Werner,
Another theory I have heard espoused for black pad is that it occurs when there is preferential leaching of nickel into the gold plating bath from small pads electrically connected via traces and vias (sorry, pun) to larger pads. The large pads plate out properly, and first, leaving the smaller ones in contact with the relatively corrosive gold plating chemicals. This would explain why the process in localized and why the higher phosphorus concentration. However, I have no clue as to what would cause the potential difference between the big and small pads. And if people are seeing black pad with low phosphorus, this theory cannot explain that.
I agree a technique - non-destructive, quick and inexpensive (where is a fairy godmother when you need one?) is needed. ENIG is a good surface finish, but getting bitten, even at the overall average occurrence in the ppm level is NOT FUN. At a previous company we ended up having to scrap over 2000 completed circuit packs because of black pad. My problem with the invitation to participate is that it INITIALLY came from students and there was no background given in the invitation. Now it did come from Siggi's e-mail address, but I think we should have got an introduction by him of the students, if they are really interested in getting people to participate. If he is on holiday, then it should have been held off until his return. Siggi, are you there?
regards,
Bev Christian
Research in Motion
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: August 27, 2003 8:28 AM
To: [log in to unmask]
Subject: [TN] ENIG--Black Pad
Hi TechNetters,
I had hoped to get a response/discussion to my posting repeated below. I
would like to get peoples experiences to see whether the fit into scenario A or B.
Repeat:
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
may be happening is as follows [please correct me, if I got it wrong and
compare to your experience]:
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 scenarios A and B.
3A. Ni with too little P is subject to more corrosion during Au-plating--thus
"Low-Phos" EN is bad--sulfur being 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 [I have not seen
this].
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. [this is more consistent with my observations].
In either case, I do not know what one would be looking for on the
as-received PWBs.
Please give my--and the TechNet--youe feedback as to your observations; in
particular, it would be interesting to get some Fab houses to respond with their
experience.
Werner Engelmaier
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