Doug
I agree with you up to a point. That point is that ion chromatography is
no use in identifying hygroscopic organics, such as residues from
various diols and their derivatives used, for example, in HASL or, dare
I say it, some no-clean solder pastes. My experience has shown that this
kind of molecule is just as likely to show a drop of SIR, but relatively
little loss of ECMR, than an ionic contaminant.
This is why I suggested that the first step should be to identify all
the contaminants. If they are ionic AND your ion chromatograph is
equipped to identify all the cations and anions present (rarely the
case), then this is an excellent tool. Another useful tool which may be
better for non-ionic contaminants is the so-called acetonitrile test
(!!! CAUTION !!! Highly toxic substance) with IR spectrometry. A similar
test done with ultra-pure water may be necessary to complete the
picture, although there are few organics which are insoluble in
acetonitrile or methyl cyanide, to give it its synonym, which are
soluble in water.
Best regards,
Brian
[log in to unmask] wrote:
>
> Currently we have a PCA (Printed Circuit Assembly) which is sensitive to
> high humidity. Due to the sensitivity of the circuit we believe the failure
> mode is possibly related to the presence of contamination on the PCA.
> To measure the level of contamination on the PCA and to control the level
> of contamination to industrial standards, can anyone recommend the standard
> that should be referenced, the level of contamination that is common in
> industry and the method of testing.
> The PCA is a double sided SMT assembly using no clean solder paste and no
> clean solder wire for the few through hole components. For this type of
> assembly should I use the Omega Test or the SIR Test or any other test and
> what is the acceptable industrial levels.
>
> *Suresh,
> You are looking for a magic bullet test - one that will give you a simple
> test method, with a universal pass-fail level that will tell you if your
> hardware is good or bad. Unfortunately, my friend, the test you are
> looking for does not exist. Here is why.
>
> The Omegameter, and other instruments like it, have a relatively benign
> extraction method, and the pass-fail level in all the standards is over 25
> years old. The test will not tell you if you have good hardware or bad
> hardware. Its only valid use is as a process control tool for day to day
> production, not product acceptance. Read EMPF report RR0013 "An In-Depth
> Look at Ionic Cleanliness Testing". That report will show how bogus this
> test can be. I have seen far too many people rely on Omegameter (or
> similar) results and still get corrosion and metal migration. You also
> indicate that you are using low solids materials (erroneously called
> no-cleans). The residues from these materials will cause high Omegameter
> results, but are not a problem on your board.
>
> The SIR or electromigration tests might show you if you have a problem with
> electrical leakage at high humidity levels, but you already know that.
> Also, SIR and electromigration tests are based on test patterns with a set
> geometry, such as comb patterns. You can run such tests, but you no longer
> have pass-fail numbers that you can use, and you have to use engineering
> judgement. In addition, those tests usually do not have the same mix of
> metals and solder mask that you have.
>
> The test that I would do in your situation is ion chromatography. It is a
> good test to find out what ions are present and in what amount. In the
> hands of a skilled analyst, and with knowledge of your materials and
> process, the data can indicate if you have a problem, the source of the
> problem, and how to fix it. Unfortunately, there are no nationally
> "approved" standards for ion chromatography data. Again, you are left with
> engineering judgement. You can find some guidelines in J-HDBK-001, section
> 8.
> My old company, CSL, can probably help you in this analysis. They have 10
> years of ion chromatography experience in troubleshooting, and they came up
> with the guidelines in J-HDBK-001. You can reach them at 765-457-8095 or
> [log in to unmask]
>
> If you need more direction, let me know and I will try to help further.
>
> Doug Pauls
> Rockwell Collins
> Chairman, IPC Diet Mountain Dew Committee 9-45r
>
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