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Date: | Wed, 19 Mar 2008 15:07:38 -0400 |
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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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