The problem with drop tests is not the gravitational force. Any drop of a PCB will cause a shock wave running through the PCB. This results in a deformation front with waves perpendicular to the surface of the PCB running across the board, similar to the rings that form on water if you drop a stone in it. Since BGAs are very stiff and since they have a large surface the wave travelling through the PCB induces large differences of height across the BGA causing brittle fracture in the intermetallics. 

Best regards


Günter


>-----Ursprüngliche Nachricht-----
>Von: TechNet [mailto:[log in to unmask]] Im Auftrag von John Parsons
>Gesendet: Freitag, 19. November 2010 21:32
>An: [log in to unmask]
>Betreff: [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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