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May 2004

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 6 May 2004 10:34:52 +0300
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Interesting, Tom.

However, I cannot see what reaction will convert trichloroethane,
trichloroethylene, methylene chloride and carbon tetrachloride to
carboxylic acids. To hydrochloric acid, yes, and this is far worse than
carboxylic acids! I find it hard to imagine how a -COOH group could form.

Brian

Tom Gervascio wrote:

> Are the residues soluble in a polar solvent (ie isopropyl alcohol). THis might be a crude to test to see if it is a cupric oxide or a cupric chloride. Alpha metals has an interesting paper on their website dealing with ISR testing and green residues.
>
>  I 've found out the hard way that the green residues can not only react with organic or halides in flux but with long exposure times or concentrations  to saponifiers. It's a problem that I'm working now with some silver plated copper wires that sometimes turn green when exposed to a 18% saponified wash.
>
>
>  An internet search turned up this information related to copper carboxylates
>
>
> "Residual organic compounds that remain on copper ... during production and fabrication are able to advance to ant-nest corrosion only with the simultaneous presence of moisture, air and the decomposition of the organics to acids. It is concluded that the mechanism of the ant-nest corrosion is a modified pitting process involving a very small pit (termed a "micro-anode") where the copper base-metal oxidizes and dissolves according to:
>
> (1) Cuş -> Cu+ + e-
> In the presence of carboxylic acid (e.g., formic acid) the copper ions react to form an unstable copper (I) complex:
>
> (2) Cu+ + HCOO- -> Cu(CHOO)
>
> This species is further oxidized to form a copper (II) carboxylate (e.g., copper (II) formate) and copper (I) oxide (cuprite):
>
> (3) 4 Cu(CHOO) + ½ O2-> 2 Cu(CHOO)2 + Cu2O
>
> Copper (II) formate has a monoclinic crystalline form and is blue in color.
>
> Micro-cracks develop and radiate outward within the pit due to the wedging effect of the deposited copper (I) and copper (II) complexes. The micro-cracks expose more surfaces of copper and the process proceeds within the micro-crack to give the copper (I) complex according to:
>
> (4) Cu(CHOO)2 + Cuş -> 2 Cu(CHOO)
>
> Thereon, reactions 3 and 4 repeat over and over until tunnels are formed leading to ultimate through-wall penetration.
>
> If, however, there is any disruption in the presence of the basic three components (i.e., moisture, air and organic acid), then the micro-cell mechanism shuts down and ant-nest corrosion ceases to propagate.
>      It may also be noted that materials that react with water to form carboxylic acids include trichloroethane, trichloroethylene, methylene chloride and carbon tetrachloride, which are commonly used as cleaning solvents. Further, borofluoride-soldering fluxes and/or hydrogen fluoride fumes or other fluorides with refrigerant gases can all be air-oxidized to carboxylic acids
>
> Source: http://www.corrosionlab.com/Failure-Analysis-Studies/formicary-corrosion.htm. "
>
>
>
> Tom Gervascio
> Senior Process Engineer
> Sparton Electronics
> (352) 540-4040
>
>
>>>>[log in to unmask] 05/05/04 10:46AM >>>
>
> Hi Charles!
>
> Finally got your pictures up. Go to: http://www.stevezeva.homestead.com and
> look at "Green Residue 1 and 2". Looking at the pictures, it sure looks like
> something is reacting with the copper. Maybe some flux got accidently splashed
> or spilled on the board after it was cleaned. How many assemblies exhibit this
> problem?
>
> -Steve Gregory-
>
>
>
>>When we do the hand solder it takes about 1.5 hr. so we clean with IPA then
>>wash with water. The flux core is alcohol based and cleans fairly well with
>>IPA but we do hand clean after with saponifier and water. I did a little
>>experiment with new copper. It looks like it might be the WS flux, but the
>>parts that have the green are not hand soldered and the parts that are hand
>>soldered do not have it.
>>
>
>
>
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