🚀 go-pugleaf

In article <77230005@teecs.UUCP> smithr@teecs.UUCP (Robert Smith) writes:
>I agree with Jerry, usually solder joints have the seeds of failure
>implanted during manufacture. If the joint is protected from the
>atmosphere it shouldn't go bad. Thermal expansion will not kill a well
>made joint; we thermally shock all our products from +85C to -40C with
>a transition of about 4 seconds for 30 cycles to test solderability.

"Experience has shown that most of these failures are produced by a
mismatch in the thermal coefficients of expansion (TCE) of the different
types of materials typically used in electronic assemblies." -- quoted
without permission from "Cooling Techniques for Electronic Equipment"
by Dave S. Steinberg, a foremost industry expert on reliability issues,
page 249.

"The standard mechanical reliability concern for solder joints is the
measurement of resistance to failure by low cycle fatigue due to thermal
cycling." -- quoted without permission from "Solder Joint Reliability",
edited by John H. Lau, page 267.

I can probably dig up dozens of quotes to this effect.

Thermal cycling is the primary cause of solder joint wear-out. Even a
well made solder joint has a finite thermal fatigue life. If you have
manufacturing defects, they make the problem much worse.

And thermal shock DOES NOT simulate thermal cycling. It does not allow
solder creep, which is the root cause of thermal fatigue and occurs over
much longer periods of time, several minutes to hours. In fact, the slower
you ramp the temperature and longer you hold at temp. extremes (upto a
limit), more you damage the solder joints. This is the nature of low
cycle fatigue and a well known fact, supported by theory and experimental
studies.

In general, solder joints in large surface mount devices (leadless being
worse than leaded) have a much shorter fatigue life than plated through
hole type devices. Since the current trend in the industry is towards
surface mount technology, solder fatigue will become more critical in
the future.
-- 
Pardeep K. Bhatti                         pkbhatti@kopth004.delcoelect.com
Delco Electronics Corporation
GM Hughes Electronics
***  The opinions expressed are mine, not necessarily of my employer.  ***