Should is the operative word in your last thought. There are many possible
reasons those vias could fail. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Thursday, January 31, 2008 1:29 PM
To: [log in to unmask]
Subject: [TN] BGA underfill opens- the saga continues

 
Back in December I posted about a problem with failures under
underfilled BGAs. Just to continue the saga .
Brief update on current failure analysis on U10 failures (169 pin
plastic OMPAC)
 
No common defects have been identified- ability to trace to pin level is
limited. Four boards were sent back to the customer to trace down to
specific pins. None of the defects had anything in common- all opens but
on different pins. 
 
1. Oven reflow profiles and underfill profiles were verified and found
to be acceptable per manufacturers recommendations.
 
2. Acoustic Microscopy completed by Sonolab on three assemblies with
known U10 failures. This included one failure without underfill and two
assemblies with underfill. These were debugged by the vendor. There was
no defect detected on the non underfilled part- there was some slight
(<5%) voids in underfill but these do not track with the pins found to
be open. There was no delamination or bond wire defects found.
 
3. Serial number 2043- non underfilled BGA U10 - was removed and
replaced with a newer date code (0509). There board was retested and the
same test failures occurred. This was a differnet part heated with two
different reflow processes and still had the same test failures as
orginally found.
 
4. Serial Number 2043. U10 was again removed. The defect on this board
was tracked by the vendor to an open between U19 pin 3 and U10 pin P16.
When a continuity check was done by me I found an open. Could be an open
on board- Don't think it was reworked induced simply due the fact that
the same functional test failures were seen before and after rework.
Asked Bill Mitchell to verify this.
 
5. Serial number 2217 will be milled off by machine shop . No evidence
of any cracks at the BGA pad to solder ball was observed. The defective
pads were tested and found to be open. It was found that the via under
the BGA was intermittent open.
 
The vias under the BGA are tented on the top and bottom. I thought that
maybe the underfill might be getting into the via and maybe causing a
CTE strain in the Z axis and fracturing the plating. So much for that
theory. What is interesting is that the boards go through electrical
test at the board vendor and we see the opens only after a limited
environmental test (there is a laminography after SMT and flying probe
and functional test before the final confidence test). The final plated
hole is 0.010" and the board is 0.062" so the aspect ratio is only 6:1-
should be able to plate it well. 
 
Any ideas
 
 
 
Tom  
 
 

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