Hi Tom!

This isn't a mixed metal job, is it?  In C51a I detect a bit of a
"waist" just above the pads on the pc board.  If so, that is not good.
You need thorough alloy mixing as evidenced by one continuous convex
surface.

Also, you mention pushing on "the vias under the joints".  Is this a
via-in-BGApad job? If so, as pointed out to me on this forum, you could
have nasty intermetallics caused by metals in a bad via fill material
migrating up towards the joint.  

As I've mentioned before to you, you will have your answer in very short
order if you simply mechanically separate the joint and inspect the
interface where the disconnects are.  If you suspect nasty
intermetallics, you can have this interface analyzed to see what the
constituents are.  Or you may just find pad contamination due to poor
solder mask developing.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Tuesday, December 18, 2007 5:08 PM
To: [log in to unmask]
Subject: Re: [TN] BGA failures after OMPAC underfill

Just thought I followed up on the fallout of OMPAC style BGAs after
underfill.  All the packages are from one date code- 47th week of 2003.
Most of the verfied failures were traced to open solder joints- they
seem to be in no particular area of the parts- for example, the open
pins under the BGA were P16, J12, N7, M8. They can press on the vias
under the open joints and get electrical continiuty but don't know where
in the solder joint the cracks are occuring.  I am having acoustic
microscopy being done on the parts right now but we normally do a
underfill on a BGA mounted on a glass slide and all the samples do not
show any obvious defects.
 
Is there a way of determining the CTE in x,y and z axis of an soldered
and underfilled part? 
 
As an experiment we did ICT test on non underfilled parts and the
particular failure was at pin M8. I examined the periperal joints and
Steve graciously posted them - they are
http://stevezeva.homestead.com/sideNearC51a.JPG
 
http://stevezeva.homestead.com/sideNearC51b.JPG
 
To me the solder joints to the PCB look well wetted but I'm more used to
a more squashed looking ball - the profile of the solderball to the
package seems to have a more pronounced curvature. Would like some
inputs from the experts - maybe I'm reading more into this.  The profile
indicates that the temperature under the center BGA joints are 214.5 C
and a time above liquidus of 84 sec and the edge of the BGA is 216.4 C
and a TAL of 86 seconds.
 
Will also microsection failed joints after acoustic microscopy and plan
to do SEM/EDS at the ball to package interface and all the cracks. 
 
Would black pad on the nickel termination on the BGA package contribute
to a failure at this interface?
 
Thanks
Tom  

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