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====<!--7.1.2.2 Immersion Processes=-->Electroless Plating===The immersion processes are usually applied in the plating of the metals gold, silver, and tin. If the material to be coated is less precious, i.e. exhibits a negative standard potential against the metal ions in the surrounding solution, it goes into solution releasing electrons while the more precious metal ions are reduced by absorbing electrons and being deposited on the electrode. This process can continue until the complete surface of the substrate is covered with a thin layer of the more precious metal. This limits the maximum achievable layer thickness to approx. 0.1 μm ''(Table 7.5)''.

Table 7Electroless plating is defined as a coating process which is performed without the use of an external current source.5: Immersion Gold Electrolytes<table border="1" cellspacing="0" style="border-collapse:collapse"><tr><td><p class="s8">Type It allows a uniform metal coating independent of Electrolyte</p></td><td><p class="s8">pH-Range</p></td><td><p class="s8">Coating Properties</p></td><td><p class="s8">Application Ranges</p></td></tr><tr><td><p class="s8">Type the geometrical shape of Electrolyte</p></td><td><p class="s8">pH-Range</p></td><td><p class="s8">Hardness</p><p class="s8">HV 0the parts to be coated.025</p></td><td><p class="s8">Punity</p></td><td><p class="s8">Application Ranges</p></td></tr><tr><td><p class="s8">Immersion Gold Because of the very good dispersion capability of the used electrolytes</p></td><td/><td/><td/><td/></tr><tr><td><p class="s8">AUROL 4</p><p class="s8">AUROL 16</p><p class="s8">AUROL 20</p></td><td><p class="s8">3, also cavities and the inside of drilled holes in parts can be coated for example.8 - 4.2</p><p class="s8">5.8 - 6.2</p><p class="s8">5.8 - 6.2</p><p class="s8">5.8 - 6.2</p></td><td><p class="s8">60 - 80</p><p class="s8">60 - 80</p><p class="s8">60 - 80</p><p class="s8">60 - 80</p></td><td><p class="s8">99.99% Au</p><p class="s8">99.99% Au</p><p class="s8">99.99% Au</p><p class="s8">99.99% Au</p></td><td><p class="s8">Thin gold layers on NiIn principal, Ni alloys,</p><p class="s8">Fe and Fe alloys two different mechanisms are employed for PCB technology electroless plating: processes in which the carrier material serves as a reduction agent (Immersion processes) and technical applications</p></td></tr></table>those in which a reduction agent is added to the electrolyte (Electroless processes).

====<!--7.1.2.3 Electroless 2-->Immersion Processes====The electroless metal immersion processes are usually applied in the plating with adding reduction agents of the metals gold, silver and tin. If the material to be coated is less precious, i.e. exhibits a negative standard potential against the electrolyte isbased metal ions in the surrounding solution, it goes into solution releasing electrons while the more precious metal ions are reduced by absorbing electrons and being deposited on the oxidation electrode. This process can continue until the complete surface of the reducing agent substrate is covered with release a thin layer of electrons whichthen in turn reduce the more precious metal ions. To achieve a controlled deposition from suchsolutions This limits the metal deposition has maximum achievable layer thickness to happen through the catalytic influence ofthe substrate surfaceapprox. 0.1 μm. <xr id="tab:Immersion_Gold_Electrolytes"/><!--(Table 7.5)-->.

Otherwise a “wild” uncontrollable deposition would occur. In most casespalladium containing solutions are used for the activation which seed thesurfaces with palladium and act as catalysts in the copper and nickel<figtable id="tab:Immersion_Gold_Electrolytes">electrolytes<caption>'''<!--Table 7.5:-->Immersion Gold Electrolytes'''</caption>

The {| class="twocolortable" style="text-align: left; font-size: 12px"|-!Type of Electrolyte !pH-Range!colspan="2" style="text-align:center"|Coating Properties !Application Ranges|-!!!Hardness HV 0.025!Punity !|-|colspan="5" |'''Immersion Gold electrolytes contain besides the complex ion compounds of the metals to'''|-|AUROL 4<br />AUROL 16<br />AUROL 20|3.8 - 4.2<br />5.8 - 6.2<br />5.8 - 6.2<br />5.8 - 6.2|60 - 80<br />60 - 80<br />60 - 80<br />60 - 80|99.99% Au<br />99.99% Au<br />99.99% Au<br />99.99% Aube deposited also stabilizers|Thin gold layers on Ni, buffer Ni alloys, Fe and accelerator chemicals, Fe <br />alloys for PCB technology and a suitabletechnical applications|}reduction agent.</figtable>

These electrolytes are usually operating at elevated temperatures (50° – 90°C)====<!--7.1.2.3-->Electroless Processes====The deposits contain besides electroless metal plating with the metals also process related foreign inclusionssuch as for example decomposition products addition of the reduction agentsto the electrolyte, is based on the oxidation of the reducing agent with release of electrons, which then in return, reduce the metal ions.The electroless processes are used mainly for copperTo achieve a controlled deposition from such solutions, nickel, and golddepositsthe metal deposition must happen through the catalytic influence of the substrate surface.

====7Otherwise a "wild" uncontrollable deposition would occur.1In most cases, palladium containing solutions are used for the activation, which seed thesurfaces with palladium and act as catalyst in the copper and nickel electrolytes.2.4 Electroless Deposition of Nickel/Gold====

Electroless deposited nickel coatings with an additional immersion layer of goldare seeing increased importance in the coating of printed circuit boards (PCBs).The process sequence is shown in ''(Fig. 7.2)'' using electrolytes contain, besides the example complex ion compounds of theDODUCHEM processmetals to be deposited, also stabilizers, buffer and accelerator chemicals and a suitable reduction agent.

After the pre====<!--cleaning (degreasing and etching) a palladium sulfate activator isused which activates the exposed copper surfaces on the printed circuit boardand thus facilitates the nickel deposition7.1. The electroless working chemicalnickel electrolyte contains – besides other ingredients – Sodium2.4-hypophosphite,which is reduced to phosphorus in a parallel occurring process andincorporated into the nickel deposit. At the temperature of 87 – 89°C a veryhomogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is depositedwith layer thicknesses > 5 μm possible. During a consecutive processing stepa very thin and uniform layer (< 0.1 μm) Electroless Deposition of gold is added in an immersionelectrolyte. This protects the electroless nickel layer against corrosion achievinga solderable and well bondable surface for thick or fine aluminum bond wires.Nickel/Gold====

It 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 possible to enhance this layer combination further by adding 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 immersionpalladium layer between 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 gold coatingnickel 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 (DODUBOND process< 0.1 μm)of gold is added in an immersion electrolyte. This Pd protects the electroless nickel layer acts as against corrosion, achieving a diffusion barrier solderable and allows theusage of this well bondable surface combination also for gold wire bondingthick or fine aluminum bond wires.

As It is possible to enhance this layer combination further by adding an alternative, for gold wire bonding applications a thicker immersive palladium layer between the electroless nickel and the gold layer of 0.2 –coating0.5 μm can be applied using an electroless (DODUBOND process). Typical electrolytes work atThis Pd layer acts as a temperature of approx. 80°C with deposition rates of 0.3 – 0.4 μm per 30minutes. There are however limitations with these electroless electrolytesconcerning their stability diffusion barrier and allows the robustness usage of the process compared to otherelectroplating processes which reduces their wider usage ''(Fig. 7.3)''this surface combination also for gold wire bonding.

FigAs 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. 780°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 compositionof a printed circuit board withreductively enhanced gold"/><!--(Fig. 7.3)-->).

<figure id====7.1.2.5 Immersion Deposition "fig:Coating composition of Tin====a printed circuit board">A tin coating by ion exchange is usually not possible since copper is the moreprecious metal. By adding thio-urea the electro-chemical potential [[File:Coating composition of copper isreduced to a level (approxprinted circuit board. 450 mV, significantly lower than tin) that allows theexchange reaction. Using jpg|right|thumb|Figure 2: Coating composition of a suitable electrolyte composition and enhancersolutions like printed circuit board with the DODUSTAN process ''(Fig. 7.4)'' tin coatings can bereductively enhanced gold]]produced that, even under usually unfavorable conditions of copperconcentrations of 7 g</l in the electrolyte, are well solderable.figure>

Fig====<!--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. 7450 mV, significantly lower than tin) that allows the exchange reaction.4Using 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> The immersion tin deposition is suitable for the production of a well solderablesurface on printed circuit boards and electronic components. It is also used asan etch resist against ammonia based solutions or as corrosion and oxidationprotection of copper surfaces.