Hi Phil! Take a look in the TechNet archives as this topic was ground thru
pretty well a couple of weeks back but you do have a dilemma. The majority
of the published literature shows that if you completely reflow the Pbfree
solderball or allow diffusion to create a uniform microstructure then you
will create a reliable solder joint. So the simple answer would be to just
reflow your entire assembly using a Pbfree reflow profile - but - the
moisture sensitivity levels (per JSTD-020B) for your Sn/Pb BGAs are going
to most likely decrease a couple of levels (e.g. a MSL 1 could drop to a
MSL 3). This decrease in MSL rating means you could be causing damage to
the components by using the Pbfree reflow profile. I would recommend
caution in running the Sn/Pb and Pbfree BGA components at the same reflow
profile. Many companies are "rebumping" the Pbfree solderspheres with Sn/Pb
solderspheres to avoid the issue. Good Luck.

Dave Hillman
Rockwell Collins
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             DUTTON Phil
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                                                                   Subject
             02/10/2005 10:17          [TN] Mixing Lead free BGAs with
             PM                        normal BGAs


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                DUTTON Phil
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Hello,

We've run into a problem where a PB Assembly will have a mixture of
BGAs, some with regular tin/lead solder balls, some with lead free
solder balls.
This raises the question as to what temperature profile to run. Do we
need to ask our loader to do a Pb free profile first, followed by a Pb
profile? Or will it be ok to solder both together using the regular,
lower temperature Pb profile?
Personally I'd like to avoid the extra run through reflow.

thanks,

Phil Dutton C.I.D.

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