Hi Werner,
How does the Tg relate to the temperature range?
This seems to imply that the delta between extremes changes.
How does this effect the number of cycles required to emulate the life?
Thanks, Rich.
Werner Engelmaier <[log in to unmask]> on 07/07/99 12:49:54 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
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To: [log in to unmask]
cc: (bcc: Richard G Wenda/ES/HSC/HUGHES)
Subject: Re: [TN] [LF] "Drop-In" Replacement
In a message dated 7/7/99 9:35:30, [log in to unmask] writes:
>Hi, Werner,
>What tests are sufficient to ensure "reasonable" reliability [of solder
joints]?
>Carol
Hi Carol ,
This sure is a loaded question, and can not be answered by a brief statement
without being flip
ůso, you asked for it!
Personally, and from experience, I rather assure solder joint reliability
using an adequate modeling approach, taking into account both the
physics-of-failure and the statistical failure distribution. The reason is
that it is no more error-prone than testing and much less expensive and
time-consuming. The time and cost factors of testing has led to test
short-cut, with sometimes catastrophic consequences.
Using a 'Design for Reliability (DfR)'-approach, of course, requires a
reasonably good model. For near-eutectic Sn/Pb solders and their derivatives,
we have the data on which a number of modeling approaches used in the
industry are based. But for many of the newer soldering alloys, including all
of the lead-free solders, we do not have sufficient data to determine a
fatigue reliability model. Thus, until such time, any modeling requires
assuming that the solder in question behaves similar to Sn/Pb and multiplying
some safety (actually ignorance) factor (>=2xlife to acceptable failure
probability).
For the new solders, side-by side cyclic testing with eutectic (or 60/40)
Sn/Pb solder from -20<->+100C (125C if substrate glass transition temperature
is at least 150C) with 15 minute dwells at each temperature extreme (24
cycles/day) with conventional chambers (or 5 minute dwells with chambers
modified to provide temperature uniformity in the whole test volume; ~100
cycles/day) with at least 32 equal continuity daisy-chains monitored with an
Anatech Event Detector for both solders, is necessary. This will give an
adequate data base to draw valid comparisons with near-eutectic Sn/Pb
solders, and, to me more importantly, from which a fatigue reliability model
can be derived.
Tests that are sufficient to ensure "reasonable" reliability will be totally
dependent on the product design together with its application. But to design
such a test, you have to have an appropriate fatigue reliability model from
which a valid acceleration factor for the test vis-a-vis the use conditions
can be determined. Electronic application vary from easily met reliability
reqirements for consumer products (~1,000 cycles (3 yrs) @ delta-T's ~10C &
10% acceltable failure probability) to difficult to meet requirements for
low-earth-orbit satellites (~90,000 cycles (10 yrs) @delta-T's ~35C & 0.001%)
or automotive-under-hood equipment (~10,000 cycles (~3 yrs) @ delta-T's ~100C
& 0.1%).
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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