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May 2004

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Subject:
From:
Charles Caswell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 5 May 2004 12:37:55 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (189 lines)
There was some soldermask touch up but not anywhere near the parts in
question. I am thinking that since the board was assembled in one area and
bagged to bring it in for cleaning that there might have been some WS flux in
the bag. That seems the most likely right now.

-----Original Message-----
From: Crepeau, Phil [mailto:[log in to unmask]]
Sent: Wednesday, May 05, 2004 12:25 PM
To: [log in to unmask]
Subject: Re: [TN] Green residue.


is there any chance there was solder mask touch up going on in this area?

-----Original Message-----
From: Charles Caswell [mailto:[log in to unmask]]
Sent: Wednesday, May 05, 2004 8:55 AM
To: [log in to unmask]
Subject: Re: [TN] Green residue.


I cannot get the board back to check if they are soluble. It will not be put
into service, we already have new drawings and boards but I do need to try to
find out what is going on here. We have not seen this stuff on anything else
we build.

-----Original Message-----
From: Tom Gervascio [mailto:[log in to unmask]]
Sent: Wednesday, May 05, 2004 10:18 AM
To: [log in to unmask]
Subject: Re: [TN] Green residue.


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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