-----Original Message-----
From: anilkher [mailto:[log in to unmask]]
Sent: Tuesday, August 26, 2003 12:51 PM
To: 'Werner Engelmaier'; TechNet E-Mail Forum.
Subject: RE: Black Pad Detecting Machine
Rather than AOI could we not use a random scan EDX for P/NI ratio under
Gold layer?? Anil Kher
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Monday, August 25, 2003 10:38 PM
Subject: Re: Black Pad Detecting Machine
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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