I have also experienced opto coupler failures due to chlorine/ chloride residue
inside optocouplers.
The chloride residue was traced to the solder paste and amount of chlorine in
the paste flux. Two opto coupler manufacturers data sheets do not recommend
using flux with greater than 0.2Wt% of chlorine. Since plastic IC packages are
not considered to be hermetic, and the flux in the solder exceeded the
optocoupler manufactures recommendation, we concluded the source of the chloride
was from the solder paste.
Mike Bogden
________________________________
From: "Forrester, Michael (H USA)" <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, February 1, 2011 7:56:34 PM
Subject: Re: [TN] PCBA Water Wash
Doug,
We sent defective parts back to the IC manufacturer. During
their failure analysis of the parts (involved opening the
part) they showed clear pictures of gold dendrites between two adjecent
pads as well as thinning of the gold bond wires
near the area described as migration of the gold to "fuel" the dentrite
growth. A surface analysis in the area of the wire bond
to external pin showed Chlorine residue. The dendrite growth was
confirmed on other samples sent to a 3rd party lab. The
growth causes intermittent and failed parts. They are CMOS optocouplers
that fail within 10 - 20 days under power.
I will try and contact Mike Bixenman. Thank you for the contact.
Best Regards,
Michael Forrester
Sr. Product Engineer
________________________________
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, February 01, 2011 12:41 PM
To: TechNet E-Mail Forum; Forrester, Michael (H USA)
Subject: Re: [TN] PCBA Water Wash
Michael,
I will give it a shot.
1) Documented, as in a standard "thou shalt use DI water"? Not really,
no. It would be considered as a Best Manufacturing Practice though.
Most standards and specifications are based on the theory of we don't
care how you clean it, it just has to meet "X".
2) Chloride on electronics is as common as nitrogen in air. The issue
is how much is present and how free is the chloride to react and do its
dirty work. What method did you use to determine chlorine
contamination? I know it takes quite a bit to make gold electromigrate.
In your case, ROSE is worthless, worthless I tell you. You need to have
your bare boards analyzed by ion chromatography in their incoming state,
and your assemblies tested by ion chromatography after your city water
cleaning process. That will tell you where your chlorides are coming
from and who to go after when you arm the villagers with torches and
pitchforks. You can do the same thing on components in the incoming
state to see how much they contribute to the picture.
3) Is 80-100 psi too strong? Depends. Do you want components on the
board when you are done? I would suggest a lower pressure, but a higher
volume. Mike Bixenman of Kyzen Corporation gave an excellent paper on
this at the IPC Cleaning and Coating Conference in December.
Doug Pauls
"Forrester, Michael (H USA)" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/01/2011 11:08 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Forrester, Michael (H USA)" <[log in to unmask]>
To
[log in to unmask]
cc
Subject
[TN] PCBA Water Wash
I have three questions I hope someone can help me with:
1) We have a vendor that uses "city water" directly into their SMT
PCA washer. I believe DI water is industry standard.
Does anyone know if it is documented anywhere? Spec?
2) We had an issue where there was Chlorine contamination and gold
dendrites found inside some standard gull-wing 6 pin SMT ICs.
This issue seems to be batch related. The chip manufacturer
claims that chlorine is never used in their process. Should the SMT
process expect to handle the possibility that the wash solution
may get into a part? The PCA vendor uses unfiltered "city water",
with no cleaner, in the wash process and does a ROSE test on one
board of each lot. Since the ROSE test passes, the PCA vendor
is blaming the part manufacturer for having "leaky" parts, since
the thinking is the wash is getting into the part and trapped.
3) One issue I have is the water pressure in the wash is 80-100 psi.
I think this is too strong, and should be half of that? You want to
flood the board with water, not hit the board and bounce off?
Thank you in advance.
Best Regards,
Michael Forrester
Sr. Product Engineer
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