Main Articel: [[Electroless Plating| Electroless Plating]]
====7.1.2.4 Electroless Deposition of Nickel/Gold====
Electroless deposited nickel coatings with an additional immersion layer of gold
are seeing increased importance in the coating of printed circuit boards (PCBs).
The process sequence is shown in ''(Fig. 7.2)'' using the example of the
DODUCHEM process.
Tabelle
After the pre-cleaning (degreasing and etching) a palladium sulfate activator is
used which activates the exposed copper surfaces on the printed circuit board
and thus facilitates the nickel deposition. The electroless working chemical
nickel electrolyte contains – besides other ingredients – Sodium-hypophosphite,
which is reduced to phosphorus in a parallel occurring process and
incorporated into the nickel deposit. At the temperature of 87 – 89°C a very
homogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is deposited
with layer thicknesses > 5 μm possible. During a consecutive processing step
a very thin and uniform layer (< 0.1 μm) of gold is added in an immersion
electrolyte. This protects the electroless nickel layer against corrosion achieving
a solderable and well bondable surface for thick or fine aluminum bond wires.
It is possible to enhance this layer combination further by adding a immersion
palladium layer between the electroless nickel and the gold coating
(DODUBOND process). This Pd layer acts as a diffusion barrier and allows the
usage of this surface combination also for gold wire bonding.
As an alternative, for gold wire bonding applications a thicker gold layer of 0.2 –
0.5 μm can be applied using an electroless process. Typical electrolytes work at
a temperature of approx. 80°C with deposition rates of 0.3 – 0.4 μm per 30
minutes. There are however limitations with these electroless electrolytes
concerning their stability and the robustness of the process compared to other
electroplating processes which reduces their wider usage ''(Fig. 7.3)''.
Fig. 7.3:
Coating composition
of a printed circuit board with
reductively enhanced gold
====7.1.2.5 Immersion Deposition of Tin====