George,
        What are the other reasons ENIG has solderability issues?

        I have an ENIG board that will initially wet (with an iron), then
dewets after a pass through the reflow oven. The dewetting only occurs on a
handful of BGA pads.  The dewetted pads do appear black.  Not that it is
"black pad", rather, the surface topology is rough and doesn't reflect
light.

        I am very familiar with the papers Jarmo mentioned, but I am not a
board fab guy, so I don't know all the issues.  Any ideas?

Thanks

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]


> -----Original Message-----
> From: George Milad [SMTP:[log in to unmask]]
> Sent: Wednesday, May 31, 2000 1:24 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Black nickel barrier problem w/immersion Ni/Ag
>
> The Black Nickel issue is a very low level defect (ppm), with big
> ramifications when it is encountered.  In absence of a good understanding
> of
> how this occurs, it has become a  "Catch all" for any solderability issues
> associated with ENIG.  Many defects that may have other explanations go un
> investigated once they are labeled "Black Nickel".
>
> The immersion gold step is a controlled corrosion reaction.   Occasionally
> this could go out of control and may create a  "Black Ni" pad, under the
> immergent Au.  This gold will have a tendency to peel if tape tested,
> depending on the degree of severity of the corrosion. The underlying  Ni
> is
> compromised and may not wet.
>
> Some of the ways that this can occur are:
> Uneven Ni deposit:
>     An uneven Cu surface (pitted, corroded etc)
>     Solder mask residues on the incoming Cu surface
>     Cu/tin intermetallics as above
>     Uneven catalyzation.  Catalyst bath operated out of control.
>     Ni bath chemistry out of the specified operating range
>
> Gold bath too aggressive:
>     Excessive dwell time in the Au bath
>     High Temperature (outside the control limits)
>     Au bath chemistry out of specified range.
>
> Most board shops today are fully aware of this condition and are making
> sure
> that their ENIG processes are running within the specified vendor
> recommendations.
> George Milad
> Shipley
>
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