Agree, but the solder mask adhesion to gold is not as good as on copper.
We had problems with solder mask adhesion during rework, and loosing solder
mask around pads on a large ground area or beteen the pad and the via is not
much fun when replacing BGA's.
Gaby
----- Original Message -----
From: "Tony Steinke" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, August 27, 2003 3:45 PM
Subject: Re: [TN] ENIG--Black Pad
> Werner, Bev,
>
> The soldermask process prior to ENIG plate is very critical(poor vacuum
> during exposing, developing, improper cure, poor oven ventilation,
> soldermask design, etc) and with my experience is probably a major cause
of
> the black pad syndrome. We process all ENIG boards as SMOG (soldermask
over
> gold). This eliminates alot of potential problems(yes, some may argue it
is
> a band-aid), but we have not experienced 1 panel with the black pad
> syndrome. The cost is a bit higher but it certainly out weighs a
> dissatisified customer(especially when you are building high end boards).
>
> Tony Steinke
> AIT-Atlanta Inc.
> ----- Original Message -----
> From: "Bev Christian" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, August 27, 2003 8:44 AM
> Subject: Re: [TN] ENIG--Black Pad
>
>
> 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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847-509-9700 ext.5315
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