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

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________