Steve

Immersion silver was originally introduced into North America in 95 by 
AT&T/Lucent. I installed the first line. There are now millions of immersion 
silver boards in the field. No one I am aware of has reported failures 
caused by embrittlement of the solder joint, so I am dubious of the failure 
mode.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389


----- Original Message ----- 
From: "Steve Gregory" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, May 24, 2007 8:44 AM
Subject: [TN] Silver Embrittlement?


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


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