It is almost certainly skip plating.

Problem here is that skip plating takes two forms, the one you can see
(that you describe) where there is no nickel under the gold, the surface
effect you are seeing being that the gold is dissolving into the copper
due to no nickel barrier layer, and the one you can't see where the
nickel deposit is very thin and has been hyper corroded by the gold
bath.

Starting point for defect root causes to look at are as follows:

1       Examine attached via holes and see if they are blocked with
debris or mask as this can cause the issue due to insufficient fluid
turnover in the via.

2       Section the attached via hole on the affected pad and look for
traces of residual tin using SEM EDX.

3       Finally confirm on a board showing the problem before using the
eraser, that there is little or no nickel on the pad compared to the
unaffected pads around it.

This can be done using standard PCB house XRF.

And finally the acid test - would I allow the use of this board in an
assembly that was going to go into my wife's car braking
circuits................. No way, I wouldn't use them.

Kind regards,

John Burke



John Burke
Senior Manager - Operations , Optichron
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of sumxp
Sent: Friday, January 13, 2006 7:48 AM
To: [log in to unmask]
Subject: [TN] enig bad board

Hi,

We noticed few ENIG bga pads black in color instead of gold.It turn
silver
when use eraser to remove away the black color pad.What is the defect
mode?
Any reliability issue?Is it advisable to continue using these pcbs?

Thanks......

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