Bev,
The problem I encountered was during removal/replacement of the
component. At that point the gold had already dissolved into the
tin/lead paste during initial soldering. The metal remaining on the
pad was the nickel and it was not solderable. The abrasion I did was on
the nickel oxide in preparation for re-attaching the new component.
Jim Kittel
>-----Original Message-----
>From: Bev Christian [SMTP:[log in to unmask]]
>Sent: Wednesday, March 31, 1999 1:12 PM
>To: [log in to unmask]
>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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