You might want to get in touch with a Quality Engineer from Cisco or look up on the internet all the issue with ImSn. 

As I recall, about 15 years ago Cisco bet the farm on IMSn and lost, for the reasons you stated. 
----- Original Message ----- 
From: "Chris Mahanna" <[log in to unmask]> 
To: [log in to unmask] 
Sent: Tuesday, November 23, 2010 10:00:14 AM 
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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