Hi Dennis,

I'm not a reliability expert myself, but from what I've read the
compatibility between SAC balls and Sn-Pb solder is questionable due to
the fact that reflow temperatures are typically lower than the 225
degrees Celcius that's needed for proper reflow and self-alignment. 
If you do increase the temperature at the lead-free BGA to above 225 °C
there will not be a reliability issue, as long as you manage to keep the
temperatures for other parts within reasonable limits. With good
profiling, a good reflow oven, and a design that permits low delta T's
you should have fine results.  
I'm sure this is a situation that many of us will have to face,
probably faster than you think !
Lead-free paste combined with PbSn BGA's are a much bigger reliability
risk due to the nature of the alloys and the intermetallics formed.

Daan Terstegge
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net


>>> [log in to unmask] 10/31/03 12:09am >>>
hi all

Just a question to all the BGA experts out there ;)

How would you tackle BGA assembly where there are both Pb Free BGAs and
the
standard Sn/Pb BGAs on the same board?
Our paste manufacture recommended we maximise the peak as far as
possible
but my concern is how would this affect the other BGAs (not Pb Free) on
the
board. Would this degrade the joint due to excess heat and possibly
longer
time above Eutectic for other components? What would be a good
compromise
peak temperature to obtain a reliable joint?

Has anyone done something like this that could share their experience.
Thank you in advance


kind Rgds
Dennis


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