Good that you carefully qualified the process, Glen. What data do you
have on the effects on the life of the components themselves?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glen Herzog
Sent: Monday, June 18, 2007 1:58 PM
To: [log in to unmask]
Subject: 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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