ENiG or IAg? Which is better?

Black pad with IAg??? Tarnish is NOT black pad. Tarnish is usually just some silver sulfide on the surface and can also usually be soldered through, by using a little additional, or slightly more agressive, flux, and still make a good quality solder joint to the copper under the silver. Black pad is an ENIG feature, not an ImAG! you are soldering to the thin gold layer on top of the nickel - the nickel has oxidised underneath. There is no real bond strength between the gold solder joint and the nickel layer underneath, which is what you are SUPPOSED to be soldering to. That is why components can be just flicked or knocked off the board. I would take ImAg over ENIG any day.

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Samir, To correct some misconceptions on the answers to your question, and "what you know from experience": With both ENIG (Electroless Nickel/Immersion Gold-IAu) and IAg (immersion silver), soldering does not take place to the gold or the silver. When the molten solder contacts the gold in the case of the ENIG finish, the 3-5 uinches of gold is immediately and completely dissolved into the solder, and the molten solder then forms a bond to the nickel underplating. A small amount of nickel dissolves into the solder to form the Intermetallic Formation (IMF).

Likewise, when molten solder comes in contact with immersion silver (IAg), the 10-18 uinches of silver immediately dissolves into the molten solder, and then a small amount of the copper underneath the silver dissolves into the solder also, again forming the IMF. One does not solder to either the gold or the silver. Those noble metals are there simply to prevent the nickel from oxidizing and the copper from oxidizing. The process of ENIG plating is much more complex and harder to control than IAg, and to make a long story short, a certain amount of phosphorus (P) is required for the ENIG plating process. If there is too much P, a nickel oxide layer will form underneath the gold, on top of the nickel. This is basically the cause of the "Black Pad". This layer of oxidized nickel poses problems in that it prevents the solder from dissolving a certain amount of nickel. Standard solder fluxes have no effect on nickel oxides, they are not formulated for that purpose.

There are no black pad issues related to immersion silver finish. Copper's rate of dissolution into solder is much faster than nickel's rate of dissolution. So, when soldering to IAg finish (immersion silver over copper) you are forming an IMF with copper, and copper readily dissolves into the solder. Thus, it is far easier to form a good IMF with copper than it is with nickel. Nickel's rate of dissolution into the molten solder is much slower than that of copper. Thus, when you are soldering to ENIG, you need slightly higher temperatures and for a longer time in order to dissolve enough nickel to form a good IMF with the molten solder. For this reason, many prefer to solder to immersion silver finishes. There are also other issues with solder to ENIG I have not discussed here.

There is no reason why one cannot rework IAg solder joints. Your statement that it cannot be done is completely untrue. Solder rework is performed on both of these finishes by the thousands every single day, and if done properly there are no reliability issues with a reworked solder joint versus a virgin sj on either finish. Finishes must be chosen during the design stage for many more reasons than just solderability. Careful consideration must be given to many other field performance issues. I have written many papers on this subject that I can share with you should you desire. If you change the finish, it should only be done after a careful qualification.

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As a rework house to contract manufacturers, a majority of the BGA rework we receive is failing due to Black Pad issues. Recently we have been taking in a lot of consumer products that have parts just falling off the lead free boards that are assembled in China due to poor reflow. Just a matter of time before things start to fall out of the sky I guess. Someone should notify the US Govt. of this before they start to send any more contracts overseas. At least that's my spin on it.

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