All the issues relating to ENIG relate to process control, this being a complex process and one that is sensitive to process parameters. However, overall, the process can be correctly carried out, providing a reliable product, although this is easier to achieve on an automated line.
The main solderability issue relates to the phosphorus content in the nickel plating bath, which needs to be appropriate for the plating solutions used. If correctly carried out, solderability is no longer an issue.
There are two ways in which ENIG materials might be suspected of giving weak joints. The first is a concern that the gold will produce a brittle intermetallic with the tin and the solder. This is indeed a mechanism which can create weak joints, but this only happens when the percentage of gold in the resulting joint is 3-5%. The ENIG process produces an extremely thin layer, which contains insufficient gold for embrittlement to occur.
The second, and very real, problem relates to ‘black pad’. Again this seems to be related to the nickel layer, with certain pads becoming susceptible to corrosion. The problem from the quality point of view is that this is a spasmodic fault, which is difficult to detect by normal statistical control. However, substantial research has been carried out, indicating complex relationships to design, process and material issues, and the consensus is that this problem can be kept under control. This is certainly an area where any right-minded Quality Manager would want to check with the supplier both their understanding of the problem and the precautions taken to guard against it.