Hi Gregg - I use the term Brittle Nickel failure a bit sloppy but George
and I are focused on what is happening at the solder/IMC/pad interface. As
George pointed out, soldering to copper (i.e. immersion silver or
immersion tin or HASL) creates a stronger solder joint than soldering to
nickel (i.e. ENIG) because of the IMC structure that is formed. Werner had
a couple of good published articles detailing/contrasting some of the
inherent differences of the two combinations. I always view Brittle Nickel
failure as a soldering issue and not a plating/fabrication issue since the
formation of that interface and the condition of the IMC is the critical
aspect of how the solder joint was formed. Brittle Nickel failure would
have the solder joint cracking at the IMC/pad interface - looking at
Andy's pictures, I assumed that was where the crack was but George has a
point in that a higher magnification photo is really needed for a more
accurate assessment. You can failure a perfectly good ENIG solder joint
with excessive stress due to the brittle nature of that interface. The
good thing for the users of ENIG is that the majority of the time the ENIG
solder joint is capable of withstanding the product use environment
stresses. Hope that helps.
Dave
From: Gregg Owens <[log in to unmask]>
To: <[log in to unmask]>
Date: 11/27/2013 12:04 PM
Subject: Re: [TN] Separation between the BGA solder ball and pad on
the PCB
Sent by: TechNet <[log in to unmask]>
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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