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March 2008

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Subject:
From:
"Igoshev, Vladimir" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Igoshev, Vladimir
Date:
Wed, 19 Mar 2008 15:43:17 -0400
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Hi Tom,

Unlike Werner, I wouldn't "blame" Black Pad right away  :-) Is it
possible to see cross-sectional images of the joints? 

Regards,

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Wednesday, March 19, 2008 3:08 PM
To: [log in to unmask]
Subject: [TN] Case of the popping BGAs

Want to ask the advice of the Learned Forum members. Had an experience
where BGAs were falling off CCAs after shipment. Both the solder and the
solder balls are Sn63/Pb 37 solder alloy.  Had the parts and boards
tested by a third party lab. Their results are 
 
"The following report is in regards to the failure analysis of the two
largest BGA components on failed
assembly . The following lists the analyses performed in the attempt to
determine the root cause of the separation of the BGA components from
the assembly: 
 

- SEM/EDS surface analysis of the failed PCB sites 

-SEM/EDS surface analysis of the corresponding failed BGA component
sites 

-Micro-sectional/SEM evaluation of the failed PCB sites 

-Micro-sectional/SEM evaluation of the corresponding failed BGA
component sites 

All evaluations were performed with a Camscan MV2300 thermionic emission
scanning electron microscope (SEM), fitted with an Oxford EDS system
(elementalanalysis). The images obtained during the analysis can be
found in the following pages. 
 
Note: All micro-section samples were exposed to a chemical tin and
copper etch, to better visualize 
the intermetallic layer (IMC). 
 
Results: 
 
SEM/EDS surface analysis 
The following tables show the EDS results (in weight%) of five randomly
chosen sites on the failed PCB and failed component site respectively.
All analysis was obtained at 10kV acceleration potential 
(ND = Not Detected): 
 
PCB site/small BGA 
 
Pad     Ni     Sn     P      C     O 
1        74.5   3.9    6.6   13.8  1.2 
2       75.7    3.7    6.9   12.7  1.0 
3       78.8    4.0    7.3   9.9    ND 
4        77.7    4.1   6.8   11.4   ND 
5        81.1   3.6    7.9   7.5    ND 
 
PCB site/large BGA 
 
Pad    Ni     Sn      P      C          O 
1      80      3 .2     4 .7   11.3     0.9 
2      81.1    2.4      5.3    11.3     ND 
3       72      4.3      6.7     15.9     1.1 
4     77.5     4.9      7.4     10.3      ND 
5      78       5.5      7.5      8.9      ND 
 

Component site/small BGA 
 
Pad     Ni         Sn         Pb         C      O           P 
1         42.9     24.        8.1        23.6     7.1       0.6 
2         25.1     23         5.1         32.7     13.7     0.3 
3         42.8     26         2.4        22.4     5.9     0.5 
4         49.8     28.3     1.5         16.5     3.3     0.6 
5         35.5     20.7     1.4         31.1    10.8     0.5 
 
Component site/large BGA 
 
Pad         Ni         Sn     Pb         C         O         P 
1             42.3     27.3     1.7     23.2     5.0        0.4 
2             37.9     23.3     1.3     28.6     8.3         0.6 
3             40.7     26.4     1.8     25.2     5.4         0.5 
4             47.4     29.2     2.4     16       4.3         0.6 
5             38.7     25.3     2.8     24.7     8.4         0.2
 
 
Small BGA: 

PCB site; very thin areas of IMC were detected, but too thin to measure.
Nickel thickness was measured to be approximately 133 micro-inches. No
signs of nickel corrosion were detected. 

Component site; an average of 2.09um of IMC layer thickness was measured
on a total of three BGA solder balls evaluated. All solder balls showed
proper ball collapse and IMC formation, indicating a sufficiently hot
assembly reflow profile. 

Large BGA: 

PCB site; very thin areas of IMC were detected, but too thin to measure.
No signs of nickel corrosion were detected. 

Component site; an average of 1.52um of IMC layer thickness was measured
on a total of three BGA solder balls evaluated. All solder balls showed
proper ball collapse and IMC formation, indicating a sufficiently hot
assembly reflow profile.
 ,
 
 
What levels of Phosphorus would be expected in a nickel plating that
exhibits Black Pad? Any other ideas relating to the failure mechanism
and methods to validate them?
 
Thanks
 
Tom  

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