ENIG (thru hole ring turns black after wave soldering)

From the pic, it looks like you have a bad board; ENIG process issue. It used to be called black pad for SMT but I'm sure it's the same condition for T/H.

Any good electronic lab should be able to id the black stuff. I would also send raw boards back to the supplier along with this picture.

my2cents

Good luck.

Mufflerbearings

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Looks like "black pad" issue. More about this here:

http://www.ipc.org/feature-article.aspx?aid=Black-pad

Definitly PCB supplier need to improve the process.

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As my fellow members are saying this is a bad mfg process, you need to talk to your supplier to see if theres something contaminating the ENIG plating, this is a very critical issue you must act on it before some problems arise in the field where can be problematic to your product.

cheers

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ASPIS (Advanced Surface Protection for Improved Reliability PCB Systems) The three year Aspis ‘Research for SMEs’ project has been conducted to address problems found with the nickel gold (ENIG) solderable finishes that are used by the electronics industry to enable components to be soldered to printed circuit boards (PCBs). ENIG has several key advantages over other types of PCB solderable finishes and these are highly significant, making ENIG the preferred choice for many applications. However, while ENIG coatings have a good reputation for excellent solderability, they are also prone to a number of well known reliability problems, such as ‘black pad’, whose formation mechanisms were still poorly understood. [http://cordis.europa.eu/result/rcn/153841_en.html].

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On a previous black pad experience we soldered a wire to a fiducial and did a pull test to failure. On the black pad board the wire and solder pulled of clean leaving the fiducial on the board. On good boards the copper came off with the wire. If you have local fiducials you can get a reading near your BGA. Ultimately everything in that board lot will have the issue despite wishful thinking that something can be done to fix things. [Don Adams, Bose Corp, USA]

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As an OEM repiar tech I would run into "Black Pad" on occasion, though mostly on surface mount pads.

The only way I could remove the oxidation was to apply water soluble flux and add solder, then wick away the excess. This left me with a suitable surface to solder down parts.

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