Janice,

I also recently experienced a pad situation similar to what you
described on a 10 layer nickel/gold immersion PWB surface.  We assembled
a prototype board for engineering using RMA paste and an
aqueous/saponifier cleaning process.  The board contained two 432 pin
BGA's, one of  which I had to replace for electrical reasons (lab power
supply voltages had been reversed).  I used an Airvac hot nitrogen gas
system to remove the BGA.  Upon removal, 90% of the pads were completely
stripped of solder and would not wet with an iron.  They appeared dark
and looked like passivated nickel.

I showed these pads to our fab PWB vendor.  He wanted to do some
checking on his nickel tanks, but thought the gold may have been too
thin, allowing passivation of the nickel under the gold, prior to
assembly.

In reworking the pads I tried a 10% solution of HCL, which didn't help
solderability.  I finally resorted to using an erasure to polish
(abraid) the pads.  Solder iron testing proved the pads readily accepted
solder.  I then screened an aggressive water soluble paste on them and
reattached a new BGA.  The process was successful.

Some day I hope to retrieve this board from engineering and remove the
second BGA.  Hopefully this pattern will exhibit the same phenomena and
I can do some cross sections and more detailed analysis.  I am concerned
as you are about long term reliability, and need to understand this
situation better.

Maybe other readers will share their experiences.

Jim Kittel

>-----Original Message-----
>From:  Janice M. Pelchat [SMTP:[log in to unmask]]
>Sent:  Tuesday, March 30, 1999 5:11 AM
>To:    [log in to unmask]
>Subject:       [TN] Ni/Au "black pad" problem
>
>We are looking for experiences with a problem that apparently manifests
>during the immersion nickel/gold process.
>
>As I understand it (pardon lack of technical expertise) the narrower fine
>pitch pads have less nickel surface than larger pads and during the
>nickel/gold exchange, there is some chemical process that results in
>entrapment between the remaining nickel and gold surface.
>
>This is not visible to the naked eye.  It survives the "tape test" at
>incoming inspection, it is not lot specific, it does not manifest as
>"blackened pads" until being exposed to the flux/reflow/cleaning process.
>
>These pads are "reworkable", but the rework requires component removal,
>slight abrasion, "retinning" of the pad, etc.
>
>We have seen this across our PWB supplier base (providing the Ni/Au
>immersion boards).  It is apparently inherent in the chemistries involved in
>the process.
>
>This is a relatively new problem for us.  Does anyone out there have any
>long term experience with this?  What about reliability issues?  Latent
>failures?  Have you determined when a board should be reworked or scrapped?
>
>
>Our major issue is that we cannot get recovery from some suppliers when this
>problem manifests.  Retooling charges for various boards is prohibitive.  We
>are exploring OSP as an alternative.
>
>Janice M. Pelchat
>SR Quality Engineer
>Benchmark Electronics
>Hudson Division
>
>Email:  [log in to unmask]
>Voice:  603-879-7000 ext 2412
>Fax:     603-879-7157
>
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