John,
I wish I could say I am surprised at your results, but only a little. BGAs perform very poorly in drop shock testing. Both the original JCAA-JGPP study at
http://www.acqp2.nasa.gov/LeadFreeSolderTestingForHighReliability_Proj1.html
And now the results from NASA/DoD phase two - papers by Polina Snugnovsky and Tom Woodrow at SMTAI last month show that BGAs fail first of all the compents on the test boards!! JGPP data shows that over 60% of the failure cause on JGPP boards is (drum roll, please) - the kind of component!!! BGAs fail first in most all kinds of stress - thermal cycle, drop shock, vibration, combined environments, but by far the first in drop shock.
In the SMTAI report data from Polina Snugnovsky at Celestica - all but one BGA failed to pass 20 successive drops from JEDEC test procedure JESD22-B110A. On the same board, all 90 chip scale components passed the same test.
My theory about the BGA failures is that the weight of the balls, and the distance the chip is from the board creates tremendous shear on the ball solder joints when dropped. BGAs do significantly better in thermal cycle than the early failure demonstrated in drop testing.
That said, a one foot drop, one time sounds pretty mild and I would still examine the board closely. Notoriously, ENIG finishes have suffered from "black pad". You will need some EDAX elemental analysis of the board surface to get a feel for how much phosphorus shows up.
I don't think the mask defined pad can be the only issue, but typically, the mask defined pads are on the package, and solder is allowed to completly wet the pad on the board. I trust the pad sizes are about the same on the package and on the board? Finally, you may have access to a dye syringe, so that dye-and-pry testing can be done on the assemblies to see if there is already cracking of the BGA joints from thermal problems in assembly, or if there is a "head-on-pillow" issue of poor wetting of balls to the paste applied to the board for assembly. (presumes the assembler is using paste stencilling, and not just tacky flux direct to the ENIG pads on your board.
Denny Fritz
SAIC, Inc.
-----Original Message-----
From: John Parsons <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Fri, Nov 19, 2010 3:30 pm
Subject: [TN] BGA Falling off PCA
I am not seeing a lot of the usual Friday quiz traffic so I know you
uys-n-gals are just itching for some real work :o).
We are a PCB fabricator and one of our customers has returned some
ssemblies to us for feedback. What they noticed when someone accidentally
ropped an assembly is that the BGA (352 pin Cirrus Logic EP9312-CBZ) popped
ff the board. They then proceeded to drop a few more from a height of
bout a foot with the same results. All the other components on this
b-free assembly stay fast.
We have received three PCA's and 5 BGA's. On one of the BGA's a single ball
as remained on the PCB. The rest of them all the balls have broken cleanly
rom the PCB leaving a very flat (under x10 magnification) dull gray
urface. I sure wish that I could take photo's but lighting is critical and
don't think that I have the resources to get photo's that would do justice
o what the naked eye is seeing.
Also of note is that the BGA lands are mask defined with a 31mil circuit pad
nd 26mil solder mask relief.
Does anyone have any ideas as to how we can determine the root cause of this
ailure. This part has been built for some time but previously the surface
inish on the PCB was a solder finish. This is the first run of the boards
hat has an ENIG finish. This doesn't look to me like a raw board issue but
hat is where everyone looks first isn't it ;o).
Regards
John Parsons
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