Yes, indeed, a solder joint (under the category of soft solders) goes through 
degradation and damaging mechanim(s) during its service life, regardless of 
alloys.

However, among the commercially-used Pb-free alloys, the solder joint 
integrity under high temperature aging or through temperature cycling excursion 
particularly a longer dwell time at the high-end temperature (in a testing setup, 
for example, 30 minutes or 2 hours in lieu of 10 minutes) is expected to 
perform as below in an ascending order:

SnCu
SnAgCu, SnAg
SnAgCuSb
SnAgCuBi, SnAgCuIn, SnAgBiIn

Note that some testing conditions or service environment may not be 
discriminating enough to reveal the difference.

Back in late 1980s, even before the research and evaluation started, we knew 
that any working Pb-free alloy must be based on Sn for both fundamental 
scientific and pragmatic manufacturing reasons. 

With all wishes to have a working binary alloy, yet we knew that none of the 
binary alloys can fill the role for reflow surface mount soldering covering 
all designs and end-use requirements. But at the time, what we did not know was 
that whether a ternary system could do the job under the established 
manufacturing infrastructure (including established flux chemistry, operating modules 
on the production floor, metallurgical phenomena, etc.). 

As such, the systematic and significant effort was directed to ternary 
alloys, particularly the SnAgBi, SnAgCu, SnAgZn, Mg-containing alloys and others. 
After a solid and systematic evaluation, in early-mid 1990s, it was becoming 
clear that a ternary alloy is unable to fulfill the mission for all applications. 
Consequently, the effort had to go further to the quaternary systems. 

Again, welcome any tests or life performance to confirm and deny the 
performance trends cited.

Regards,
Jennie Hwang

Dr. Jennie Hwang
H-Technologies Group
Asahi America, Inc.
Office: 216-839-1000
Personal e-mail: [log in to unmask]
----------
In a message dated 4/12/06 9:54:20 AM Eastern Daylight Time, 
[log in to unmask] writes:
> Hi Keith,
> Actually, the lower impact strength of SJs after T-cycling has in most cases 
> 
> nothing to do with IMCs and any possible growth of IMC layers—IMC growth is 
> an 
> old red herring. 
> It has to do with the fact that the grain structure of solders is 
> inhertently 
> unstable and will coarsen with time, temperature, and stress. After about 
> 25% 
> of the fatigue life consumed, micro-voids and micro-cracks form at the grain 
> 
> boundaries. That is why SJs exhibit declining strength with increasing 
> fatigue 
> cycles. The failures are not interfacial, but in the solder volume.
> Werner
> 


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