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Subject:
From:
Phil Bavaro <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 31 Mar 1999 17:38:37 -0700
Content-Type:
text/plain
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text/plain (217 lines)
Janice et al,

This sounds very much like a problem we chased quite extensively a few
years ago and it turned out to be residue from the LPI process that was
virtually invisible on the Ni/Au pads before soldering.

The effect of reflowing solderpaste caused the residue to darken and
basically cause intermittent dewetting of the pad below the component
leads.  As one might guess, these components could be easily pried loose
from the board.

Handsoldering would in fact heat up the residue enough that the tin lead
could get a grip on the base metal and form a bond.

This happened on PTFE boards which we were having SMOBC applied to before
the  Ni/Au finishing.

It has been awhile but I think we finally concluded that it was "scum" from
the rinsing of the LPI that was the culprit.  Tighter process controls
finally cleared up the problem, but not until it had taken its toll on our
yields.

You are welcome to contact me direct if you need some more information
regarding this.

Phil Bavaro
------------------


At 3:31 PM -0700 3/31/99, Jim Kittel wrote:
>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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