gross clean qualitative subjective test that uses a clean, dry white wipe or glove to wipe across a flat surface.  The white wipe or glove easily shows contamination of colored residues through visual examination.  Since the method is qualitative, a pass/fail criteria is usually used.

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