We have recently begun to see that the voids beneath our larger pitch
(1.27 mm) BGAs which are on pads containing the larger size microvias
consistently are exhibiting voiding which gets partially trapped in
the microvia  but stretches up into the ball as well.  It looks kind
of like Casper the friendly ghost when microsectioned.

The typical location of these voids historically has always been near
the top of the bga ball and never at the pad or sub-pad level so this
is a new development for us.  Same oven profile temps and time, just
different microvias creating different final resting spots for voids
which result.

It is hard to believe that the shape of the microvia construction
could cause the voids to not rise normally but that is my gut feeling
after looking at the microsections.  These microvia are smaller at
the pad level than the bottom of the microvia.  This ususally happens
with .004" thick dielectric material and becomes exaggerated when
.006"  material is used.

Once in a while, the entire void makes it into the ball area and
enlarges the total ball diameter,  but this is unusual.

Anyone else exhibiting this type of phenomena?

BTW, the neighboring solderball connections are void free, which
eliminates the paste suspicion as to root cause of the voids.


  At 12:54 PM -0700 1/27/00, Ryan Grant wrote:
>Jim,
>         I specifically looked for voids in microBGA's that had microvias in
>the pad.  I found voids in every pad with micro vias.  However the voids
>were very small.  I'm not entirely convinced the voids I saw were not an
>illusion from looking into a via; although I did view the voids and via at
>an angle.  (I guess if I really cared, I could cross section a joint and
>know for sure).
>
>         Voids in BGA balls can be caused by many factors.  I think microvias
>will sometimes get a bum rap because large voids are assumed to come from
>the microvia; when in fact the large void may be caused by the paste, reflow
>profile or whatever else.  While it may or may not be true that micro vias
>cause voids, they are definitely here to stay.
>
>>  -----Original Message-----
>>  From: Jim Kittel [SMTP:[log in to unmask]]
>>  Sent: Wednesday, January 19, 2000 1:11 PM
>>  To:   [log in to unmask]
>>  Subject:      [TN] Pad in Via for Micro BGA's
>>
>>  Tech Netters
>>
>>  I have a question for you on micro BGA's and I don't want this to be
>>  confused with the recent discussion on via-in-pad which may still be in
>>  process.
>>
>>  The working draft copy I have of "Design and Assembly Process
>>  Implementation
>>  for BGAs" states on page 63 that a 0.1 mm micro via may be placed in the
>>  BGA
>>  solder land to allow more space for routing of traces.  They say the only
>>  noticeable affect is a small dimple.  I seem to remember articles in
>>  literature about outgassing and large voids in the BGA solder ball
>>  centered
>>  over the embedded via.   Could anyone comment on their experience placing
>>  microvias in BGA solder lands and if they agree with this comment in the
>>  design guide?
>>
>>                  Jim Kittel
>>
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Phillip A. Bavaro
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