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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">3.8 - 4also cavities and the inside of drilled holes in parts can be coated for example.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 Ni, Ni alloys,</p><p class="s8">Fe and Fe alloys In principal 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 adding reduction agents to the electrolyte is based on the metals also process related foreign inclusionsoxidation of the reducing agent with release of electrons which then in turn reduce the metal ions. To achieve a controlled deposition from such as for example decomposition products solutions the metal deposition has to happen through the catalytic influence of the reduction agentssubstrate surface.The electroless processes Otherwise a "wild" uncontrollable deposition would occur. In most cases palladium containing solutions are used mainly for the activation which seed thesurfaces with palladium and act as catalysts in the copper, and nickel, and golddepositselectrolytes.

====7.1.2The electrolytes contain besides the complex ion compounds of the metals to be deposited also stabilizers, buffer and accelerator chemicals, and a suitable reduction agent.4 Electroless Deposition of Nickel/Gold====

Electroless deposited nickel coatings with an additional immersion layer of goldThese electrolytes are seeing increased importance in the coating of printed circuit boards usually operating at elevated temperatures (PCBs50° – 90°C).The deposits contain besides the metals also process sequence is shown in ''(Fig. 7.2)'' using the related foreign inclusions such as for example decomposition products of thereduction agents. The electroless processes are used mainly for copper, nickel, and goldDODUCHEM processdeposits.

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 <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|Electroless Deposition of Nickel/Gold]]</figure>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 deposition. The electroless working chemicalnickel electrolyte contains – besides other ingredients – Sodium-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) 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.

It is possible to enhance this layer combination further by adding a immersionpalladium layer between the electroless nickel and the gold coating(DODUBOND process). This Pd layer acts as a diffusion barrier and allows theusage 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 ata 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 and the robustness of the process compared to otherelectroplating processes which reduces their wider usage ''<xr id="fig:Coating composition of a printed circuit board"/><!--(Fig. 7.3)''-->.

Fig. 7.3<figure id="fig:Coating composition of a printed circuit board">[[File:Coating composition of a printed circuit board.jpg|right|thumb|Coating compositionof a printed circuit board withreductively 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 moreprecious metal. By adding thio-urea the electro-chemical potential of copper isreduced to a level (approx. 450 mV, significantly lower than tin) that allows theexchange reaction. Using a suitable electrolyte composition and enhancersolutions like with the DODUSTAN process ''<xr id="fig:Process flow for electroless tin deposition using the DODUSTAN process"/><!--(Fig. 7.4)'' --> tin coatings can beproduced that, even under usually unfavorable conditions of copperconcentrations of 7 g/l in the electrolyte, are well solderable.

Fig<figure id="fig:Process flow for electroless tin deposition using the DODUSTAN process">[[File:Process flow for electroless tin deposition using the DODUSTAN process. 7.4: jpg|right|thumb|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.