SMTCleanliness Testing Methods & Conformal Coating

3. Cleanliness test methods

3.1 Visual examination:

Boards are inspected under an optical microscope at 5 to 10X for flux residue and other contamination. The main disadvantage of this method is that the flux residues trapped under large components cannot be inspected. Therefore, for process characterization, the components must be removed to allow visual examination. The visual method is qualitative and is valid for gross contamination levels only. It is difficult to visually detect minute amounts of residue.

3.2 Solvent Extraction:

The solvent extraction method involves immersing the board in a test solution and then measuring its ionic conductivity in terms of micrograms per square unit of board area. For this method to be effective, the solution (isopropyl alcohol and deionized water) must remove the contamination from under every component.

Without sufficient agitation of the solution, it is questionable whether all the flux residues are being removed from under the components that reside close to the board surface. Recently, equipment has become available that allows agitation of solvents during the test. Some commercially available test equipment are Omega meter and Ionograph. This method is commonly used to monitor the cleanliness of conventional assemblies.

4. Conformal coating

Sometimes, conformal coating is applied to the cleaned board, for the purpose of resisting, or at least minimizing the performance degradation caused by hostile environmental influences such as humidity. Because there is no single coating that provides perfect protection, many coating types are available, e.g. acrylics, poly urethanes, epoxies, silicones and polyimides, which may be applied in various ways. The application of a conformal coating is important for boards with very fine conductive patterns, which are sensitive to dust particles, and on which harmful effects of reduction of surface resistance, voltage breakdown and migration are observed.