Voids can be caused by a number of things, but the most common cause is
the moisture content of the solder paste, and its condition.
That being said, a certain paste that has been known to be the mainstay
for many years currently is the worst one today as far as voiding. A
different paste manufacturer made an excellent paste that was formulated
to prevent voiding.
I recently performed a paste evaluation of standard 63/37, a leadfree
water soluble, and a leadfree no clean paste. There were 27 different
tests, with 5 vendors. On the standard 63/37, the paste formulated to
prevent voiding had ZERO voids in 1330 BGA joints, multiplied by 15
samples. The previous paste vendor had voids in every single BGA joint.
All variables were strictly controlled, and I did not know which sample
was being tested (they were numbered by someone else to prevent any
bias).
While I do not want to give out vendor names on the forum, I would be
glad to share this info with anyone offline.
The application engineer who told you not to worry about voids should be
taken outside and shot. While some data exists that says voids are OK if
they are small enough, you should certainly strive for an alloy that
provides you with an excellent grain structure free of voiding, with
evidence of good IMF at the pad/ball interface. Sorry, pal, but you
cannot determine this with an X-ray machine. It can only be done with a
good microsectional analysis.
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stephen Gregory
Sent: Monday, July 18, 2005 1:07 PM
To: [log in to unmask]
Subject: [TN] BGA voids

Good Day Technetters!

I have a question about BGA voids. We are bidding on some work and it
looks like they have taken statements that are in the IPC-7095, and
placed them as notes on their drawing.

One of the notes has to do with a limit on the percentage of balls on a
BGA device that are allowed to have voids. For example; "Less than 5% of
the balls can have voids, and the percentage of the void area must meet
class
3 requirements."

I can understand the void area being critical, but the percentage of the
number of balls that are allowed to exhibit them?

The reason that I'm asking, is that I was told last week by a
application engineer from a x-ray machine company (who shall remain
nameless), that seeing voids is normal. Just as long as they aren't too
big, or being concentrated at either the pad/ball interface, or the
ball/device interface.
He said you should worry if you don't see any voids on BGA's with
eutectic balls.

Just curious as to what you all think about this?

Kind regards,

-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
__________________________________________________________________
This message may contain information that is privileged and confidential
to LaBarge, Inc.  It is for use only by the individual or entity named
above.
If you are not the intended recipient, you may not copy, use or deliver
this message to anyone.  In such event, you should destroy the message
and kindly notify the sender by reply e-mail.

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