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Subject:	Re: Encapsulated Ions
From:	[log in to unmask]
Reply To:	TechNet E-Mail Forum.
Date:	Tue, 31 Jul 2001 07:56:17 -0500
Content-Type:	text/plain
Parts/Attachments:	text/plain (84 lines)

How do you carry out a non-destructive test in a no-clean process. Ions are
encapsulated as part of the process, surely methods used in the standards
will release the ions and then fail the test?

*Cathy, Andrew Hoggan had some good answers and I will add to what he said.
If you do a suitable extraction-based test, and by suitable I mean the
right composition of solvents, time, and temperature, then you WILL get
ions. Either from the surface residues from the assembly process, or ions
leached out the base materials. Examples are chloride (present in all
epoxies), bromide (present in most flame retardant laminates), carbonate
(from many solder masks), etc. I don't consider the existing ROSE/SEC
tests to be suitable, since the times are short (10 minutes), the
temperatures relatively benign (45C max) and it is not ion specific.

The key to evaluating ROSE/SEC or any extraction-based test is to
understand what your signal means. For far too long, people have looked at
the value of 10 (suitably modified per machine) as the divider between
"good" and "bad" and never understood what the response was telling them.
When manufacturers started shifting to no-clean assembly processes, they
did not understand that the low solids flux residues dissolve in
isopropanol/water and become electrically conductive when they do so.
People used to seeing 4-5 now saw 40-50 and, to put it Biblically, there
was much wailing and gnashing of teeth. They did not understand what the
signal was telling them. That is why I prefer ion chromatography as a tool
for analyzing ions. People have been using it now for over 10 years to
help determine if the ions they see are good ions or bad ions. (Every time
I say that I think of Glinda, the Good Witch of the North in the Wizard of
Oz asking if Dorothy was a good witch or a bad witch. Where was I?)

Any time that you have a cleanliness test, whether ROSE or ion
chromatography, you have to determine several things: (1) is the test
repeatable, (2) am I seeing good residues or bad residues, and (3) how
clean is clean enough for my product in my end use application. As my
friend John Sohn puts it "the company has to do the hard work".

You can use an Omegameter type instrument on a process, provided you throw
out any preconcieved notions of what is good and bad and form your own
process control limits. You can analyze by ion chromatography, but you
have to do correlations with either field failures or accelerated life
testing in humid conditions to determine what ions are present and how
robust your assembly is to various ions.

Overall, do not look to IPC specifications to tell you what "clean" or
"dirty" is; you won't really find it. There are no more golden numbers.
No one size fits all figure. We could do that when we all manufactured
with RMAs and Freon cleaning, but there is far too much diversity in
products and materials to do that now. The best that you can hope for is
guidance from IPC on how a company should do the "hard work" to determine
their own limits. What limits are good for Rockwell Collins may not be
good for you. Clean enough for RF work might not be clean enough for
biomedical.

This is an issue that one of my task groups is working on (Bare Board
Cleanliness). I think we have good tools for measuring bare board
cleanliness, but now we have to define a protocol for how a manufacturer
might determine how clean their boards need to be for their product. A
tough task.

Well, now that my fingers are warmed up.....

Doug Pauls
Rockwell Collins
Court Jester to Jack Crawford

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