Skip Plating is a term used to describe the absence of Ni deposition on a
pad surface.
It is occasionally encountered in processing parts down an ENIG line. It is
not the norm, and that is why ENIG continues to be a viable surface finish
with  increasing market share.

Skipping in the Ni occurs when the palladium catalyzation on the copper
surface fails. Without the immersion Pd coating on the copper pads Ni deposition
does not initiate.

Pd is deposited on the Cu surface by an immersion exchange reaction. In  this
reaction the cupper gives up an electron and is oxidized to the Cu ion  and
goes into solution and the Pd ion in solution picks up the electron and is
reduced to the metal and deposited on the copper surface.

The times that this reaction fails to proceed is when the copper surface
carries a delta charge that interferes with this electron transfer. usually some
form of a localized galvanic setup. This charge would be shared by other pads
on  the same net.

Things that are known to cause this are:
Entrapped residual oxidant in a partially plugged via adjacent to the pad
that skips.
The presence of residual tin in the hole. Tin is used as an etch  resist and
must be stripped.

The awareness of those 2 mechanisms, would go a long way in eliminating the
problem where it occurs.

I hope this helps.

 Regards
George Milad
National Accounts Manager  for Technology
Uyemura International Corp (UIC)
249 Town Line Rd
Southington CT 06489
[log in to unmask]
Cell: (516) 901  3874

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