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Created page with "====7.1.2.2 Immersion Processes==== The immersion processes are usually applied in the plating of the metals gold, silver, and tin. If the material to be coated is less precio..."
====7.1.2.2 Immersion Processes====
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 7.5: Immersion Gold Electrolytes
<table border="1" cellspacing="0" style="border-collapse:collapse"><tr><td><p class="s8">Type 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 of Electrolyte</p></td><td><p class="s8">pH-Range</p></td><td><p class="s8">Hardness</p><p class="s8">HV 0.025</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 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 - 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 Ni, Ni alloys,</p><p class="s8">Fe and Fe alloys for PCB technology and technical applications</p></td></tr></table>
====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 the
surfaces 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 inclusions
such as for example decomposition products of the reduction agents.
The electroless processes are used mainly for copper, nickel, and gold
deposits.
====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====
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 with the DODUSTAN process ''(Fig. 7.4)'' 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. 7.4: Process flow for electroless tin deposition using the DODUSTAN process
The immersion tin deposition is suitable for the production of a well solderable
surface on printed circuit boards and electronic components. It is also used as
an etch resist against ammonia based solutions or as corrosion and oxidation
protection of copper surfaces.
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 7.5: Immersion Gold Electrolytes
<table border="1" cellspacing="0" style="border-collapse:collapse"><tr><td><p class="s8">Type 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 of Electrolyte</p></td><td><p class="s8">pH-Range</p></td><td><p class="s8">Hardness</p><p class="s8">HV 0.025</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 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 - 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 Ni, Ni alloys,</p><p class="s8">Fe and Fe alloys for PCB technology and technical applications</p></td></tr></table>
====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 the
surfaces 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 inclusions
such as for example decomposition products of the reduction agents.
The electroless processes are used mainly for copper, nickel, and gold
deposits.
====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====
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 with the DODUSTAN process ''(Fig. 7.4)'' 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. 7.4: Process flow for electroless tin deposition using the DODUSTAN process
The immersion tin deposition is suitable for the production of a well solderable
surface on printed circuit boards and electronic components. It is also used as
an etch resist against ammonia based solutions or as corrosion and oxidation
protection of copper surfaces.
