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Date: | Wed, 22 Dec 2004 16:25:06 -0600 |
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Hi Sri! If an immersion silver pwb finish has a purple hue/tint then it
typically has become tarnished to due exposure to a sulfur source resulting
in the formation of silver sulfide. If the silver sulfide thickness was
relatively thin and the immersion silver plating was reasonable thick then
the creation of acceptable solder joints would still be possible. Also
immersion silver pwbs will undergo some surface tarnishing when processed
in reflow soldering. I recommend that you investigate how the pwbs are
being shipped/stored (wrapped in silver saver paper in a sealed bag is one
acceptable method) to prevent similar future issues. Many, many packaging
materials are "good" sources of sulfur which result in tarnish. Sending a
photo to Steve G. for posting may be helpful for additional suggestions.
Good Luck.
Happy Holidays
Dave Hillman
Rockwell Collins
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smaganti
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12/22/2004 01:39 [TN] Purple pads / corrosive ? pads
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I received some PCBA assembled at one of our contract manufacturer This is
double sided / mixed tech assembly. There are some locations where one
could either use a BGA or a QFP. Currently we are using the BGA version.
The stencil does not have aperture opening to print QFP ( they use
different stencil ), however on the QFP pads, I see some purple / brownish
pads, almost exhibiting corrosive traits. The PCB are supposed to immersion
silver. I see some small solder balls here and there. Test via pads look
good. The SMT solder joint has transparent residue typical of No clean
paste
The contract manufacterer should be using eith AIM NC 253 or 293 or 291 or
Indium NC SMQ 92J.
I can share some photos. Any ideas as to what could have happened /
chemistry guess as to the flux / paste ? I am still in the preliminary data
collection mode
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