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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 anegative standard potential against the metal ions in the surrounding solution, itgoes into solution releasing electrons while the more precious metal ions arereduced by absorbing electrons and being deposited on the electrode. Thisprocess can continue until the complete surface of the substrate is coveredwith a thin layer of the more precious metal. This limits the maximum achievablelayer 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>

====<!--7.1.2.3-->Electroless Processes====The electroless metal plating with adding reduction agents to the electrolyte is based on the oxidation of the reducing agent with release of electrons which then in turn reduce the metal ions. To achieve a controlled deposition from such solutions the metal deposition has to happen through the catalytic influence of the substrate surface. Otherwise a "wild" uncontrollable deposition would occur. In most cases palladium containing solutions are used for the activation which seed thesurfaces with palladium and act as catalysts in the copper and nickel electrolytes. The electrolytes contain besides the complex ion compounds of the metals to be deposited also stabilizers, buffer and accelerator chemicals, and a suitable reduction agent. These electrolytes are usually operating at elevated temperatures (50° – 90°C).The deposits contain besides the metals also process related foreign inclusionssuch as for example decomposition products of the reduction agents.The electroless processes are used mainly for copper, nickel, and gold

====<!--7.1.2.4 -->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 ''<xr id="fig:Electroless Deposition of Nickel Gold"/><!--(Fig. 7.2)'' --> using the example of theDODUCHEM process.DODUCHEM <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 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 processand 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.

After the pre-cleaning (degreasing and etching) As an alternative, for gold wire bonding applications a palladium sulfate activator isused which activates the exposed copper surfaces on the printed circuit boardand thus facilitates the nickel depositionthicker gold layer of 0.2 – 0. The 5 μm can be applied using an 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 Typical electrolytes work at a temperature of 87 – 89°C a veryhomogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is deposited80°C with layer thicknesses > 5 μm possibledeposition rates of 0. During a consecutive processing stepa very thin and uniform layer (< 3 – 0.1 4 μm) per 30 minutes. There are however limitations with these electroless electrolytes concerning their stability and the robustness of gold is added in an immersionelectrolyte. This protects the electroless nickel layer against corrosion achievingprocess compared to other electroplating processes which reduces their wider usage <xr id="fig:Coating composition of a solderable and well bondable surface for thick or fine aluminum bond wiresprinted circuit board"/><!--(Fig. 7.3)-->.

It is possible to enhance this layer combination further by adding <figure id="fig:Coating composition of a immersionprinted circuit board">palladium layer between the electroless nickel and the gold coating(DODUBOND process)[[File:Coating composition of a printed circuit board. This Pd layer acts as jpg|right|thumb|Coating composition of a diffusion barrier and allows theprinted circuit board with reductively enhanced gold]]usage of this surface combination also for gold wire bonding.</figure>

As an alternative, for gold wire bonding applications a thicker gold layer of 0====<!--7.1.2 –0.5 μm can be applied using an electroless process-->Immersion Deposition of Tin====A tin coating by ion exchange is usually not possible since copper is the more precious metal. Typical electrolytes work atBy adding thio-urea the electro-chemical potential of copper is reduced to a temperature of level (approx. 80°C with deposition rates of 0450 mV, significantly lower than tin) that allows the exchange reaction.3 – 0.4 μm per 30minutes. There are however limitations Using a suitable electrolyte composition and enhancer solutions like with these the DODUSTAN process <xr id="fig:Process flow for electroless electrolytesconcerning their stability and the robustness of tin deposition using the DODUSTAN process compared to otherelectroplating processes which reduces their wider usage ''"/><!--(Fig. 7.34)''--> tin coatings can be produced that, even under usually unfavorable conditions of copper concentrations of 7 g/l in the electrolyte, are well solderable.

Fig. 7.3<figure id="fig:Process flow for electroless tin deposition using the DODUSTAN process">Coating compositionof a printed circuit board with[[File:Process flow for electroless tin deposition using the DODUSTAN process.jpg|right|thumb|Process flow for electroless tin deposition using the DODUSTAN process]]reductively enhanced gold</figure>

====7.1.2.5 Immersion Deposition of Tin====A The immersion tin coating by ion exchange is usually not possible since copper deposition is suitable for the moreprecious metal. By adding thio-urea the electro-chemical potential production of copper isreduced to a level (approxwell solderable surface on printed circuit boards and electronic components. 450 mV, significantly lower than tin) that allows theexchange reaction. Using a suitable electrolyte composition It is also used as an etch resist against ammonia based solutions or as corrosion and enhancersolutions like with the DODUSTAN process ''(Fig. 7.4)'' tin coatings can beproduced that, even under usually unfavorable conditions oxidation protection of copperconcentrations of 7 g/l in the electrolyte, are well solderablesurfaces.