Hi Andy - if you can send the pictures to Steve Gregory, he can post them 
so we can all view them. What you describe sounds like Head on Pillow or 
possibly Brittle Nickel Fracture but having the pictures will assist in 
making an accurate assessment. One action you can take that will be very 
informative is to run shadow moire on the BGA to measure how much warp 
happens during the reflow process. Excessive package warp could be the 
root cause of your issue.

Dave Hillman
Rockwell Collins
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From:   Andrzej Zielinski <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/26/2013 09:34 AM
Subject:        [TN] Separation between the BGA sodler ball and pad on the 
PCB
Sent by:        TechNet <[log in to unmask]>



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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