In a message dated 5/27/2001 11:17:43 PM US Eastern Standard Time,
[log in to unmask] writes:

Dear Chuck

I was away this last week so sorry for the delay in responding to your
questions on ionic contamination.  I have seen and published a number of
articles and case studies on the subject of ionic contamination and
electrical leakage due to fabrication residues and processing fluxes that are
partially reacted.  On our Web site we have a case study titled "Leakage,
Leakage, Leakage" dealing with the issues that you describe.  I have seen
that the cleanliness of the bare board, components (0402 components) and how
complexed are the assembly fluxes for a low solids (no clean) assembly
process are very important, but more importantly we have also seen the action
of rework and hand soldering using extra rework flux and not activating
(heating) the area not associated with the joint.  These unreacted fluxes can
create moisture absorbing sites and set up a leakage path.  This leakage path
on critical circuitry is as detrimental as a corrosion cell with an
electromigration short.

We use a tool call Ion Chromatography to identify and quantify the ionic and
organic residues extractable from the surface of the assembly,  this method
has shown correlation to electrical field performance and SIR testing.

<<
 My questions are:
 1.  Does anyone have experience that indicates that the 10ug/sq.in. level
 for ionic contamination is OK for PWAs that use 0402 sized components?  Or
 does experience indicate that a lower level is more appropriate and if so,
 what level?  (It don't know the exact pitch of this board, but am waiting
 for the answer.)

CSL Response: We have found that the ROSE data even has problems electrically
with levels at 2.0 ug/in2 of NaCL equiv.

 2.  Is it possible to determine an impedance above which one should be
 concerned with a level of ionic contamination for a given voltage?

CSL Response: We have our own level of cleanliness per Ion Chromatography.

 3.  If it turns out that for this design, RH and the level of ionic
 contamination are causing the problem, can the situation be improved by
 conformally coating the board or areas of the board?  (If so, can someone
 please point me to the applicable standards for conformal coating?  Thanks.)


CSL Response: We have found that coating is a delay mechanism that if there
is a conductive or corrosive residue below the coating moisture will pass
through over time and exposure to higher RH conditions.  We have seen
coatings work well to minimize the effects of condensing moisture and general
humidity conditions.


I hope this helps

Terry Munson
CSL Inc.
[log in to unmask]
www.Residues.com

765-457-8095 P
765-457-9033 F

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