Hi Chris,

I've uploaded a top-down SEM shot showing a good example of the Imm Sn wetting defect where we found excess conversion of the original unconverted Sn to Cu-Sn IMC's. Once we had cleaned the flux residues, SEM inspection showed islands of unconverted Sn on an otherwise fully converted Cu-Sn "cobblestone" background. I have some micro-section shots also, I'll post these shortly. 

http://www.themcsgroup.co.uk/images/gallery/042_immersion_tin_solderability_defect_-_non_wet_pad_surface_sem_5kx.jpg

Kind regards,

Stewart

Stewart McCracken
MCS Ltd.
Centre House
Midlothian Innovation Centre
Roslin
Midlothian
EH25 9RE
 
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: 23 November 2010 15:00
To: [log in to unmask]
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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