 How
clean is clean? Three test methods for determining cleanliness are commonly
used in the electronics industry: |
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visual examination, |
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solvent extraction, and |
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surface insulation resistance (SIR) measurement. |
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| Visual Examination |
| This method entails inspection of circuit boards under an optical microscope at
2X to 10X magnification in order to identify flux residues and other contamination.
The main limitation of this method is that flux residues trapped under large components
cannot be identified microscopically. |
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| Solvent Extraction |
| The solvent extraction method involves immersing the circuit board in a test solution
and then measuring the ionic conductivity in terms of micrograms of NaCl equivalent
per square unit of the board area. For this method to be effective, the test solution,
typically isopropyl alcohol and deionized water, must remove the contamination
from under every component. Solvent extraction is commonly used to monitor the
cleanliness of conventional assemblies. J-STD-001 requires ionic contamination
to be less than 10.06 μg/in² (1.56 μg/cm²). This standard applies to
all fluxes, including no-clean fluxes. |
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The NaCl equivalent standard was developed
to allow one test measurement to yield a value with a common
meaning no matter what type of contaminant is present. The value
is a calculation of the amount of NaCl that, if dissolved in
the solution, would produce the same measured resistivity. The
value does not mean there is necessarily any NaCl in the solution.
dsi ’s standard for assemblies is less
than 0.5μg/cm². |
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| The solvent extraction process is widely employed primarily due to its simplicity.
The equipment to perform this test is inexpensive and does not require a highly
skilled operator. Consequently, this test is often used at the end of a process
line to characterize the cleanliness of a board. In itself, the figure (μg/cm²
of NaCl) is not directly translatable to a specific contaminant. However, if
the process has been approved for use with a NaCl equivalent cleanliness test,
then the number generated by the cleanliness test can be used as a process indicator
for total cleanliness and a range of acceptability can be established. |
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| Surface Insulation Resistance Measurement |
| SIR measurement is widely used for determining the insulation resistance of laminates,
assessing the compatibility of fluxes with circuit board material, and testing
the cleanliness of circuit board assemblies. The equipment used to measure SIR
values consists of a high resistance meter, generally referred to as a megohm
meter, and a humidity chamber. |
| The primary advantage of the SIR measurement method is that it is direct and
quantitative. It provides useful results when applied to boards with an aggressive
flux. SIR tests also flag problems with adhesive curing. If an adhesive is cured
rapidly, voids are generated that may entrap flux. Inspection and extraction
methods cannot adequately detect flux entrapment, but SIR measurement can. |
| The major disadvantage of SIR measurement is the need to design additional
circuitry on the surface layers of the circuit board to conduct the measurements
effectively. Another limitation of this approach is that the trace pattern must
be standardized to either a “Y” or a “comb” pattern. Differing component sizes
on the board can make it difficult to standardize one particular SIR pattern.
Also, to obtain a representative indication of the contamination under the components,
the selected SIR pattern must appear in all areas of the board. |
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| Selecting the Right Method |
| The two test methods described in this article can generate very useful results
if they are interpreted properly. The SIR approach yields a figure that can
be correlated directly to the concentration of a specific contaminant. However,
the test equipment is sophisticated and does not lend itself to the factory
floor. Further, the product must have been designed to accommodate this test.
In contrast, the solvent extraction method is a representation of total cleanliness
and does not give specific information. However, this test is ideally suited
for inclusion in a process line and does not require advance planning to test
the product. Failure of the solvent extraction test clearly indicates that the
process is out of control and the product should not be shipped. |
| Sources: |
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1. |
IPC. Post Solder Aqueous Cleaning Handbook.
IPC-AC-62A, 1996. |
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2. |
Prasad, Ray P. Surface Mount Technology: Principles
and Practice, Second Edition. Kluwer Academic Publishers, 1997. |
