Is there a standard for acceptable levels of chloride on assembled PCBs and
on bare boards?  What sources of chlorides exist in a board fabrication
process, and in an assembly process?

We are using a no-clean low-solid flux.


*Cheryl, the short answer to your first question is NO.  There are no
standards yet for "acceptable" levels of chloride (or other anions) in
printed wiring assemblies, and I doubt there ever will be.  The main reason
is that most of us realize that there are no more golden numbers, no one
value that can be blindly applied across the board without considering the
function of the assembly or the end use environment.  Most of us can
remember back to the "good old days" of rosin flux and Freon cleaning,
where we could use the military specs and the single pass-fail value for
ROSE testers, regardless of whether it was a garage door opener or a
controller board for the radar on an Aegis warship.  Them days is gone and
so is a single pass-fail value.  I think the best you will get is
standardized protocols on how to determine how clean you need to be for
YOUR product.  It is harder work for you, but better for your product in
the long run.

What sources of chloride exist?  Consider chloride in the workplace to be
as prevalent as nitrogen in the air, as corruption in Washington, as empty
Diet Mt. Dew containers in Doug Pauls' cubicle..... but I digress.
Chloride is in your finger salts, metal plating salts, fluxes, cleaning
solutions, and as constituent elements of most resins.  It is everywhere.
What YOU need to do is determine how much chloride is bad for YOUR hardware
in YOUR end use environment.

The recommended chloride levels used by CSL are a good starting point, and
I emphasize starting point.  You need to do correlation studies between ion
chromatography and environmental stress tests in humid environments on
actual product to determine the amount of acceptable chloride.  If you have
an in-house ROSE/SEC tester (or as I call them, artificial reef materials),
then you can expand the correlation study to determine what ROSE values you
should be using for process control for your hardware.

People have asked where the CSL numbers come from.  As I was technical
director there for 8 years, I'll tell you.  Over half of the work done at
CSL was failure analysis.   Products coming back from the field with
electrochemical failures (corrosion, metal migration, leakage).  Ion
chromatography was done on the failures to determine "bad"  levels of
anions.  Then we would work with the client to examine the process and
determine where the big contributors to the bad anions were.  Cleaning them
up to a "good" level solved the problem.  Over time, you build up a
histogram of "bad" and "good" levels.  The recommended CSL levels are the
general dividing lines between "good" and "bad".

I will correct Mr. Sedlak on his response.  Ion chromatography is ion
specific and can give you accurate chloride levels.  A ROSE or SEC tester
(e.g. Omegameter) gives outputs in micrograms of sodium chloride
EQUIVALENCE per unit area, and has nothing to do with the actual amount of
chloride or sodium on the board.  It is merely a reference to an easily
ionizable salt.  So you can't use an Omegameter (or others) to determine
how much chloride is present.

I would suggest that you take a look at J-HDBK-001, the companion handbook
to J-STD-001.  Section 8 goes into more detail on cleaning and cleanliness.
The IPC Ionic Conductivity / Ion Chromatography Task Group, chaired by
Terry Munson, CSL, is working on a tutorial or state of the art paper on
ionic measurement, which just kicked off a few weeks ago.

Doug Pauls
Rockwell Collins

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