Hi Richard,
Just some food for thoughts. I've been looking at perfectly healthy
solder joints on ENIG finished boards and measuring the content of P in
the layer of electroless Ni layer quite a few times and every time the
reading was way above 3%. Atotech has a few publications on the topic.
In addition, doing quantification on a fracture surface isn't a very
good way of getting some numbers, as the surface is obviously not
perfectly flat.
Regards,
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, March 19, 2008 4:22 PM
To: [log in to unmask]
Subject: Re: [TN] Case of the popping BGAs
This analysis reads very similar to lab analysis reports I have had done
in the past that turned out to be levels of grey pad or black pad. The
key is the widely varying levels of P on different pads on the same
board. This can cause anything from flat-out black pad to brittle
failure. Anything over a P content of about 3% will begin to cause
problems.
Be sure to monitor the PWB lot codes. I hate to tell you this, but the
problem will probably affect more than 1 assembly. As the assemblies see
thermal excursions in their service life, the P barrier will continue to
grow, causing more failures. Monitor the failures so you can detect a
trend in the PWB lot numbers, this way you can be somewhat proactive in
determining which assemblies have potential reliability issues.
Where is that guy who told us that Black Pad was no longer a problem?
Anybody heard from him lately?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Wednesday, March 19, 2008 2:08 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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