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November 2010

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From:
Stewart McCracken <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stewart McCracken <[log in to unmask]>
Date:
Tue, 23 Nov 2010 16:47:49 +0000
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Hi Chris,

I've dug-out one of the micro-section shots which I was talking about and uploaded here:

http://www.themcsgroup.co.uk/images/gallery/044_immersion_tin_pcb_solderability_defect_-_microsection_pcb_pad_-_sem_10kx.jpg 

Shows that in this case, much of the pad was fully converted to Cu-Sn IMC with "islands" of thicker Sn scattered across the surface. Once the ratio of oxidised Cu-Sn IMC to unconverted Sn becomes too high, overall behaviour is for solder to de-wet. Analysis of the remainder of the board found a range of conditions, not all in poor condition so plated layer consistency over the panel was a key issue. 

Kind regards,

Stewart 

Stewart McCracken
MCS Ltd.
Centre House
Midlothian Innovation Centre
Roslin
Midlothian
EH25 9RE
 
m. +44(0)7711 541735 
t.   +44(0)131 440 9090
f.   +44(0)131 440 9085
e.  [log in to unmask]
w. www.themcsgroup.co.uk
 

-----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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