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 [log in to unmask] 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. ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________