There are some new immersion tin formulations recently announced that supposedly address the issues raised on the Technet. We have started testing and will see where it leads.
Regards Steve Kelly
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: November-24-10 7:36 AM
To: [log in to unmask]
Subject: Re: [TN] ImSn finish, BGA solderability
How polite!
Suggest to add to delicacy and short shelf life from oxidation/formation of
unsolderable intermetallics:
Batch to batch variation/Inconsistency
Inconsistency means not just between batches but worst case within batches
and on same boards.
INMHO: Best you can say for Immersion Sn is that it is a supplier dependent
product.
Regards
Mike
ü Please consider the environment before printing this e-mail
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, November 23, 2010 4:08 PM
To: [log in to unmask]
Subject: Re: [TN] ImSn finish, BGA solderability
Hi Chris - like all of the pwb surface finishes, immersion tin has both
advantages and disadvantages. One primary disadvantage is that immersion
tin is not very robust to oxidation degradation. Stewart detailed the
issue with oxidized IMCs which can be a big issue. SERA would be one
technique to determine what level of oxidation (both Sn and IMC) is on the
pads dependent on the BGA pad size. And yes, I would do a solderability
test after the pwbs have undergone one reflow pass - that thermal
excursion may be enough to push the immersion tin from solderable to
unsolderable. Good Luck.
Dave Hillman
Rockwell Collins
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Chris Mahanna <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/23/2010 09:00 AM
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Subject
[TN] ImSn finish, BGA solderability
Due to the holiday time crunch, I'm looking for some brainstorming on ImSn
solderability forensics.
I'm thinking that there may be some low hanging fruit that I'm missing
because we just have little experience with ImSn.
We've found random one-up Pb-free BGA balls not wetted to the lands.
Finish is ImSn. Basic question in my mind is Sn oxides, SnCu
intermetallics, or misprint.
Representative bare boards passed J-STD-003 solderability with flying
colors, but it is a double sided SMT. Haven't tried after reflow
simulation yet.
Both the assemblies and representative bare boards are now naturally aged,
so SERA wouldn't quantify the problem as it existed. Would it?
I've noted the coincidence of eggshell voids with the non-wet balls.
Doesn't that imply some paste/flux? Does it imply anything else? Are the
volatiles more likely to form eggshells when they are burning through Sn
oxides?
Here are some pics:
http://ipc-technet.groupsite.com/gallery/16789
Thanks!
Chris
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