Hi Glen - just one comment to add to your set of 
considerations/observations.  One possible disadvantage to increasing the 
tin/lead soldering process temperature in an effort to produce a more 
homogeneous solder joint microstructure is the potential impact on the 
other components on the assembly and the assembly laminate itself. 
Component MSL (moisture sensitivity levels) are very dependent on the 
soldering process peak temperature and some laminate characteristics can 
be impacted by the soldering process peak temperature. As you detailed, in 
some use environments and with specific material sets, the impact is 
minimal and the assessed risk low. In other situations, the impact could 
be significant.  As Doug Pauls exclaims "it depends" and having objective 
evidence that the impact is low is a good practice.

Dave Hillman
Rockwell Collins
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Re: [TN] BGA's and lead free






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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