I guess you are wondering about a CBGA with a high Pb ball.  Those are
designed to work with eutectic SnPb paste.  It gives you a compliant joint
that improves reliability of a rigid, low CTE CBGA.

With SnPb paste and a Pb-free BGA ball, you aren't forming something you
want or desire.  If your reflow is hot and long enough, the SnPb paste will
melt the Pb-free ball and give what looks to be a fully mixed joint.  The
reliability, however, is almost always worse than the comparable SnPb joint
or Pb-free joint.

Personally, I would rather switch to a flux only assembly process that gives
a smaller but better joint.  But that means going much hotter in reflow so
you might damage your component or PCB and you have to worry more about the
effect of component warpage on your assembly yields.  So most will end up
having to mix SnPb and Pb-free alloys because they have too.

Regards,

Daniel Blass
Process Research Engineer
Area Array Consortium
Surface Mount Technology Lab
Universal Instruments Corporation
http://www.uic.com/wcms/WCMS2.nsf/index/Global_Svc_Support_31.html

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
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
-----------------------------------------------------