Without trying to be too sarcastic, the easiest way I know
Tom,
Without trying to be too sarcastic, the easiest way I know to find out
if Werner is correct is look at the pad and see if it is black or dark
gray in color. I've said this many times, in my way of thinking "Black
Pad" isn't a name that describes a failure mechanism, it's a name that
describes what you see after the failure occurs. The results you
described are typical of analytical labs. They have lots of analytical
data to try an impress the customer that they got their money's worth
but they don't state what might be obvious. I think from the analysis
the Ni on the pads is probably a sure give away that the surface finish
on the PCBA was probably ENIG. There wasn't an analysis of what was on
the BGA side of the failure but my guess is when the BGA's fell off the
solder balls stayed attached to the BGA and the IMC was probably thicker
on the BGA side of the fracture than the pad side which would indicate
that your fractures were close to or at the electroless nickel / IMC
interface. Like I indicated if the pads look back or dark gray in color
you probably have a "classic" case of "Black Pad". If the pads are not
black and you don't see the "mud cracking" also said to be an indication
of "Black Pad" then you probably still have a stress induced brittle
ENIG fracture at or near the electroless nickel / IMC interface.
Regards,
George
George M. Wenger
Andrew Wireless Solutions
Senior Principal FMA / Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office] (732) 309-8964 [Cell]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir
Sent: Wednesday, March 19, 2008 3:43 PM
To: [log in to unmask]
Subject: Re: [TN] Case of the popping BGAs
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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