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February 2001

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Subject:
From:
Douglas Pauls <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 28 Feb 2001 17:19:23 -0600
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*<sigh>  I always try to avoid responding to these questions, but it draws
me like a moth the the flame......

 refer to IVF Research Publication 96846 - Cleanliness and Reliability:
Evaluation of Test Methods and the Impact on Contamination from Production
Processes on the Reliability of Printed Circuit Board Assemblies - Dr
Per-Erik Tegenhal 1996

He states:

"It was demonstrated that the accuracy of solvent extract conductivity
measurements, i.e. the most commonly used method for cleanliness
measurements, is very poor. In one case, it was found that less than one
percent of ionic contaminants present on a hot solder levelled PCB of FR-4
type was detected by a commercial tester. The main reason for the poor
accuracy is that flux residues are absorbed into the epoxy resin during the
soldering process. Therefore it is very difficult to clean FR-4 boards but
also to analyze the cleanliness of such boards. The use of equivalence
factors for various testers is not a feasible way to handle this problem."

*I agree with Dr. Tegehall in that the repeatability and reproducibility of
these automated ROSE or SEC testers is very poor.  The IPC ionic
conductivity task group, in conjunction with the EMPF proved this in the
early 90s.  IPC has the 200+ page report if desired.  So does EMPF.  I
would disagree that the reason you don't see the flux is that it completely
absorbs into the laminate.  Some FR-4 laminates will absorb flux.  Some
solder masks will.  Others don't.  The better the chemical resistance of
the laminate and mask, the more it will resist absorption of flux.  The
main reason that you don't get all the flux detected is a solubility, time
and temperature phenomenon.  ROSE and SEC are extraction tests.  If you
can't dissolve the contaminant, you can't detect it.  Room temperature
IPA/water will extract a certain amount.  The ROSE testers that use heated
solutions get a higher value because hot IPA/water and turbulent actions
tend to increase the extraction into solution.  It is not really difficult
to clean an FR-4 board, we have been doing so for years.  But it is
difficult to get an accurate measure of cleanliness and even more difficult
to determine how clean is clean enough.

If the 1% of ionic contaminants are removed from the surface of the board
then what is to stop the highly mobile chlorides from migrating out of the
"spongy" FR-4 substrate after cleaning in the direction of the newly
established concentration gradient and eventually reaching pre-clean
contamination levels? What effect does this have on reliability?

*If you DO have absorbed flux in the laminate and mask at appreciable
levels, then you can get electrical leakage under humid conditions,
electrolytic corrosion, conductive anodic filament growth, and
electrochemical migration.   You probably won't ever get to pre-clean
conditions.  The thermal processes of soldering have an effect on the
materials, so you can never truly get back to your starting condition.

Logically then, taking chlorides for example, the reliability of the PCB
will NOT correlate with the concentration of the surface chloride levels,
but with the relative humidity and "wetting" of the PCB surface. A "wet"
surface would allow the chloride to migrate out of the substrate,
compromise the SIR and this may correlate more highly to PCB failures.

What levels of chlorides have been found in and around the solder pads of
HASL/FR-4 type boards?

*You need to keep in mind the difference between chloride residues on the
board, and the results of the ROSE test.  When the ROSE test gives you a
result, it is in micrograms of sodium chloride equivalence per unit area.
This is NOT the amount of chloride or sodium you have on your board.  If
you want to learn more about this, look in the recently published J-STD-001
Handbook, where I give a full run down on why this is so.  To answer your
latter question about chlorides on HASL boards, CSL has done extensive work
in that area using ion chromatography as the measurement tool.  Much of
that work was published in the monthly Circuits Assembly magazine column,
Process Rx, between Sept 96 and Feb 01.  You can also access that through
the CSL web site at www.residues.com.

In this situation. If the spot levels of chlorides in and around the solder
pads of HASL/FR-4 boards is high (how high I do not know), what then is the
relative contribution to failure rates by environmental chloride
contamination? Environmental contamination may only contribute a fraction
of the total ionic contamination found in an around the solder pads.

*Chlorides are a nasty electroactive material and are one of the primary
contributors to electronics failures.  If high, you can see the
electrochemical failures I listed above.  Environmental chloride can be an
issue, depending on whether you conformal coat the boards and how excessive
the chloride can be.  It all depends.

Doug Pauls
Rockwell Collins

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