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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.
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.
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.
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