That is a nice summary, Werner. However, I don't think that is the
problem Ioan is facing. If you look at the picture, then you won't see
any "puddle or pillow" shaped solder on the board side, which tells me
that the balls have never been in "mechanical" contacts with the solder
paste. That is why I suspected that not enough solder paste might have
been printed in the first place and warpage would just add to the
problem.

Regards,

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Wednesday, November 15, 2006 2:03 PM
To: [log in to unmask]
Subject: Re: [TN] Separation in BGA joints

Hi Ioan & All,
You need to consider what is actually happening when reflow soldering
the SJs
underneath a BGA.
(1) the BGA is a 'cold' heat sink;
(2) the PCB pad underneath the BGA gets heated by conduction of the PCB;
(3) the paste heats up with the PCB, the fluz activates, the tiny solder
paste balls, become pasty above Solidus [at this time the BGA solder
ball is way
below solidus];
(4) the solder paste melts and forms a
puddle/pillow/whatever-you-call-it,
taking on the shape of the still solid BGA ball;
(5) conduction heat transfer takes place between the molten solder paste
solder, and the solid BGA solder ball;
(6) the BGA solder ball reaches Liquidus on the bottom and gets pasty
[the
rest of the BGA ball is still below Solidus;
(7) the BGA ball reaches Liquidus on it surface and melts;
(8) of the consistency/surface tension of the molten BGA ball solder is
right, the solder from the paste and the solder from the BGA ball join
together to
form a solder joint.

Ioan, you got stuck somewhere between (4) and (7).

Werner

---------------------------------------------------
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
-----------------------------------------------------

---------------------------------------------------
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
-----------------------------------------------------