Hi Technet - just a sidenote to Bev's comment - you would be "amazed" at
the amount of embedded eraser particles the eraser use leaves on a surface.
Metallurgists have been using the "pearl pink eraser analysis" technique
for a long time as one method of determining if a the problem area has a
surface only or subsurface contaminate. However, the use of an eraser was
never intended as a repair process or rework method - an FTIR analysis of
an erased surface is very disturbing.
Dave Hillman
Rockwell Collins
[log in to unmask]
Bev Christian <[log in to unmask]> on 03/31/99 02:11:50 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Bev Christian <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [TN] Ni/Au "black pad" problem
Jim,
If we are talking about a problem that is occurring at the gold/nickel
interface I do not understand how cleaning the top surface of the gold with
an eraser is going to help you soldering. Please explain.
regards,
Bev Christian
Nortel Networks
PS Yes, we are experiencing this problem. Rarely, but when it happens it
is
a BIG mess.
> -----Original Message-----
> From: Jim Kittel [SMTP:[log in to unmask]]
> Sent: Wednesday, March 31, 1999 8:50 AM
> To: [log in to unmask]
> Subject: Re: [TN] Ni/Au "black pad" problem
>
> 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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