Charles Barker@I-O INC
05/22/98 01:06 PM

Pat,

Your question is VERY interesting. I would really like to know what answers
you get.

Hopefully, some of the gurus on here can provide additional insight.




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Subject:  [TN] Via Corrosion




Greetings from sunny Arizona:
     This is my first posting to the net so bear with me a bit please.
     I recently had the question posed to me "we have a concern about
the possibility of autocatalytic corrosion in vias due to residual RMA
flux activator due to tenting of one side of the vias; what corrosion
rate might we expect".
     I am familiar with the term "autocatalytic corrosion" as it applies
to crevice corrosion in stainless steels exposed to aerated chloride
solution.  This is where chloride ion diffuses into the crevice and is
regenerated as freshly generated metal chloride hydrolyzes to chloride
and insoluble metal hydroxide.  As chloride concentration increases so
does corrosion rate.  Apparently this can also occur in copper and other
metals.  I don't think  this is the case with via corrosion since the
chloride concentration is fixed.
     My approach was to assume the activator is diethylamine
hydrochloride, which I assume hydrolyzes into hydrochloric acid.  I also
assumed that the acceptance limit of 1.55 micrograms per square
centimeter NaCl equivalent (MIL-STD-2000A) was the coverage within the
via.  I next assumed that the via was filled with rinse water and that
99.5% of this water evaporated.  This left (fortuitously) 0.1N HCl in
the via, for which Uhlig and Revie happen to give an initial current
density for copper electrode of 2E-3 A/sq meter.  I assumed this rate of
reaction remains unimpeded (i.e., no polarization) and calculated time
to penetrate 4 mils of Cu.  I got about 20 years.  I must admit that I
assumed NO amine-caused stress corrosion cracking and no other fast
reactions.
     Does anyone have experience or alternate analytical approach?
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