John,
I am using leaded solder with lead-free components and am quite satisfied with 
the results.  I am a small OEM with one SMT line.  I could not afford to be an 
industry leader in lead-free, RoHS and WEEE.  So I studied the literature for 
the last several years, and attended several APEX/IPC EXPOs and SMTA 
conferences to learn from those who could experiment.  I also had the 
opportunity to run one concluding experiment before going ahead with mixed 
alloy backward compatible soldering.

I found that the literature was mixed.  However, it was also clear that those 
people relatively new to processing, and many of those in large companies 
where any risk is unnacceptable were strongly opposed to backward 
compatible soldering.  It wasn't "the best."  I had to consider a risk.  I studied 
the research and found that in those experiments where the peak temperature 
was 220C and above and the time above 217C was in excess of about 45 
seconds the lead-free SAC ball melted and combine completely with the leaded 
solder.  When the melting is complete the solder joints looked good.  In some 
experiments they were slightly rough.

Reliability appeared to depend on peak temperature and the final use 
environment.  Joints made at higher temperatures (230C) were stronger than 
those made at 220C.  However, the intermetalic area is somewhat thicker.

Mixed alloy joints were often higher reliability for higher differential temp 
cycling and thermal shock where Lead-free joints were more reliable for lower 
temp cycling and thermal shock.

There are tradeoffs.  In general pure leaded or pure unleaded joints are more 
reliable, but mixed alloy may be - may be - much more reliable than required.  
You need to know your application environment.

Backward compatibility metals melt at 206C - 208C.  We are processing boards 
at 230C with significant success.  

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