Hello Wener,
SEE COMMENTS BELOW.
JEAN-PAUL
In a message dated 4/21/2006 6:08:01 P.M. Eastern Standard Time,
[log in to unmask] writes:
Hi Pratap,
Side-by-side ACT of SnPb and LF SJs is comparing apples and oranges, because
of the significantly different creep rates.
JUST TO ADD TO YOUR COMMENT, THERE IS ALSO THE GRANNY SMITH AND THE GOLDEN
APPLES AND MANY MORE.
THE LF SOLDER JOINT RELIABILITY ISSUE HAS BECOME MORE COMPLEX BECAUSE OF
THE MULTITUDE OF ALLOYS BEING USED. E.G.: SAC396/SAC387, THEN SAC305, LATEST
COMPOSITION I HEARD BEING USED IS SAC105. AND DON'T FORGET SN-CU(NI), SN-BI,
AND MANY OTHERS, INCLUDING QUARTENARY ALLOYS.
SO UNLESS THE ALLOY AND THE CONDITIONS ARE BEING SPECIFIED, I HAVE NO IDEA
WHAT "LF RELIABILITY" REFERS TO.
TALKING ABOUT SAC396/SAC387 UNDER THERMAL CYCLING CONDITIONS, A LOT OF DATA
IS AVAILABLE AND THE TREND IS PRETTY CLEAR:
THE SAC TO SN-PB LIFE RATIO GOES DOWN, AND CAN BE LESS THAN 1, AS FOLLOWS:
1) WHEN THE DELTA T (TEMPERATURE SWING) INCREASES
2) WHEN THE DWELL TIMES BECOMES LONGER
3) WHEN THE MEAN TEMPERATURE (AVERAGE OF HOT AND COLD TEMPS.) INCREASES.
That is why it is impossible to
extend IPC-7901 to LF-solders until a generally valid acceleration model for
LF-solders exists.
Werner
I THINK WE DISCUSSED THIS BEFORE ON THIS FORUM. FOR SAC396/SAC387, OVER A
DOZEN THERMAL CYCLING RELIABILITY MODELS HAVE BEEN DEVELOPED. I GAVE REFERENCES
IN MY SMTAI'05 PAPER LAST YEAR AND THERE IS MORE COMING OUT. I DON'T
BELIEVE IN 'GENERAL' MODELS (all of them require some kind of component specific
calibration), SO I USE AT LEAST TWO MODELS WHEN I CAN AND THEN I TRY TO
UNDERSTAND WHY PREDICTIONS CAN BE DIFFERENT (which is not unusual for SAC and was
not unusual for SnPb either).
Quote of the month:
"Success is the sum of details", Harvey S. Firestone.
_______________________________________________
Jean-Paul Clech
EPSI Inc., P.O. Box 1522, Montclair, NJ 07042, USA
tel.:+1 (973)746-3796, fax: +1 (973)655-0815
_http://www.jpclech.com_ (http://www.jpclech.com/)
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