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—so, 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