A person I know who works at CALCE (www.calce.org) has spent a lot of years in studying solder joint reliability.  She told me the other day that she thinks that IAg is better than ENIG because ENIG has "black pad" problems and I should switch to IAg.  

Here are several article on ENIG problems

http://circuitsassembly.com/cms/content/view/5042/95/
 
According to George Milad and Gerard O’Brien of the IPC Plating Process subcommittee, “Black pad is a low-level nickel corrosion defect formed during the immersion gold deposition step. It takes a compromised nickel surface and prolonged dwell in the immersion gold bath for it to occur.” Black pad is well known to fabricators and EMS suppliers, and as Milad and O’Brien added, the subcommittee “was convinced that with good definition and a proper specification, the black pad phenomenon could be virtually eliminated.”

-this is not a problem IF you have a decent board vendor.  We have never seen black pad nor has our assembler except for one time (and that vendor is no longer in business).
 
The other problem is a weakness in the nickle coating as is described in this article
http://www.empf.org/empfasis/feb04/0204help.htm

When an ENIG surface finished PWB undergoes soldering, the solder must adhere to the underlying electroless nickel plate. This is because the immersion gold is so thin that all of the gold dissolves into the solder upon soldering. The solder bonds directly to the electroless nickel. 

If that nickel surface is contaminated in some way, the resulting joint will be weak and the locus of failure will be just below the surface of the nickel, leaving both nickel and phosphorous (a normal constituent of electroless nickel) on both the pad and the failed solder joint surface. 

The EDAX analysis of the failed solder joint showed the presence of nickel at both the fracture surface of the solder joint and the pad from which the solder joint had lifted (see Figures 4-1 and 4-2) This is a positive indication that contaminated nickel was the cause of the failure. 
 
Our assembler (who I have used for several decades) has not had a good experience with IAg.  A client gave them boards with silver saver paper between each board (SMT both sides). They ran half the boards (which ran fine) then sealed the rest in a dry seal bag.  Six months later they ran the rest of the boards (tried to) and the results were terrible.

Gold has been successfully used for decades (before SMT). Tektronix and HP both used gold plated PCBs and they were serviceable decades later (when repairs were needed).  Silver in the ceramic terminal strips in Tektronix scopes (I'm a former Tek and HP Field Engineer) would tarnish and require cleaning to be used.

If a board needs to be serviced or modified at some time in the future, or the runs of boards are not completed when the boards are delivered, I would never use IAg and that is why we use ENIG.

Bob Landman, President
Senior Member, IEEE PES, Reliability Society
H&L Instruments, LLC
Electro-Optics for Industry & Science
34 Post Road, PO Box 580
North Hampton, NH 03862-0580
(tel) 603-964-1818 (fax) 603-964-8881
www.hlinstruments.com

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