Dave,
My bag is humor, you're SAC is hope. Good work. I hope you will be allowed
to share the data.
Dewey
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of David D. Hillman
Sent: Tuesday, December 07, 2004 12:16 PM
To: [log in to unmask]
Subject: Re: [LF] Transition questions, Pb to Pbfree wave soldering ??
Hi Genny! Well, since ya asked for an example! Rockwell Collins may present
a paper on the issues of having a SAC solderball in a typical Sn63
soldering reflow profile at the Pbfree Conference in Toronto in May (yes,
Bev, I know I owe ya the abstract!). We have conducted a couple of tests to
see what can happen. The issue with the SAC solderball in a Sn63 profile is
that you get a very nonuniform microstructure that, depending on your use
environment, will impact solder joint integrity. The SAC solderball never
melts so that the Sn63 solderpaste diffuses into the solderball from the
bottom up. This diffusion pattern results in lead (Pb) being segregated at
the SAC solder alloy interdendritic (aka grain boundry in a simplistic
sense) regions causing wetting defect issues. This SAC solder ball in a
Sn63 issue has already been reported in the industry literature as problem
for some use environments. Some of the published literature shows that the
SAC solderball goes molten then the nonuniform microstructure issue
resolves itself. I recommend Carol take a good look at her microsections
to determine if the solder joint integrity has been compromised. I'll send
Steve a photo to post of a cross section illustrating what happens to the
solder joint microstructure. Also, the JCAA/JGPP consortia has included
several variations of SAC solderballs/Sn63 reflow profile situation on
their test vehicles - that data will be very useful to the industry and
should be available in early 2005. Hope this helps.
Dave Hillman
Rockwell Collins
[log in to unmask]
Genny Gibbard
<Genny.Gibbard@VC
OM.COM> To
Sent by: Leadfree [log in to unmask]
<[log in to unmask] cc
>
Subject
Re: [LF] Transition questions, Pb
12/07/2004 12:40 to Pbfree wave soldering ??
PM
Please respond to
"(Leadfree
Electronics
Assembly Forum)"
<[log in to unmask]
>; Please respond
to
Genny Gibbard
<Genny.Gibbard@VC
OM.COM>
I am curious, Carol, who provided the choices you could pick from? And why
did you choose None of the Above?
We have a customer who tells us the product we make for them is exempt so
they don't want leadfree, (Server) but their lead free policy states that
leadfree BGA's are not backwards compatible with a SnPb process. So if the
BGA's we use are suddenly not available with SnPb, we will be forced to go
lead free. No choices. Even if we could provide the documentation you are
currently gathering on your BGA, I doubt they would change their mind
I have heard from many directions that Pbfree BGA's should NOT be soldered
with standard PbSn solder. Does anyone know of a good reference paper or
such that can explain clearly why?
-----Original Message-----
From: Carol Spiers [mailto:[log in to unmask]]
Sent: December 7, 2004 12:23 PM
To: [log in to unmask]
Subject: Re: [LF] Transition questions, Pb to Pbfree wave soldering ??
Thanks for the input Rick, I totally agree with you. Makes sense since the
two swipe thing would be totally inefficient (costly) as I would have to
mask of the footprints for the leaded parts, put the non leaded parts in
after the first Pbfree swipe and then wave in th SnPb. Now for my next
question.
Here's a real life scenario that I am living with. I now have a board that
is populated with SnPb parts, the component supplier has converted the BGA
to lead free. I had the choice to
a) change my process to high temp SnPb solder at smt (and adjust oven to
higher temp and assume I am still within scope of thermal limits of all
components on board) to help ensure joint reliability or
b) convert the whole smt process to lead free paste and subsequently adjust
ovens to even higher temp, (at this point not sure if thermal limits on all
parts can withstand the potentially 20 Deg C adjustment) for this product.
I opted to go with
c) ran the BGA in my current process with no adjustments.
We have completed all functional testing with no failures, but, I have sent
samples to a lab for xray and cross sectioning and comparison evaluations
to
determine long term reliability.
I haven't received the results back yet.
What would you have done?
Carol
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