Black pad, from what I've researched and been told, is caused by electrical
imbalances in the Nickel plating process, especially if the plating bath
concentrations are wrong, ususally by not changing the chemicals in the
bath often enough. The solution is normally Phosphor Nickel, and if the
phosphor levels are too high, there is an increased risk of black pad
occurring.
The solution is to gain an understanding of the problem (Ingemar at
Eriksson was very helpful to me with this) and talk over the problem with
your fab house to assure yourself that they are capable. I attach a
relevant instruction that I include to fab houses whenever I have ENIG
boards fabricated. It may be of some use, and is intended to produce a
long-term-solderable and reliable board with the minimum risk of black pad
occurring. Note the stated phosphor levels. The Nickel layer is thicker
than some might like, but it reduces the speed at which copper
intermetallics grow through the nickel and spoil the solderability.
"Board finish shall be 5 microinches (0.12 microns) of Gold over minimum
235 microinches (6 microns) Nickel in accordance with IPC 4552. Surface
finish of copper should be 39 microinches (1 micron) or better prior to
Nickel plating and surface plating of Nickel should be 39 microinches (1
micron) or better prior to plating of gold. Phosphor levels shall be
maintained at between 6% and 8% at time of Nickel plating if using Phosphor
Nickel for plating process. Boards must not be allowed to dry between
plating processes."
Hope it helps.
Peter Duncan
"Kirsch, Clif"
<Clif.Kirsch@SYCAMO To: [log in to unmask]
RENET.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask]> Subject: Re: [TN] ENIG v Immersion Ag
10/03/01 09:43 PM
Please respond to
"TechNet E-Mail
Forum."
George,
The BGA was removed using an Air-Vac rework station. The pads varied in
appearance from untouched (as though the gold was still present) to dull
gray/black. At that point we did not place a BGA as the pads would not wet
with a hand held iron. The BGA was originally placed and reflowed in a
standard Fuji SMT line. You speak of "black pad" which I am beginning to
hear about. Can you tell me the cause? Is it inherent in the fab
construction/chemistry or in the paste reflow process?
Thanks,
-Clif
-----Original Message-----
From: Wenger, George M (George) [mailto:[log in to unmask]]
Sent: Wednesday, October 03, 2001 7:39 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG v Immersion Ag
Clif,
When you indicate that "Once the BGA is removed some of the pads
indicate they did not / will not wet", what do you mean? How was the BGA
removed (mechanically or thermally with a repair tool?) What did the
problem pads look like (were they still covered with gold or was the pad a
dull gray or black color?). Did you print solder paste on the pads before
placing and reflowing the BGA?
The answers to the above would be help assessment of the problem. If I had
to guess off the top of my head you might be experiencing a brittle
fracture
like the "Black Pad" problem rather than a soldering problem.
As for immersion silver (IAg) Lucent's been in volume production with IAg
boards since 1997. It is our surface finish of choice.
Regards,
George
George M. Wenger (609) 639-3210 [log in to unmask]
Celiant
Route 569 Carter Road PO Box 900
Hopewell, NJ 08525 Princeton, NJ 08542-0900
-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Wednesday, October 03, 2001 7:07 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG v Immersion Ag
Be aware that the wetting forces are lower and wetting too is slower on
ENIG
than with HASL or Immersion Silver, or for that matter bare copper. This is
because you are soldering to nickle, not copper. Make sure that the time
above liquidus is long enough and the that temperature under the BGA is hot
enough. You should be able to solder to the ENIG.
Our experience with Immersion Silver has been positive. I prefer it over
Immersion Tin.
But I do not have data to answer your other questions.
Guy Ramsey
Senior Lab Technician / Instructor
E-Mail: [log in to unmask] <mailto:[log in to unmask]>
Ph: (610) 362-1200 x107
Fax: (610) 362-1290
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Kirsch, Clif
Sent: Tuesday, October 02, 2001 4:45 PM
To: [log in to unmask]
Subject: [TN] ENIG v Immersion Ag
We are running into solderability issues with ENIG boards, especially under
dense (500+ pins) BGA devices. Once the BGA is removed some of the pads
indicate they did not / will not wet.
Does anyone have experience with immersion silver finished boards from a
reliability and NEBS compliant standpoint? Also are there surface
resistance
issues for test pads that are not reflowed with solder?
Clif Kirsch
Mfg Eng
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