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→Immersion Deposition of Tin
Electroless deposited nickel coatings with an additional immersion layer of gold gaining increased importance in the coating of printed circuit boards (PCBs). The process sequence is shown in <xr id="fig:Electroless Deposition of Nickel Gold"/><!--(Fig. 7.2)--> using the example of the DODUCHEM process.
<figure id="fig:Electroless Deposition of Nickel Gold">
[[File:Electroless Deposition of Nickel Gold.jpg|right|thumb|Figure 1: Electroless Deposition of Nickel/Gold]]
</figure>
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.
(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 (<xr id="fig:Coating composition of a printed circuit board"/><!--(Fig. 7.3)-->).
<figure id="fig:Coating composition of a printed circuit board">
[[File:Coating composition of a printed circuit board.jpg|right|thumb|Figure 2: Coating composition of a printed circuit board with reductively enhanced gold]]
</figure>
====<!--7.1.2.5-->Immersion Deposition of Tin====
A tin coating by ion exchange is usually not possible, since copper is the more precious metal. By adding thio-urea the electro-chemical potential of copper is reduced to a level (approx. 450 mV, significantly lower than tin) that allows the exchange reaction. Using a suitable electrolyte composition and enhancer solutions like within the DODUSTAN process (<xr id="fig:Process flow for electroless tin deposition using the DODUSTAN process"/><!--(Fig. 7.4)--> ). The tin coatings produced in that way, even under usually unfavorable conditions of copper concentrations of 7 g/l in the electrolyte, are well solderable.
<figure id="fig:Process flow for electroless tin deposition using the DODUSTAN process">
[[File:Process flow for electroless tin deposition using the DODUSTAN process.jpg|right|thumb|Figure 3: Process flow for electroless tin deposition using the DODUSTAN process]]
</figure>