As described by others plating defects can cause the solder void. 
However a more common problem we see is failure of the uVia to wet 
out.  I've got pics of both conditions going back years now.

This due to a final shape of an otherwise well formed and plated 
uVia. The via needs to wet out. If it doesn't wet, the air will be 
held in by the surface tension of the liquidus solder. The temp will 
finally rise enough for the air pressure to overcome the surface 
tension. That's when the solder ball expands and forms a bridge to an 
adjacent ball.

In order to wet out consistently, the uVia final shape should be a 
bit more open at the top than at the bottom.  If the via sidewalls 
are too near vertical or if the plating has a cornice where plating 
closed down narrower at the knee of the via it will cause 
voiding.  The liquid solder forms a meniscus that hangs up on the 
knee of plating.

Simple fix?  Specify solder copper filled uVias in 
outerlayers.  Tougher fix? Agree on an acceptability spec with your 
suppliers for final uVia shape. Many of the top tier high volume 
offshore uVia fabs already have a uVia shape standard that's been 
applied to them by a large OEM or two (:-)) who've gone before you.


At 06:34 PM 8/26/2009, Chris Mahanna wrote:
>Hello netters-
>
>Have we a term for vias-in-pad blowing solder ball bubbles to the 
>point of bridging?
>Regardless, is there a unique cure for the problem?
>I wonder because- how does one get just enough pressure to blow the 
>large bubble, but not blow-out?   Bad luck?
>Or is it not that unusual, and just usually gets screened before it 
>gets to me?
>
>Chris
>
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to 
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to 
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site 
>http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional 
>information, or contact Keach Sasamori at [log in to unmask] or 
>847-615-7100 ext.2815
>-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------