However, in spite of many advantages over conventional solder levelling
it has only been
adopted for certain application areas.
Nickel gold is utilized in particular where PCB surfaces are exposed to aggressive
environments,
for instance in mobile phones, or automobile electronics.
Due to the consistent chemical precipitation of metal, gold is the obvious choice for all
fine-pitch
assemblings with under 0,5 mm grid spaces.
Due to its smooth excellence, the immersion nickel gold surface is a better alternative to
HAL,
particularly for narrow SMD grids on both side of PCB`s.
The actual wiring of a component is effected by means of a 4-6 my thick layer of nickel,
and
not by means of the extremely fine 0,05 to 0,2 my layer of gold which merely serves
as an oxidation
and diffusion blockage in order to extend the soldering conditions.
Thicker gold layers, up to 0,3 my, are only targeted for thermosonic wire bondings using gold
wire.
Procedures and processes
Not only the expense of the material makes a high gold plating
undesirable; the problem
of soldering bath pollution is also a reason for as thin a coating as
possible.
It is not only the use of a precious metal, gold, which increases the price but especially
the
processing costs. The number of machine hours alone is far higher than for solder
levelling as
the facilities required are similarly complex to those for galvanization, which
additionally
require a complex waste water processing infrastructure.
The immersion nickel gold process also
requires staff intensive attention as the
chemical baths call for a great deal of analysis and
supervision due to the narrow
processing slit. The process of solder mask coating as well as deoxidation
of bare
copper surfaces (activation) must perfectly match the gold plating.
To prevent the risk of poor
metallic deposition on the gold surface, vias must be
completely free of solder mask coating or
be completely closed by means of a special
filling process. Curing of the solder mask coating must
also concur with that of the
chemical gold plating as it is exposed to hydrogen in the 90° C
nickel baths.
One has the choice between an expensive coat-ing system or a subsequent infrared
solidification which, in turn, also calls for further investment in facilities.
The potential for a lead-free alternative
For a lead free altenative provided the PCB manufacturer ensures
optimal conditions for
immersion nickel gold coating, the results of wave soldering are equally
good to solder
levelling.
Good storage ability is also achieved by covering the pad and smd sides with gold. In our
opinion,
however, immersion nickel / gold will never become an established alternative
to HAL.
As already mentioned, the processing costs are the greatest handicap for this method.
However,
due to the lack of other alternatives in the lead-free future immersion nickel /
gold will retain
its position in the area of fine pitch and COB applications.
Cost disadvantages of the immersion nickel / gold process could be balanced, in
particular in
the production of large series (consumer goods), if the use of chip-on-board
is considered.
It is often worth purchasing “bare” chips and con-necting them directly
to the PCB with bonding wire. The amount saved on the chip casing could then pay off.
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