Hi Inge,
The images are nice and clearly prove we won't have to use the "B" word any more (at least not in that case).
But I got even more confused :-)
My uneducated guess would be that the first two images were taken in the backscattered electron topo mode. That would explain why there are plenty of blubs/hillocks (or whatever one'd prefer to call them) on the surface. That is how surface CAVITIES (not protrusions) can look like in that particular mode.
It's sort of an optical illusion.
Your other pictures are in the secondary electron mode and here the cavities do look like cavities.
If I'm right, then I'd have to understand what those black spots are, as they don't look like voids (voids woud look like protrusions and I'd expect voids to be more rounded).
The interface on the board side looks normal on all images (and from my understanding that is the interface you are having the problem with).
I don't see any Ni on the lead side and I don't like how the intermetallics looks like there (there is kind of a "demarkation" line inside the layer, which might bring up a couple of speculations on is origin).
The intermrtallics is most probably (Ni-Cu)/Sn, not Ni/Sn as you have a huge source of Cu (cut off lead). It even looks like Ni-Cu/Sn, not Ni/Sn.
And now we got down to the most "misterious" part :-) - those cavities in the surface. I might be wrong, but I think I either heard or read somewhere that Pb is etched away during polishing (reaction with water). That would explain why you see the cavities. To prove (or disprove) my point, I'd take pictures from the same areas in backscattedred COMPOSITIONAL (not topo) mode.
And after all, I'd love to see how a failed joint look like in cross-section (with the lifted lead) :-)
BTW, did you see those Sn spheres? :-)
Regards,
Vladimir
-----Original Message-----
From: Hfjord
To: Vladimir Igoshev; 'TechNet E-Mail Forum'
Sent: Tue May 08 14:44:42 2007
Subject: SV: [TN] SV: [TN] Metalurgists, need assistance
Vlad & TN gnus,
Hey, curious? I'm still concerned, but at a higher level, as said
Newton. To begin with, I Xsectioned samples from failed part AND known
good references. Both finished with 0.1 micron diamond milk. Etched for
10 second, 5 seconds, 1 second and no etching at all. Same result: voids
in the failed parts, absolutely flat with references. I'll send photos
to Mr Wallman, alias Steve.
Pic new xsections_1 : typical solder joint with void concentrations
under the lead's foot. Not etched.
Pic new xsections_2 : details from under the lead's foot. Nickel plating
seen clearly, and a very thin nickel IMC layer. All bright areas are
predominated by Tin, very little Lead. Black spots are voids that
contains a little of everything debris from polishing. Not etched.
Pic SEM_3 shows following: from bottom copper pad / solder joint with
voids / lead with nickel finish. Not etched.
Pic SEM_1 shows upper part of solder joint, with lead copper uppermost.
Not etched.
Pic SEM_ 2 shows typical solder joint at higher magnification with
predominating Tin and also little Lead and Copper. Not etched.
As I said earlier, the references show no voids at all like the above.
Preliminary, I have excluded Black pads and Skip plating. Furthermore, I
don't think there are brittle copper IMCs either, even if there are
relatively high Cu peaks everywhere. Think Cu peaks come from
preparation. Nor is it likely with Ni IMCs, they belong to the interface
parts. So, what is this? Someone mentioned a insufficient soldering
process, and that is finally what I begin to think too. Against this
speaks the very experienced and big company that makes these boards (one
of the big elephants, not Ericsson, not American). On the other hand,
even the best can fail.
The failure appears randomly around the world, much annoying for the
customers, and the technicians scratch their heads.
Finally, I remind you all about the fact, that the fractures start close
to the solder pads and spread to the middle of the solder joints. And
only on the large FPGAs. No other components are hit.
Inge
-----Ursprungligt meddelande-----
Från: Vladimir Igoshev [mailto:[log in to unmask]]
Skickat: den 8 maj 2007 15:47
Till: TechNet E-Mail Forum; Hfjord
Ämne: RE: [TN] SV: [TN] Metalurgists, need assistance
So inge,
What the cross-section looked like? :-)
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Monday, May 07, 2007 4:57 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] Metalurgists, need assistance
Diet Coke! Man! Does not remove black pads on your teeth..
New samples are ordered, will try both oxides on paper and diamond on
nylon.
If the customer pays, I may try even FIB or TOFSIMS. Both are superior,
and even if the time tax is high, I must say that polishing, and
repolishing and endlessly repeated polishing can be quite expensive as
well.
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För David D. Hillman
Skickat: den 7 maj 2007 14:23
Till: [log in to unmask]
Ämne: Re: [TN] Metalurgists, need assistance
Hi George! Good comments! It does seem that the industry, at times,
spends
more time in describing a phenomena rather than resolving the root cause
so that we can avoid a reoccurrence. As for Inge's Monday dilemma, I
would
wait and see what the cross-section results reveal. If the observations
are that the nickel plating grain boundaries look bad, then I would work
with my pwb fabricator on what went wrong in the plating bath. It the
observations are that the nickel plating grain boundaries look good, I
would chase either a poor solderability or a poor soldering process root
cause. As you noted, the plating chemistry suppliers have worked
extremely
hard on making the ENIG plating process more robust and "real" black pad
problems are less prevalent today. I have always viewed the solution to
black pad as one of two choices: don't use ENIG or work very closely
with
the pwb fabricator. Here is an article reference which I think really
does the best job to date, of describing the root cause of black pad
(one
of the contributors is our good fried Dick Coyle!).
Zeng, Steirman, Abbott, Murtuza, "The Root Cause of Black Pad Failure of
Solder Joints with Electroless Ni/Immersion Gold Plating", JOM, June
2006,
pp. 75-79
The reality for us may be that no amount of plating bath control will
completely eliminate the potential of having a black pad situation and
that a risk assessment on a product basis is the final solution. Ouch,
way
too much thinking on a Monday before I have had a Diet Coke!
Dave
"Wenger, George M." <[log in to unmask]>
05/04/2007 07:56 PM
To
"TechNet E-Mail Forum" <[log in to unmask]>, <[log in to unmask]>
cc
Subject
RE: [TN] Metalurgists, need assistance
David,
I hate the "B" word because it describes an optical appearance rather
than a failure mechanism. I've heard the "B" word as well as terms like
"weak Nickel" and "Black Line Nickel" used many times to describe ENIG
failures. The reason I don't like these terms is because they are
adjectives to describe what one thinks they see and don't describe the
failure mechanism that causes the brittle interface failures. We use
ENIG on an old telecommunications legacy product because it wouldn't be
profitable to do any engineering evaluations to qualify another surface
finish. However, since the root cause of the ENIG solder joint failure
mechanism has never been understood or resolved to the point where one
can turn it on or off, we avoid the use of ENIG surface finish on all
new products. Even though the suppliers of ENIG chemistry have done a
great job of monitoring their chemistry and educating board shops on the
proper controls to reduce the occurrences of brittle solder joint
failures they haven't been able to eliminate these type of failure.
Ingemar is one of the few people I know of on TN that not only has a
great understanding of solder joints but appreciates the process control
details one needs to follow in PCB fabrication as well as PCBA to avoid
un-reliable solder joints. His current problem (i.e., not being able to
understand the failure mechanism or predict when it is going to happen)
is the major reason we avoid the use of ENIG surface finish.
Let's just say Ingemar does a cross section and confirms that there is
"mud flat" nickel grain boundary attack, what does he do on Monday?
Your advice about having a beer because it is Friday and not worrying
about this until Monday only will help this weekend but come Monday what
does he do. I assume that he's already using a "good" board shop and a
"Good" ENIG chemistry. I know what we did and it worked for us but as
an engineer I'd really like to what the ENIG failure mechanism is and
how to avoid it.
Regards,
George
George M. Wenger
Andrew Corporation Wireless Network Solutions
Senior Principal FMA / Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office] (732) 309-8964 [Cell]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, May 04, 2007 4:45 PM
To: [log in to unmask]
Subject: Re: [TN] Metalurgists, need assistance
Hi folks and thank you Steve for being "TechNet Picture Central"! Sorry
but just to warn everyone but I am going to use the "B" word shortly.
Inge
- take a close look at picture 4. As Werner pointed out the gold is
non-wetting the nickel in number of locations. What really jumped out
at
me is that "mud flat" appearance on some of the nickel surface regions -
a
"mud flat" appearance is a classic indication of black pad! I recommend
you complete a cross-section to confirm that you have nickel grain
boundary attack.
But since it is Friday, go have a beer and worry about this on Monday!
Dave Hillman
Rockwell Collins
[log in to unmask]
Steve Gregory <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
05/04/2007 01:45 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Steve Gregory <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Metalurgists, need assistance
Hi Inge!
Got your pictures posted. Took me a bit, I was swapping our wave
solderpot from 63/37 to HMP. Done now. I'm getting quicker, I can do it
in 20-minutes now. Took me about 45-minutes when I first did it...
Anyways, here's the links:
http://stevezeva.homestead.com/files/Pic1.jpg
http://stevezeva.homestead.com/files/Pic2.jpg
http://stevezeva.homestead.com/files/Pic3.jpg
http://stevezeva.homestead.com/files/Pic4.jpg
http://stevezeva.homestead.com/files/Pic5.jpg
http://stevezeva.homestead.com/files/Pic6.jpg
-Steve-
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Friday, May 04, 2007 12:22 PM
To: [log in to unmask]
Subject: [TN] Metalurgists, need assistance
Have an issue with (randomly) lifted FPGA leads.
Pic 1: good joint after pull test
Pic 2: ditto but high magnification
Pic 3: "rotten" joint after pull test
Pic 4: ditto but high magnification
Pic 5: "rotten" cross section
Pic 6: ditto but high magnification
60Sn on ENIG, obviously a brittle region. Suspect Kirkendall voids, but
the pockets with zillions of Tin spheres (0.1 to 2 um in size) points at
insufficient peak temperature to make 100% melt and good solidifying. If
these little balls were Lead, I could understand, but they are Tin!
Never seen like. Any "quickhead" out there, need fast advice.
Note: Pic 2, you can see the underlying board pad nickel as dark areas.
An experienced analyst will hopefully recognize. No Phosphorous peaks!
My Friday headache.
Inge
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