Hi Steve!

Never saw anything about silver embrittlement, and since Sn62 is still
a popular alloy, carrying about 2% silver, I doubt this is much of an
issue.  I've soldered some SAMTEC headers like that using Sn62 and
haven't seen a problem.

With respect to the TSOP-66 parts:

Just pry up one of the unconnected leads while watching what happens
under a microscope.  If it comes up real easy, which it should, then
it's probably mostly copper, like they've gone to on big PQFPs.  Also,
if it comes up real easy, then it would cause me to consider other
possibilities other than differential expansion of the circuit board as
the failure mechanism.

When you carefully examine the pried up lead, then you will probably
see the interface the crack propagated at, and you will see how the
wetting went.

Ideas based on your present data:

-Lead too stiff (alloy 42)
-board XY CTE too high
-bad component lead plating
-bad board plating
-bad solder or solder process
-bad design idea (although it would seem that almost anything should
pass at least a few dozen thermal cycles)

Might be simple and easy to jam a thermocouple up against the hottest
and coldest spots on the problem part while that hot BGA is fired up.

Wayne

>>> [log in to unmask] 5/24/2007 8:44:40 am >>>
Mornin' all!
 
I know this subject has been talked about before, but my memory is
getting rusty. I was wondering if it was possible to get solder joint
embrittlement on a board that has immersion silver finish and
components
that have a flash gold finish? As you might have guessed, it's those
pesky Samtec SMT connectors...
 
We're experiencing another issue on that board as well. We're seeing
solder joint problems with some TSOP-66 DDR SDRAM that are on the
topside of the board. They're arranged close around two XILINX
1152-ball
BGA's that get very hot when the assembly is under power. Both BGA's
have large finned heatsinks mounted to the top of them. The BGA's get
so
hot that if you power the board up without the heatsinks, and touch
one
of the BGA's, you will burn your finger. There have been some cases
that
during testing the board will fail and the failure points to one of
the
SDRAM TSOP's. If you take something like the end of a wooden Q-Tip and
press on a suspect lead, the board passes test. The solder joint looks
fine upon initial inspection, but there's obviously an open
connection.
I remember at one time TSOP leadframe material used to be made of
Alloy-42, is that still the case? These are parts from Micron
(MT46V16M16TG-6T), and I haven't yet found what the leadframes are
made
of. I'm wondering if the stiffness of the leads may be contributing to
the solder joint failures that we're seeing. My theory is that because
the BGA's run so hot, they heat the PCB and cause board to expand in
the
area around them (where the SDRAM are mounted).
If the leads on the SDRAM are stiff and non-compliant, the solder
joints
might fracture.
 
Crazy theory I know, but I'm trying to figure what's going on with
these
boards...
 
Steve Gregory 
	

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