Hi Richard - your comments provide good additional ideas to keep in mind.
The folks at Celestica (Dr. Polina Snugovsky and crew) have published a
couple of papers since my paper first was released to the public which are
very good and give a better overall bandwidth discussion on the different
aspects of achieving good solder joint integrity in a mixed situation. One
issue I know that all of the "camps" include in their discussions is the
impact of the increased reflow temperature on the integrity of the
tin/lead solder process qualified components. Your "Camp C" option is a
good attempt to find a solution and balance all of the competing
advantages/disadvantages. Richard Coyle published a good paper at the 2008
SMTAI Conference with contained some excellent data on the impact of
mixing. I am hoping that we get a few more papers this year from the
industry research that show the impact of a hybrid situation in a product
type test.
Dave
"Stadem, Richard D." <[log in to unmask]>
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10/21/2008 12:03 PM
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Re: [TN] Lead Free BGA'S
I echo Dave Hillman's response, with some additional considerations. I
am of Camp C, which sort of agrees with Camp A and also Camp B. But Camp
C is very concerned with trying to reflow lead-free BGAs using a 63/37
paste and profile, albeit slightly higher, which I think is the "hybrid"
reflow profile which John's assembly house is probably using. The
concern here is that the circuit board material and the components
themselves were probably not selected and/or qualified to be subjected
to the higher reflow temperatures and longer times needed to achieve the
completely mixed (homogenous) alloy. In addition, I know of no studies
or industry data that tested this particular alloy combination.
I would have to say that if you are attempting to reflow this
combination, and are also using ENIG as a surface finish, the risks of
running into issues with the solder joint reliability, the via
reliability, and the rest of the component's reliability are stacked
against you.
I cannot say enough good things about Dave's paper on the subject. I
used it as a reference document in my paper published in this month's
Circuits Assembly magazine. I think if you read both papers you will
come away with a better understanding of the risks of assembling the
Pb-free BGAs using leaded solder. What will become very clear to you is
that you should never attempt to do this without fully qualifying the
process to ensure you are not doing something harmful to the rest of the
components and/or the circuit board itself, so you do not end up with a
huge problem later. As you can see by some of the other respondents,
that is what happened to them, and they are certainly not the only ones
that this happened to. I have been called in to look at this issue more
than once, and it gets real expensive in a hurry.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, October 21, 2008 11:23 AM
To: [log in to unmask]
Subject: Re: [TN] Lead Free BGA'S
Hi John! Well, I have a response that probably isn't what you are
looking for - " it depends" (cha-ching, Doug gets another Mt. Dew).
There are two basic camps out in the industry: Camp A: don't use a
lead-free BGA in a tin/lead solder process because you just don't know
what the resulting solder joint may be; (2) Camp B: increase the reflow
temperature so that the lead-free BGA in a tin/lead solder process can
reach a uniform distribution of Pb throughout the solder joint. Some of
the published reports also indicate that the BGA construction specifics
(i.e. material type, die size, I/O, ball metallurgy, e.g.) can play a
significant role in whether the solder joint integrity would be
compromised. The
2006/2007/2008 SMTAI Conference proceedings are good sources of
published papers on the topic. Good Luck.
Dave Hillman
Rockwell Collins
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John Foster <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
10/21/2008 10:21 AM
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[TN] Lead Free BGA'S
Our assembly house recently assembled some boards with Lead Free BGA's
for
us
and when I asked them if they used a lead free process they responded
with.
We use leaded solder (Sn63Pb37) with a hybrid oven profile to
accommodate
the lead-free BGA. I hope this answers your question. Please let me know
if you have any more questions."
Is this a common process, is this a reliable process
Any input is appreciated
Thank You
John Foster
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