My guess is that the TSOP packages do have Alloy 42 lead
Steve,
My guess is that the TSOP packages do have Alloy 42 leads and that you
may be seeing a failure that has gotten the name "Double Reflow" I hate
that name almost as much as I hate the name "Black pad". What do the
solder fillets look like on your TSOP packages? If they look good but
your testing is indicating an intermittent and you can pass test by
pushing down on a TSOP lead I think you most likely have a brittle
interface solder joint fracture. It will look like a perfect surface
mount solder joint with nice feathered solder fillets like it is wet
well to the PCB pad, and it probably was when it was formed. However,
then the temperature is elevated and there is any mechanical stress the
solder joint can facture at the IMC bulk solder interface. You won't be
able to see the fracture unless you view it at a low angle under high
magnification. The "Double Reflow" name was coined by IBM Austin back
in 1979 presentation at NEPCON. They called it "Double Reflow" because
their surface mount solder joints look great after surface mount reflow
soldering but failed (fractured) after the boards were wave soldered
(i.e., failed at the second reflow). I've attached their NEPCON paper
as well as one we did describing a solder joint fracture we had using
QFPs with Alloy 42 leads. We also saw similar failures on another
product on corner leads after the initial surface mount reflow. Those
failures occurred during the first reflow and had nothing to do with
double reflow. The solder joint fractures occurred on corner leads
during the surface mount reflow cooling process.
Regards,
George
George M. Wenger
Senior Principle FMA / Reliability Engineer
Wireless network Solutions
Andrew Corporation, 40 Technology Drive, Warren, NJ 07059 (908) 546-4531
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Thursday, May 24, 2007 8:45 AM
To: [log in to unmask]
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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