 How
clean is clean? Three test methods for determining cleanliness
are commonly used in the electronics industry: |
|
 |
visual
examination, |
|
|
solvent
extraction, and |
|
|
surface
insulation resistance (SIR) measurement. |
 |
| 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. |
|
| 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. |
|
| Mr.
Tech Dweeb Tech Tip |
.jpg) |
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. |
|
|
|
| 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. |
|
| 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. |
|
| 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: |
|
1. |
IPC.
Post Solder Aqueous Cleaning Handbook. IPC-AC-62A, 1996. |
|
2. |
Prasad,
Ray P. Surface Mount Technology: Principles and Practice,
Second Edition. Kluwer Academic Publishers, 1997. |
.jpg)