Jim
It sounds like you have "Double Reflow". Although I hate the name, IBM
coined the term in their 1994 NEPCON WEST paper and it has stuck. I think
the paper that best describes what you are experiencing is "Double Reflow -
The Stress Fracture Phenomenon of the '90s" by Wenger, and Furrow presented
at SMI 1995. It sounds like you are experiencing brittle stress induced
fractures and an intermetallic interface.
Regards,
George
George M. Wenger DMTS
Bell Laboratories Princeton, Supply Network Solutions
Engineering Research Center FMA / AQA / RCA Lab
(609) 639-2769 (Office); 3210 (Lab); 2343 (Fax)
[log in to unmask]
-----Original Message-----
From: Jim Kittel [mailto:[log in to unmask]]
Sent: Thursday, December 14, 2000 5:29 PM
To: [log in to unmask]
Subject: [TN] Solder Failure During SMT/Wave Solder
Hi,
I am encountering solder failures I have never seen before on some type II
SMT assemblies. These are about 8 x 16 inch mixed technology boards with
misc SMT parts including some 0.020 inch pitch QFP's and some thru hole
connectors. The cards are 0.062 and about 8 layers of 1 ounce copper.
Paste is 63/37 eutectic.
We paste, populate the primary side with SMT, reflow solder, clean, inspect,
and then send the board to wave solder area. In wave solder they install
the thru hole connectors on the primary side and then wave solder. From
what I know this is an industry standard process.
The problem is that on two QFP's (always the same) the leads can be easily
lifted right out of the solder fillet after wave solder. (Board has
cooled.) Sometimes the solder stays on the lead, sometimes it stays on the
pad. Solder is always fine before wave solder. We attached thermocouples
to leads on these QFP's and ran the boards back thru the wave solder. Found
a lead that peaked below melt temperature and another lead peaked above
solder melt. The leads between these are the ones that can be easily pulled
out of the solder after cooling. In some cases the solder appears dull and
grainy, but in other cases it still looks shiny.
Is it possible the solder in these connections is reaching the plastic
stage, crystallizing and then loosing it's strength? If so, why doesn't
this happen to others in the industry running this process? We have to run
about 3 feet/minute to solder the thru hole connectors to the 1 ounce
planes. Is the remedy to go even slower to reflow all the top connections?
Ran EDX on the solder in the failed connections and saw nothing unusual.
Any help would be appreciated. I am stumped.
Thanks,
Jim Kittel
L-3 Communications
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