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November 2001

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Subject:
From:
Dougal Stewart <[log in to unmask]>
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Date:
Thu, 15 Nov 2001 17:33:09 -0000
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I'm afraid that I have to disagree with Peter Duncan on his 'check the
phosphur content of the bath ' explanation. Phosphorus is codeposited with
the nickel at a concentration dependent on the rate of nickel deposition.
The percent phosphorus in the bath has no effect. The good processes
(compared to the ones prone to problems) control the nickel deposition rate
very carefully. The black pad is created by the immersion gold process,
which is a substitution reaction, and so has a corrosive attack on the
nickel. If the nickel deposit has too many crevices due to poor deposition,
or the gold reaction is too slow, then you end up with a phosphur rich
layer at the gold nickel interface (also known as black band), and this is
impossible to solder. Microsectioning the pad should show a 'toothache'
effect if you really do have black pad, and then you should talk to your
raw card supplier.
Dougal Stewart
email [log in to unmask]
telephone +44 1896 822204
mobile +44 7984 629667
-----Original Message-----
From:   Zhu, Xiang [SMTP:[log in to unmask]]
Sent:   15 November 2001 05:18
To:     [log in to unmask]
Subject:        [TN] ENIG-Black Pad

Hello All,
I encounter a touch problem with ENIG (Electroless Nickel / Immersion Gold)
PCB in assembly process. This problem maybe so-called "Black Pad".
This problem happen on a ten layers RCC build-up PCB's uBGA pads. The BGA
pad on PCB is 0.4mm in diameter with 0.8mm pitch. On some BGA pads, there
are 6 mil laser via connecting layer 1 and layer 2 (via in pad).
In ICT station, detect this problem as open. When I remove the BGA wit hot
air, find some pads didn't wetted with solder at all and have a dark/black
appearance. I think the surface of black pad is passivated nickel.
I don't know what kind of source caused this defect and how to prevent it.
PCB vendor or our assembly process cause this problem?
Is there anybody who ever encounter this kind of problem? Please kindly
offer me a hand.
Thank you in advance for your kindly help!


Best regards,
Zhu Xiang

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