Morning All,

After re-reading Ioan's post, I tend to agree with his line of thinking, if
it is indeed solder. If it is a plating defect then you would have to be
worried about the plating on the rest of the board.

Looking at the photo, Ioan is right about maybe having issues with taping
that strip. I can see that device is a Linear Technology µModule which can
be either a LGA or BGA, and there are the 0402's right along that strip.
Taping that with kapton tape would raise the stencil by whatever the tape
thickness is (kapton can be 2-4 mils thick). I don't know if I would want
that tape there when trying to print an LGA and 0402's right next to the
tape.

The last time that I had to tape off gold plated areas was when I was
working with Guy Ramsey at R & D Circuits. We built very expensive 17" X
22", 40-layer DUT boards used in the semiconductor test industry. The DUT
(Device Under Test) locations on the board were plated with hard gold, and
could not under any circumstances get contaminated with solder, otherwise
the board would become expensive scrap. I taped the DUT locations off as
soon as we got the boards from New Jersey in an area in the stockroom away
from the manufacturing area, the boards were then baked. The tape stayed on
until just before the last wash after all touch-up and hand soldering was
done. As long as that was done we never had any contamination at all.

Steve



On Tue, Jul 29, 2014 at 1:27 PM, Ioan TEMPEA <[log in to unmask]
> wrote:

> Hi John,
>
> These are definitely solder splashes coming most likely from printing. If
> the assembler is covering the gold areas, you'll need to find out if they
> apply the Kapton prior to printing. Judging by the 0603 (or 0402?) caps and
> the miniBGA in the neighborhood, something tells me they would not want the
> Kapton there during printing, as it pushes the stencil up, increasing the
> paste volume with all the goodies that this can generate (shorts,
> tombstones, etc.).
> To make it more complicated, those tiny solder balls might even come from
> the table where taping is done. If the PCB is laid face down for some
> reason, it would gather no matter what. I know this situation, I've faced
> it many, many times and it's very hard to completely get rid of those
> splashes.
>
> One question: why would you want spotless gold on that large of a trace?
> If it's for grounding, as long as the splashes don't bulge up and hinder
> contact, there's plenty of Au surface to make a very good ground. Otherwise
> said, if you don't know for sure that small splashes like this affect form,
> fit or function, it would be a lot easier to live with them than trying to
> eliminate them. Of course, I'm talking about a reasonable amount of
> splashes, comparable to what shows in your photo.
> I guess at L3 you are doing intensive qualification of hardware and it
> would be easier to prove these splashes are benign than create massive
> rework.
>
> Regards,
>
> Ioan Tempea, P. Eng.
> Manufacturing Engineer
> Satellite Systems
> MDA
>
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> -----Message d'origine-----
> De : TechNet [mailto:[log in to unmask]] De la part de John Foster
> Envoyé : Tuesday, July 29, 2014 12:13 PM
> À : [log in to unmask]
> Objet : [TN] Trying to figure out possible mechanism for contamination
>
> We have been seeing a problem recently on several different CCA'S that we
> are having assembled.
> The boards are ENIG with plug and plate.  The assembly shop is assuring us
> that these gold areas are being Protected during the entire assembly
> process with kapton tape.
> I was thinking it was flux splatter since the areas are always almost
> perfectly circular.
> The locations are very random and range from nothing to what is in the
> picture.
> Is there any other mechanism that might be causing this? On our next run I
> am going to go to the shop To witness the taping. But I was curious if
> maybe it's some other cause.
> Mr. Gregory was kind enough to post a picture.
> http://stevezeva.homestead.com/spots.jpg
>
> Any thoughts would be appreciated
> Thank You
> John Foster
> L3 Communicatons
>
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