Pete,
You have probably received boards that were improperly plated. What is
the concentration of P that is being measured?
While some P is co-plated in the electroless nickel to improve its
resistance to corrosion during the subsequent IG bath, too much
phosphorus presents a barrier to proper IMF once the gold is absorbed
into the solder during reflow. If there is too much P included in the EN
surface layer after the IG bath, then during reflow the IG is quickly
absorbed into the solder, but the underlying P/EN barrier does not allow
a proper IMF with the nickel base underneath.
The statement that subsequent multiple heating cycles do not increase or
induce black pads initially caused by this high-P barrier is incorrect.
Subsequent thermal cycling DOES lead to a continuing P barrier growth,
which is independent of the Ni/Sn IMF.
The more you attempt to rework it, the worse it can get, eventually
weakening the solder joint to the point of failure. This can also be
caused by thermal cycling due to normal operation while in service.
So your manufacturing people are probably correct, that they received
pwbs with a solderability issue. If they subjected the pwbs to a hot
bakeout prior to assembly (150 C for 8 hours, for example), this would
exacerbate the problem. The reflow process itself can continue the
growth of the P barrier. By the time you have performed a double reflow,
and perhaps a wave solder process, the parts can literally fall off of
the assembly from the P barrier growth.
Realize that some level of P must be there to facilitate the IG process,
but too much will cause problems later during assembly. However, one
must be cautious in the statement that "Black Pad" is the issue, as
there can be various levels of phosphorus, and while the amount may not
necessarily cause the black pad syndrome, a certain amount of P can and
will affect solderability to some degree.
This is the inherent problem with ENIG finish. More heat is required for
a longer time to form a good IMF with Nickel, which has a much slower
dissolution rate than copper. If a high level of P is present, the
required heat for a longer TALT gets the P level going, which leads to
some solder defects, which leads to another thermal excursion to rework
the problem, which increases the P barrier, which ........ well, you get
the picture.
The root cause is the plating process, however. If properly performed,
the subsequent thermal cycling seen from baking operations, reflow
soldering, etc., should not cause a problem.
However, this is not the only problem that can cause poor solderability
associated with the ENIG process. If the nickel barrier is too thin or
there are skips (skip plating), copper can migrate through and react
with the gold. If the gold is too thin, or there are skips, the nickel
will oxidize. The nickel oxides will also form a barrier to proper IMF
between the molten solder and the nickel base.
There are some good pictures of black pad along with a good article on
the subject on Steve Gregory's website
http://www.stevezeva.homestead.com, and I believe there are some good
pictures showing the effects of high-P levels within IPC-7095.
There are many EDX images available on the net, also, from various
papers on black pad.
AlphaMetals has an excellent article on the subject written by the
Cookson Electronic Assembly Materials group called "The Black Pad
failure mechanism-from beginning to end". The best on the subject I have
seen to date.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Matthew Park
Sent: Monday, October 30, 2006 8:52 AM
To: [log in to unmask]
Subject: Re: [LF] need some SEM/EDX BGA images
Pete,
Chances are your manufacturing is right. Multiple heating cycles do not
increase or induce black pads. You might be getting bad boards from your
board shop. It is possible nickel layers were oxidized before
electroless immersion gold process by your board shop, or gold layer
could be too thin or non-existent.
I might have some images of black pad phenomenon. Let me know. I can dig
thru. If I find some, I can email you off-line.
Regards
Matthew P.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Friday, October 27, 2006 4:38 PM
To: [log in to unmask]
Subject: [LF] need some SEM/EDX BGA images
I'm trying to interpret a report we had done on some BGA failures, and
I need some comparative images.
I've got one image of a partial joint crack. I've got a number of
images of dark line defects. I'm having some difficulty determining if
the dark lines are actually defects. I want to find some EDX images
that I can compare to those in my report. I think the ones I have are
showing proper concentrations of Phosphorous and Nickel, including
Nickel in the solder balls. Manufacturing is insisting they are getting
bad boards, removing BGAs and finding black pads all over. Since there
seems to bea good amount of Nickel still present, I tend to believe
they are creating black pads with their thermal profile or multiple
heating cycles.
Does anyone know a website where I can find some of these images?
TIA.
Pete
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