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Date: | Sun, 29 May 2005 09:25:09 EDT |
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Immersion plating is a process by which an ion in solution is reduced to the
metal at the expense of the substrate metal being oxidized to the ion. The
substrate gives up an electron (oxidized) and the electron is picked up by
the deposited metal (reduced).
This is a displacement reaction and is self limiting, meaning if no more
electrons are supplied the reaction would stop.
It is a fact that:
* One can get 8 –10 microinches of immersion gold on Ni (NOT
RECOMMENDE)
* Immersion tin deposits as much as 50 uins (ALL THE TIME)
* Immersion Silver thicknesses of 40 uins were measured in some
instances.
None of these processes have reducing agents. All are not electroless in
nature.
Clearly electrons continue to be supplied to the reaction after the expected
coverage was complete.
In the case of ENIG this could be the result of excessive Ni corrosion. The
electrons don’t have to be supplied at the point of deposition but may occurr
at a corrosion site within the system.
In the case to immersion tin, copper dissolution or oxidation may continue
thru intermetallic formation that occurs during the deposition process.
In the case of silver large ground plane areas may contribute and abundance
of electrons that may cause deposition on smaller pads at a faster rate than
on the larger pads. The IPC specifies 8 – 12 uins on pad size of 60 X 60 mils
or equivalent.
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