Hi Julie - there is a very good paper in the Journal of Electronic 
Materials (JEM) covering the issue of gold dissolution and formation of 
AuSn4 IMC due to the presence of nickel (back in 2010 I think). Before 
ENIG was introduced as a printed circuit board finish, the rule of thumb 
was if the gold was adequately dispersed in the solder joint then there 
was not chance of forming the AuSn4 IMC  as long as the overall gold 
content of the solder joint was keep below the 5% range. As you mentioned, 
the Gibbs free energy is such that when there is nickel plating layer 
present, the metallurgical kinetics favor the formation of the AuSn4 IMC 
rather than keeping the gold distributed. So our old rule of thumb needs 
some modification when using ENIG - basically smaller solder joints could 
be more prone to having a gold embrittlement issue than previously 
thought. One option for the QFN - procure them with a tin finish rather 
than a gold finish to avoid the pretinning issues.

Dave Hillman
Rockwell Collins
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From:   Julie Silk <[log in to unmask]>
To:     <[log in to unmask]>
Date:   05/29/2013 06:17 PM
Subject:        Re: [TN] Gold Removal
Sent by:        TechNet <[log in to unmask]>



A few other points:
We have found that the gold-tin intermetallics that form when the gold 
plating dissolves into the bulk solder will migrate to the nickel plating. 
 They seek out the nickel, which lowers the energy of the compound when it 
dissolves into the gold-tin, and since nickel doesn't move very fast into 
the bulk, the gold-tin moves to the interface and you'll have (Au, Ni)Sn4. 
 This gold-tin layer itself is what's brittle, not so much the gaps left 
behind from when it moved.  At least that's what we saw.  We also saw that 
when there is copper on one side of the joint (no Ni plating), the AuSn4 
didn't move into a continuous brittle layer.  The AuSn4 became (Au, Cu, 
Ni)Sn4 --  the theory is that the Cu is taken in where Ni would have gone. 
 Cu migrates faster, so is more available in the bulk.  So, you can get 
away with higher Au content when you don't have nickel plating on both 
sides of the joint.  But not too much more Au.  You should still be 
worried above 5 wt%

What I want to know is if anyone has successfully removed gold from a QFN 
without the ground pad being too lumpy to solder or the part being fried 
from excessive heat.




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