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Frequently Asked Questions: Cleaning
Chemistries
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Equipment
Q. What equipment should I have for cleaning?
A. That depends on the cleaning chemistry you plan on using and
the throughput of your shop. Basically, you can break down cleaning
equipment into three possible categories: hand cleaning; batch
cleaning; and in-line
cleaning. Of these three, in-line produces the best and most
consistent cleaning. Batch cleaning is better than hand cleaning,
but suffers from
variability in cleaning. Batch cleaning can either be something
like a batch degreaser, which uses a one rack at a time approach,
or a batch
cleaner, like an industrial dishwasher. Hand cleaning is most
often the localized application of a cleaning solvent, such as
isopropanol, with
scrubbing (e.g. toothbrush). We don’t recommend hand cleaning because
it spreads the flux residue and only infects larger portions of the board.
This is especially true of no-clean fluxes. What kind of cleaning you
do is also highly dependent on the capital investment budget and the reliability
of the hardware produced.
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Q: What does "micrograms of NaCl equivalence
per square inch" mean?
A: Most bulk ionic contamination (BIC) testers,
such as an Omegameter or Ionograph, measure the total electrical
conductivity (or inversely, resistivity) of the extract solution.
The NaCl equivalence
refers to the amount or concentration of sodium chloride (salt)
needed to produce a solution of the same conductivity. It has
nothing to do with
the amount of elemental sodium or chloride found in the test
solution. NaCl equivalence is an equivalence factor used to translate
the results
from one ionic tester to another. Q: Can my BIC (bulk ion contamination)
tester be used to analyze non-rosin flux technology assemblies? A:
The use of a BIC tester is not recommended as a process evaluation
tool, but
can be successfully used as a process control tool. The BICs
were all developed based on rosin fluxes and all of the materials
properties of
rosins. Newer flux technologies are based on very low levels
of rosin, or resin, but the solubilities are different, the chemicals
are different.
Many of the newer flux technologies have electrically conductive
elements which are not harmful to the long-term reliability of
the assembly. A BIC tester cannot distinguish between harmful and non-harmful
residues. A BIC tester can be used as a process control tool
to determine changes
in contamination levels, but it is important to understand what
the BIC response means.
Note: An excellent report, published by the Electronics Manufacturing
Productivity Facility (Report number RR0013, www.empf.org) discusses
many of the relevant issues regarding BIC testers and their applicability
to
various technologies.
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Chemistry Selection
Q. What cleaning media should I use?
A. That depends on the soils you need to remove and the hardware
susceptibilities involved. Our preferred cleaning method for any
flux is saponified cleaning in deionized water. The amount of the
saponifier, and the saponifier you use, depends on the cleaning
challenges present. A 3-5% solution of Enviro Gold 816/18.54A (EnviroSense
Tel# 408-213-2291) is good for most materials. A solution of 7-10%
solution may be needed for heavier flux loads. In our opinion,
aqueous cleaning is superior to semi-aqueous cleaning and gives all
the benefits without the drawbacks of semi-aqueous cleaning. Solvent
cleaning often does not have the mechanical action of the sprays of
aqueous cleaning and so is not as efficient a cleaner. Solvent
cleaning should only be used, in our estimation, where the hardware
is completely intolerant of water.
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Evaluation
Q. How should I evaluate which cleaner is best for me?
A. Basically, ion chromatography is the best method. You want
to characterize the residues present and determine how much residue
is left after the candidate cleaning process. In such an evaluation,
you should include uncleaned samples to show the residue level
before any
cleaning, so you have an idea of the efficiency of any process.
We also recommend that you include a known good, or baseline,
process
for comparison,
so you have an idea of whether the residue levels after the candidate
cleaning are "good" or "bad".
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