Hallo Elisabeth and Christian,
Yes, a method to detect problematic ENIG prior to assembly would be very
useful--my problem is, that I do not know what such a method would be looking for.
From my experience [but no research other than what other people write], what
is happening is as follows [please correct me, if I got it wrong]:
1. during the Ni plating, P is co-deposited,
2. the amount of P varies with the plating bath/chemistry,
At this point there seems to be some divirging opinions--maybe two different
processes--I split them up into paths A and B.
3A. Ni with too little P is subject to more corrosion during Au-plating--thus
"Low-Phos" ENi is bad--sulfur dragged into the EN bath by solder mask also is
implicated.
4A. A "loosely structured" nickel layer enriched in phosphorous is formed
under the Au layer; this is sometimes called black-line nickel (BLN).
5A. Reflow soldering fails to form a consistent Sn/Ni IMC layer and a good
metallurgical bond.
6A. "Brittle" interfacial fractures occur when loads are applied.
CONSEQUENCE: The whole PCB surface would have this condition.
or
3B. During reflow, the Ni is consumed by the formation of the Sn/Ni IMC layer
leaving P behind--the higher the P-content in Ni, the more Ni concentration
at the interface between Ni and Sn/Ni IMC--thus "High-Phos" EN is bad.
4B. The more heat [temperature x time] is available for IMC formation, the
higher the P concentration.
5B. Nickel phosphide is formed creating a "weak link" adhesion between the Ni
and the Sn/Ni IMC, even though good wetting has occurred.
6B. "Brittle" interfacial fractures occur when loads are applied.
CONSEQUENCE: The "Black Pad" condition is worse in high peak reflow profile
areas--thus it is seemingly "localised" on the PCB; and typically the soldering
pad has a crater-like rim of fractured ductile solder--rather than the
"brittle" interfacial failure, perhaps because the P-concentration is less at the
pad/ENIG area periphery.

In either case, I do not know what one would be looking for on the
as-received PWBs.

Werner Engelmaier

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