Hi, Patti,
PLCC's are a bit notorious (though not as bad as CLCC's) for their rigidity
when it comes to resisting creep fatigue and vibration. Solder joints
frequently crack in military operating conditions if care is not taken with
them. Hand soldering is a somewhat variable process unless you have a very
skilled and conscientious operator doing the soldering, so some high stress
points may be being built in to start with. I'm kinda reading between the
lines here and get the impression that you're dealing with already
assembled boards, rather than building them from scratch. Can you not
screen print and mount these suckers using a rework station? It might
produce more consistant results.
Depending on which aircraft and where in the aircraft the boards are
mounted, temperature cycling and vibration levels could be almost anything.
Do you have any specs for the temperature ranges and rates of change, and
vibration profile that the boards are experiencing? It's possible that the
spring rate of the mounting mechanism may be contributing to an enhancement
of the vibration levels, depending on the spring's natural frequency,
especially as the boards seem to be long and narrow (and thin?). I wouldn't
normally be comfortable with spring-mounting a board in a piece of military
gear unless I was very sure of the harmonics involved, but for all I know,
that portion is out of your hands.
As for a cure - well, I've had some success in the past with using water
soluble stand-offs under such components to create a bit of a space between
the component leads and the board and increasing the solder joint height
slightly. If assembly was being done using solder paste stencils to deposit
paste, I would be using something like a 10 mils thick stencil, but you can
try experimenting with solder joint heights as long as the solder fillet
form complies with IPC-A-610 or similar.
I've never tried using any of them, but there are "Fatigue Resistant
Solders" about, the best known of which is "Maxon" from AMT, produced by
Howard Electronic Instruments, I think, or one of their subsidiaries. It's
a fairly specalised solder with a particularly fine grain structure and
contains some extra dopants. The soldering temperature is slightly lower
than eutectic solder at around 180 deg C, but there are higher melting
point FRS's and lead-free ones as well. There is info available on the www
for these if you want to surf.
Hope this helps a bit.
Peter
Patricia Volkman <[log in to unmask]> 26/06/2002 05:18 AM
Sent by: TechNet <[log in to unmask]>
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] fractured solder joints
Fractured Solder Joints
My company has been doing hand assembly/soldering on a driver IC board for
a
display that is used on a military airplane. The board uses 8 Holt ICs -
HI-8010SM-32 - http://www.holtic.com/ProPDFs/8010.C.pdf
The boards we provide have been in use for between 1 year and 8 years, the
actual time is unknown, but it is likely that they have been in place for a
long period of time.
The IC is a 44 pin QFP J-lead with 0.050 center spacing on the leads.
Our customer has recently found fractured solder joints associated with
these ICs (identical to those in photo - IPC-A-610C page 12-85 )
It is assumed that the fractured solder joints occured after leaving our
facilities, caused by airplace stress cycles or ???
I am looking for any information on why these fractures would occur, the
boards are spring mounted on the plane, the boards measure about 1.5" x 5".
Most importantly, I am looking for a solution to this problem, I am
currently looking for a more ductile solder...the solder used in the past
was Kester SN60PB40. I can also change the assembly process if that is
determined to be the root cause (all hand soldered)
thanks for any information you can provide.
Patti Volkman
(1-800-823-5588)
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