This sounds like the same problem I was asked about last week in the UK??

Yes filled vias can outgas through the plating and I normally test them
using my standard oil outgassing test and happy do so on a sample. If you
have poor quality via fill or excessive dips due to poor planarization you
can also get large voids and corresponding variation in ball size seen on
x-ray with out the gassing.

As has been stated look at the via size being used and prove it out with a
microsection and also CT if you are going to cut the board up?

If you want to still use the board assembly stick with the outgassing test.

There are some other process things that can be done.


Bob Willis
2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
Tel: (44) 1245 351502
Fax: (44) 1245 496123
Mobile: 07860 775858
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas McCall
Sent: 13 October 2009 12:07
To: [log in to unmask]
Subject: [TN] BGA voids in filled & capped micro-via PWB

Hi,
We have non-conductive resin filled and capped micro-via (via-in-pad)
through the complete PWB thickness (1.6mm FR4) 8-layer, we have seen
considerable voiding in the BGA balls following assembly:
 
Q1) What is the chance of out-gassing from the PWB board (or specifically
the filled capped via) going into the BGA ball and creating a void during
reflow. 
 
Q2) What is adequate flatness on the PWB surface considering “dish-down” of
the filled resin during via filling and before Cu plating.  I think the IPC
spec.  is 70% but does anyone have differing experience of this.
 
Q3) Would a fairly “rough” topology on the surface of the capping plating
(i.e. not totally flat, bumps etc) cause any difference in the
solderability,
 
I believe that:
·         The via drilled diameter is 0.25mm
·         PWB finish ENIG,
·         Cu/Ni/Au plating thickness is 20um
·         The PWB thickness is 1.6mm
·         The PWB Tg is approx 170 deg-C
·         The paste is Pb-free,
·         The reflow profile is good to the paste suppliers recommended
profile,
·         The PWB’s have been baked-out 130 deg-c 24-hours before assembly,
·         Reflow peak reached 242 deg-C
·         Reflow soak 170 deg-C for 170 seconds,
 
There are large and small voids in the BGA balls during X-Ray and cross
sectioning. Some of the BGA balls apear slightly elongated not correct
shape.
 
In essence would you think that it would be a PWB issue or assembly problem.
 
Any feedback would be much appreciated.
 
Thanks,
D. McCall.
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