Hi Phil! Oh yes, you are correct and win the Friday PM Diet Mountain Dew
prize - sorry Doug, Phil beat ya to it. The IMC between ENIG and SAC305
will be tin/nickel (depending on the soldering kinetics it will be Ni3Sn
or Ni3Sn4). And as you described, if you don't form an IMC , you don't
form a solder joint. If you form too much IMC, you degrade the solder
joint behavior as IMC is a brittle structure. The actual structure of the
IMC depends on the solder joint composition and temperature history. One
of the big "concerns" of the leadfree soldering process is the
types/shapes of the IMCs that are formed. In the tin/lead process, we just
had to worry about IMCs of copper/tin or nickel/tin - with the SAC system,
we have silver playing a role in what IMC phases get formed.
Dave Hillman
Rockwell Collins
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Phillip Bavaro <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/15/2008 12:22 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Phillip Bavaro <[log in to unmask]>
To
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Subject
Re: [TN] Metallurgists, need assistance, closing this one
Ok, I know this one is old, but I just happened to notice something I did
not understand.
In Inge's cross section, VD08073_improved_cross_section, my assumption is
that there is copper and nickel plating on the board, and then there is an
intermetallic layer sitting on top of the nickel, followed by the solder
ball.
What is that intermetallic if the PWB was ENIG and the ball/paste is
SAC305?
And is it supposed to be a complete layer covering the nickel or merely to
be thin "fingers" which grab the solder.
My gut feeling is that the longer the connection is held at soldering
temperature, the more complete that IMC will become until it forms a
uniform bond/thickness between the nickel and solder.
If the IMC grain structure gets too long, then it becomes brittle so there
is a definite window of opportunity with respect to the reflow times.
Am I close?
Thanks in advance,
Phil
Hernefjord Ingemar <[log in to unmask]> wrote: Thanks
George,
Such words from you are technical vitamins. Indeed, I had to finish before
the winning post. I'm nicknamed the "terrier" here, usually don't let go
anything once I clench the teeth, but my client thinks he can take over
here. I have already asked for MTBF or similar that could give a better
background, but he does not like that. Your proposal to swap is brilliant,
and is already in the report, posted to another country.
"Wie get's mit ENIG?"
"Wenig." ha-ha
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: den 11 maj 2007 00:04
To: [log in to unmask]
Subject: Re: [TN] Metallurgists, need assistance, closing this one
You've done an extremely thorough job of analyzing the Ingemar,
You've done an extremely thorough job of analyzing the issue associated
with randomly lifted FPGA pads and you've listed many logical conclusions.
However, your email seems to indicate that you are getting tired of
looking through the kaleidoscope and going around in circles. That
doesn't sound like the "old Ingemar" I interacted with several years back.
However, I do understand and I think Werner would agree that all
customers are not the same and some have a tendency to make their
technical talents feel like they are going around in circles. The one
thing I did not get out of your first email posting was a sense of how
random the randomly lifted FPGA pad is. In order to do an informed risk
assessment one needs the data to input into the risk assessment. You've
certainly provided the technical knowledge of possibilities but what is
missing is the frequency or percentage of lifted pads. I'd like to make
one other suggestion that you might want to consider in the
report to your customer. Since your customer is using ENIG PCBs the only
real method I know of to determine if the lifted pads are related to ENIG
is to run a comparison to another surface finish. I would recommend that
your customer have a percentage of his boards fabricated with immersion
silver and track the occurrences of lifted pads. If the frequency of
occurrences stays the say with ENIG boards and you don't see lifted pads
with immersion silver boards your customer with then have "real" data to
allow them to make a decision of how they want to proceed; work with their
ENIG board vendor to improve the PCB surface finish quality or switch to
an alternate surface finish.
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 Steve Gregory
Sent: Thursday, May 10, 2007 4:50 PM
To: [log in to unmask]
Subject: Re: [TN] Metallurgists, need assistance, closing this one
Hi Inge!
Well folks, another Swede adventure comes to an end, but he has left in
his wake yet one more image in a kaleidoscope of SEM's and cross-sections
that has all of our eyeballs spinning:
http://stevezeva.homestead.com/files/VDO8073_IMPROVED_CROSSECTION_2.jpg
I think if you look through the many images that Inge has sent for me to
post on my web page over the years, there a few gems that could be
blown-up and framed as art...
Steve Gregory
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Thursday, May 10, 2007 2:42 PM
To: [log in to unmask]
Subject: [TN] Metallurgists, need assistance, closing this one
Well, well, anything funny comes to an end. I have to write a report and I
will do my best to convince my client. If anything goes wrong, I will say
: this is what the TechNet gurus decided....he-he
Seriously, I think I have focused on wrong objects. Gone in circles. Not
unusual in our profession. I have considered and weighted what you gnus
had to say, and made this logical conclusion:
General joint strength: Good
Leads plating: OK
Solder pad plating: OK
Solder paste: OK
Solder phases near lead: OK
Solder phases in middle of joint: OK
Solder phases near solder pad: OK
IMC at lead interface: OK
Solder voids: caused by insufficient peak dwell Solder voids vs. failure:
not primary cause Solder void content: debris caused by micro shockwaves
in water, polishing grains acts as abrasive, soft Lead partly removed
mechanically.
Solder voiding countermeasure: diamond milk, low rotation speed.
Fracture at lead interface: NO
Fracture in the middle of joint: NO
Fracture at solder pad surface: NO
Fracture at lead/IMC interface: likely
Fracture frequency: random
Fracture pattern from field : random
Black pad: not likely
Skip plating: not likely
Dewetting: not likely
IMC at solder pad interface: much varying in thickness, partly missing!
Consequence of poor IMC: local weakness in joint strength Cause for poor
IMC on solder pad: Insufficient soldering Primary failure cause: varying
flow of coating under FPGA, thermomechanical impacts overloads regions
with missing IMC.
Countermeasure: No coating allowed under FPGA, improved solder profile.
The confidence of this conclusion is better than 50/50, best I could do.
Look at last photo, which I send to Steve. Follow the IMC from right to
left. Seen even better in SEM, but this is THE END.
Thanks all
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Hfjord
Skickat: den 9 maj 2007 20:04
Till: [log in to unmask]
Ämne: [TN] SV: [TN] SV: [TN] SV: [TN] Metallurgists, need assistance
Dave,
Goofy, agree. No, I have not confirmed the ENIG finish, because my client
did not send naked boards to me. I will ask for a fresh board so that we
can check the condition. That's a good thought, to investigate the parts
before assembly, the board, the solder paste and the FPGAs. My client
denies there could be anything wrong here, but it can, of course.
I'll call them tomorrow.
As you could read, one guy pointed out that the voids can be created at
the polishing process, because of Lead dissolved by the water. Sounds
goofy too. Suppose that could happen, why don't I get that when polishing
the good references then. I've seen SnPb cross sections for years, but
never seen Lead being superseded by voids!
OK, can try oil polishing instead...
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För David D. Hillman
Skickat: den 9 maj 2007 14:12
Till: [log in to unmask]
Ämne: Re: [TN] SV: [TN] SV: [TN] Metallurgists, need assistance
Hi folks! I agree with Vlad - I don't see any "B" word characteristics in
the nickel plating. Looking over the SEM photos, something just looks
goofy. As Vlad pointed out the voids don't seem to be from a flux
outgassing root cause - they contain rough, jagged features. Inge - have
you run a SEM EDX analysis to get a chemical ID on the structures inside
the voids? My guess is that the cause of the poor solder joint integrity
is linked to the formation of your strange void structures. I was thinking
maybe gold embrittlement but if you are using ENIG then you don't have
enough gold present. Have you confirmed that the ENIG finish was correct?
Dave
Vladimir Igoshev Sent by: TechNet
05/08/2007 08:01 PM
Please respond to
TechNet E-Mail Forum ; Please respond to Vladimir Igoshev
To
[log in to unmask]
cc
Subject
Re: [TN] SV: [TN] SV: [TN] Metalurgists, need assistance
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."
05/04/2007 07:56 PM
To
"TechNet E-Mail Forum" ,
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 Sent by: TechNet
05/04/2007 01:45 PM
Please respond to
TechNet E-Mail Forum ; Please respond to Steve Gregory
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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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