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November 2010

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Werner Engelmaier <[log in to unmask]>
Date:
Fri, 19 Nov 2010 15:49:04 -0500
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 Oh boy, John,
There are a number of possible root causes—also, remember that those BGAs are the heaviest components and the thermally most massive ones. 
(1) this being ENIG, there is always 'Black Pad;'
(2) when soldering to ENIG, you are soldering to nickel—it is possible you had an insufficient peak reflow temperature to cause adequate 'wetting' of SAC solders to Ni;
(3) given SMD (solder mask defined) PCB pads, it could be a combination of stress concentration and weak interface due to either 'Black Pad' or insufficient reflow T's;
Full root cause determination will take cross-sections and SEM.
Werner

 


 

 

-----Original Message-----
From: John Parsons <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, Nov 19, 2010 3:31 pm
Subject: [TN] BGA Falling off PCA


I am not seeing a lot of the usual Friday quiz traffic so I know you

guys-n-gals are just itching for some real work :o).



 



We are a PCB fabricator and one of our customers has returned some

assemblies to us for feedback.  What they noticed when someone accidentally

dropped an assembly is that the BGA (352 pin Cirrus Logic EP9312-CBZ) popped

off the board.  They then proceeded to drop a few more from a height of

about a foot with the same results.  All the other components on this

Pb-free assembly stay fast.  



 



We have received three PCA's and 5 BGA's.  On one of the BGA's a single ball

has remained on the PCB.  The rest of them all the balls have broken cleanly

from the PCB leaving a very flat (under x10 magnification) dull gray

surface.  I sure wish that I could take photo's but lighting is critical and

I don't think that I have the resources to get photo's that would do justice

to what the naked eye is seeing.



 



Also of note is that the BGA lands are mask defined with a 31mil circuit pad

and 26mil solder mask relief.



 



Does anyone have any ideas as to how we can determine the root cause of this

failure.  This part has been built for some time but previously the surface

finish on the PCB was a solder finish.  This is the first run of the boards

that has an ENIG finish.  This doesn't look to me like a raw board issue but

that is where everyone looks first isn't it ;o).



 



Regards



 



John Parsons



 







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