Hi George! Yea, I am being a little sloppy with terms as I think about
things metallurgically but forget that not everyone spends as much time as
I peering thru a microscope at cross sections. As you detailed, ENIG
surface finishes can have a couple of different failure mechanisms: (1)
Black Pad which has its root cause in the plating process impact on the
nickel/gold interface with a couple of other factors coming into play (the
type of nickel plating and the gold bath formulation); (2) Brittle Nickel
failure which has its root cause in the metallurgical configuration of the
copper/nickel/solder joint interfaces with the soldering process coming
into play as a factor. Neither of these failure phenomenon is a one trick
pony - typically two or three conditions/processes collaborate resulting
in the solder joint failure. You do point out that the "black pad of the
1990s" is not the same black pad failures we see today and I think that is
primarily due to the industry learning, probably way too slow but
learning, on how to improve our plating and/or soldering processes to
minimize the failure phenomenon. I know that is somewhat frustrating but
sometimes the industry completes root cause analysis on a serial path. I
agree that having a higher magnification photograph focused on the crack
interface such that we can assess the IMC, solder and pad interfaces would
allow for a more refined assessment. I am certain that the TechNet
discussion will assist Andy in having a more in-depth understanding of his
failures even if we have to redefine our terms as we go. I don't think
Andy has a Black Pad type of defect but something with a root cause in the
solder/IMC/copper pad region of his solder joints. Good stuff.
Dave
From: "Wenger, George M. [Contractor]" <[log in to unmask]>
To: <[log in to unmask]>
Date: 11/27/2013 12:42 PM
Subject: Re: [TN] Separation between the BGA solder ball and pad on
the PCB
Sent by: TechNet <[log in to unmask]>
We need to be careful about how we use terms. Dave made it sound like the
failure mode is brittle nickel. I think it would be more appropriate to
indicate that there is a brittle fracture and it occurs at the IMC/nickel
interface.
I'm sure by now that most people on TN know I'm a fan of immersion silver
surface finish and the two PCB surface finishes I like the least are
immersion tin and ENIG. The problem I have with ENIG is that ever since
the early work of Zequn Mei there has been an never ending new theory
about ENIG failures. First it was due to the P content I the electroless
nickel, then it was poor control of the PCB surface finish plating
process, and more recently it is because the immersion gold plating
chemistry is aggressive and that it attacks the electroless nickel. The
other problem I have with ENIG is that I really don't think the failures
are well understood. If they really were understood then one should be
able to run a PCB fabrication process and produce the failure or run a
fabrication process and eliminate the failure but what happens is that
fabricators indicate the ENIG problem isn't a problem as long as they
control their plating process and when it does happen they indicate
something in the plating process was out of control. The other problem I
have with ENIG is that the terms (i.e., "Black Pad", "Black Line Nickel",
"Weak Nickel", etc) being used for failures are observations rather than
recognized failure modes.
I'm certainly not a metallurgist but it is my understanding that solder
joints made to nickel are not as strong as solder joints made to copper.
When you solder an ENIG board you are making a solder joint to the nickel
plating so the solder joint isn't as strong as it would be if made to
copper. If you look at the photos that Steve posted you see the fracture
starts on the side of the pad which is solder mask defined and not on the
other side where it isn't solder mask defined. When the solder wets to
the BGA pad and flows around the side it increases the interconnection
strength. So when there is a mechanical stress on the BGA solder joint
the fracture will start at the weakest area which is where the solder
didn't flow around the side of the BGA pad. One also needs to remember
that the strength of a solder joint linearly decreases with increasing
temperature. When the solder is a liquid there is a wetting tension
holding the liquid solder to the pad and if a mechanical stress is exerted
on this pad due to warpage the joint doesn't break because the liquid
ligament compensates for the stress. However, once the solder solidifies
there is mechanical stress due to warpage applied to the solder joint. The
solder joint may be below solidification temperature but it is still at an
elevated temperature and it is very weak and what happens is you get a
brittle interfacical fracture at a weak interface, which is exactly what
Andy's problem is; brittle solder joint interface fractures at the outside
corners of the BGA. Control of the PCB and BGA warpage will help prevent
the fratures.
Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gregg Owens
Sent: Wednesday, November 27, 2013 1:04 PM
To: [log in to unmask]
Subject: Re: [TN] Separation between the BGA solder ball and pad on the
PCB
For the slow of mind, what causes brittle nickel - PCB fabrication or
soldering process or both?
Gregg
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, November 27, 2013 9:56 AM
To: [log in to unmask]
Subject: Re: [TN] Separation between the BGA sodler ball and pad on the
PCB
Hi Steve - thanks for posting up the pictures. Looks like a brittle nickel
fracture failure mode to me - the failure is at the IMC/copper pad
interface and the dye & pry shows progressive crack growth.
Dave Hillman
Rockwell Collins
[log in to unmask]
From: Steve Gregory <[log in to unmask]>
To: <[log in to unmask]>
Date: 11/27/2013 11:49 AM
Subject: Re: [TN] Separation between the BGA sodler ball and pad on
the PCB
Sent by: TechNet <[log in to unmask]>
Hi Andy,
Got all of your pictures and attachments. All the cross-section pictures
are here:
http://stevezeva.homestead.com/A1.jpg
http://stevezeva.homestead.com/A16_1.jpg
http://stevezeva.homestead.com/A16_2.jpg
http://stevezeva.homestead.com/T16_1.jpg
http://stevezeva.homestead.com/T16_2.jpg
Your dye & pry is here:
http://stevezeva.homestead.com/256BGA-mod_for_TechNet.pdf
Lastly, here is the reflow profile:
http://stevezeva.homestead.com/Andy_s_Reflow_Profile.jpg
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of andi1978
Sent: Wednesday, November 27, 2013 6:56 AM
To: [log in to unmask]
Subject: Re: [TN] Separation between the BGA sodler ball and pad on the
PCB
George,
First apologize for not timely replying to this topic, I am living within
Europe time zone.
Here more facts about this condition and as well to clarify some questions
already posted.
I did re-sent pictures again and in addition will send 2 pages of our dye
& pry report document we did in March this year (from the same
subcontractor).
As George W. already indicated we can eliminate lab issue and poor sample
preparation doing the dye & pry test. Fortunately I am in possession
of such a report from March 2013 before our BGA cross-section from
September 2013. The short message is that there are evidences of dye
ingression at the PCB pad level and only existing in the corners of the
BGA package. To clarify George's question on completed inter-metallic
fusion. What I can see from pictures I sent to Steve, the solder paste
perfectly melted with the BGA solder balls and the sample board was fully
cooled. The cross-section was done after the initial reflow (single
thermal processing).
The whole board is double sided assembly but because of the need for
cross-section, only the BGA component was populated (no other parts were
assembled).
There was a request to send the reflow profile to Steve, have done that
now. That's all I know and far I can go myself.
I kind of excluded PCB warpage due to small size of the sample board
(65mm/70mm/1.5mm) and that this PCB was held in the thermal panel secured
with 4 clamps during reflow (single pass).
Regards,
Andy
27 November 2013 3:02 "Wenger, George M. [Contractor]"
<[log in to unmask]> wrote:
Andy,
Once Steve has your photomicrographs of the cross section posted it will
be much easier to provide meaningful advice. Without a photograph and
additional information one can only guess. You indicated separation
occurred after the reflow process was completed because there is evidence
of completed inter-metallic fusion. I assume when you say "after the
reflow process you most likely mean after the solder melted and then
cooled below liquidus. Is this a single sided reflow assembly or a double
sided reflow assembly? Where the separations discovered after the first
reflow or the second reflow?
Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Andrzej Zielinski
Sent: Tuesday, November 26, 2013 10:33 AM
To: [log in to unmask]
Subject: [TN] Separation between the BGA sodler ball and pad on the PCB
Dear Colleagues ..
I am facing an issue with the BGA part where in three locations we found
separation between the BGA solder balls and pads on the PCB (ENIG finish).
This condition was discovered during the cross-section that we have
requested from our local lab.
I can share few pictures I have taken from our report (can't attach the
whole document as this report is confidential to our business).
Basically this BGA is the plastic ACTEL package 896 pins, SAC305 finish,
soldered with leaded process using 62Sn/36Pb/2Ag solder paste. Temperature
was slightly elevated to accommodate SAC305 solder finish from the BGA
package.
The separation condition we have observed was only found in location A1,
A16 & T16 which are the corners of the BGA package. As well all 3
separations occurred after the reflow process was completed because there
is an evidence of completed inter-metallic fusion.
In location A16 & T16 you can see the copper is protruding of the pad
but I have been told this is not an issue and cause of the separation.
I was considering package or PCB warpage as the cause of this separation
or something related to cooling the board after reflow.
I asked our lab to measure the height of the solder balls across row A
& T and they all deem to be ok.
I have asked to check for the phosphorus content as the pad level to see
if this is related to black-pad symptom and the phosphorus was measured as
10.3%.
So far the board assembly house is adamant their reflow profile is
correct.
PCB manufacturer is confident with their PCB and pointing to the assembly
house.
There is a thought about poor sample preparation where the separation
issue would be caused by the lab (during cross-section).
We are attempting to perform another cross-section on different board but
here I am not really sure if this is the right way to go (extra cost).
Because I have never seen such a defect, I am not sure what should be the
next step to take.
Any advice guys ?
Andy
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