Cheryl,
You may have one or two failure mechanisms going on simultaneously. It is
possible that you have contamination internal to the component itself. If
the seal was compromised during component plating processes, you could have
contaminants leak into the device body or contaminate the seals. In that
case, you can clean till the middle of next month and not get the
contamination out. Have you tried to do IC on some of the incoming parts
themselves? Do they show high levels of contaminants coming in the door?
Have you taken some components and put them through reflow conditions, but
without exposure to anything like fluxes? If you saw no elevated levels on
the incoming components, but elevated levels on the reflowed components,
you have bad parts. Go find the vendor, find a dark alley, take Guido and
Nunzio with you and give a natural vent to your frustrations. Did you see
any levels of methane sulfonic acid on the component, indicative of poor
rinsing in plating.
If your components show OK, then do the same check on your bare boards.
Are they dirty coming in? Does exposure to reflow conditions show a
dramatic increase in residue levels? If so, you have a poor solder mask
that has absorbed material and no amount of cleaning will get it all. Find
your board vendor, and let Guido and Nunzio play some more. Who knows,
they might cure Steve's snake skin mask problem.
If the components are OK and so are the boards, then it is your assembly
process. Whether you employ Guido or Nunzio becomes a company policy
decision.
In this lamentable case, you may have excessive or unreacted flux, or flux
that you did a poor job of cleaning. For most fluxes, you get high
insulation resistance values if you clean it completely, or in the case of
low solids fluxes, you leave them completely alone. Anywhere in between
and you have problems. What you describe is an electrochemical failure
mechanism. Can you tell me what the analyzed levels were for your halides
and sulfates? Did you also analyze the areas for weak organic acid
activators? What flux are you using.
If your cleaner does not have saponifier, and your present methods probably
would not work well on the contaminants I suspect, you can get some samples
of a good saponifier material, do a presoak in the material and then use
your in-line cleaner as a rinse. I recommend either Kyzen Aquanox (SSA or
XJN) or Envirosense Envirogold 816. Both work well in this mode. I'm sure
Charlie Pitarys of Kyzen, who lurks in the background waiting to pounce on
unsuspecting potential customers, can help you out.
In short, well, as short as I ever get, if you have a surface residue, it
can be cleaned with the right combination of chemistry and cleaning
process. If the residue is absorbed, either in the mask or the component,
you are toast.
Doug Pauls
Rockwell Collins
Well into my 4th Dew of the Day.
Cheryl Tulkoff
<cheryl.tulkoff@N To: [log in to unmask]
I.COM> cc:
Sent by: TechNet Subject: [TN] Contamination on a SOT-23 component
<[log in to unmask]>
02/10/2004 03:20
PM
Please respond to
"TechNet E-Mail
Forum."; Please
respond to Cheryl
Tulkoff
I've been working on a tough but challenging contamination problem. The
contamination causes electrical leakage fails on a precision test
instrument. Ion chromatography analysis points to an incoming cleanliness
problem on a SOT-23 component. Elevated choride and sulfate levels were
found on the components (5x - 10x greater ug/in2 levels than anything
found elsewhere on the boards). High levels were found even on our "good"
boards.
We are trying to work with our supplier to identify and fix the problem
long term but are looking for some short term fixes as well. Repeated
water washing of the boards (high pressure spray cleaning) improves the
situation but does not fix it. Alcohol scrubs in the area further improve
it but don't eliminate the problem entirely.
We have a only a DI water wash process (no surfactant/saponifier
capability).Does anyone have any board or component cleaning
recommendations?
Are there any parts makers out there willing to tell me a little more
about parts cleaning after the lead plating process?
What tool(s) do part makers use to verify cleanliness of parts?
I don't have a tool capable of measuring what was found on the part so I
don't know where to go.
Thanks, Cheryl Tulkoff
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