Insoluble Anodes For The Electroplating Industry - OnBoard ...

PCBFABRICATION
Insoluble Anodes
For The Electroplating Industry
The movement towards a new
generation of insoluble anodes
is a very important step for the
future and evolution of the electroplating
industry. “Modified”
insoluble anodes (SIA) have produced
encouraging results in
testing, displaying several advantages
over “standard” insoluble
anodes and suggesting that they
may be suitable for other electroplating
applications such as
CrIII, Zinc and Zinc alloys (such
as Zn-Ni).
by Stéphane Menard,
MPC
Figure 1 - Additive consumption according to type of insoluble anode used
The use of insoluble anodes in
the electroplating industry is
widespread nowadays, especially
in precious metals applications.
However, it is less common in
base metal applications such as
the electrodeposition of Copper
in the printed circuit industry or
roller gravure processes, or the
electrodeposition of Tin in the
electronics industry.
The main reason for this is the
fact that, historically, the most
cost-effective method of metal
replenishment in large-scale industrial
applications has been the
anodic dissolution of the metal.
The use of an insoluble anode requires
a new approach to metal
replenishment, involving the
use of a metal salt, concentrated
metal solution or special separate
diaphragm cells for electrolytic
dissolution. All of these tend to
add to the overall production
cost. Anodic oxidation reactions
at the insoluble anodes also often
require the matrix of the electrolyte
to be adapted to ensure and
maintain the superior characteristics
of the deposit, especially if
a special type of current has been
applied (reverse pulse current,
for example).
Insoluble anodes – advantages and
drawbacks
The various phenomena associated
with the use of insoluble anodes
have been closely studied over
the years. We know, for example,
that with regard to the Copper applications
for printed circuits using
pulse periodic reverse (PPR)
technology, the use of platinized
Titanium insoluble anodes is not
an option (the Pt coating peels off
after a certain amount of time in
such current conditions when in
the presence of chloride in a sulphuric
acid electrolyte). The use of
an Iridium or Mixed Metal Oxide
(MMO) -coated Ti anode prevents
this problem, however, and allows
the performance benefits of the
insoluble anode technology to be
implemented successfully in this
application.
With this type of innovation and
adaptation, the use of insoluble anode
technology can be applied more
broadly. Indeed the use of insoluble
anodes has multiple advantages. To
begin with, the anode geometry remains
constant with time, allowing
current and therefore metal distribution
to be optimised and maintained.
Insoluble anodes do not need to be
filmed and are not subject to the
filming problems associated with soluble
anodes such as the build-up of
breakdown products, which can affect
both anode and bath performance.
This results in an increase in
the bath’s life. Anode maintenance
is also minimised and there is no
need to stop lines to clean and replenish
anodes, change anode bags
and re-film anodes, resulting in an
increase in productivity and a reduction
in labour costs. Insoluble
anodes also prevent potential problems
with metal particulates associated
with soluble anodes, which
threaten to come onto on the plated
substrate, creating a reject.
However, there still remain certain
drawbacks to using “standard” insoluble
anodes. The initial cost is
usually higher than with soluble
anodes, for instance, whilst the life
of insoluble anodes is dependent on
type and use. Furthermore there
is an increased consumption of organic
additives as compared with
the use of soluble anodes, as well as
the anodic oxidation of bath constituents
as a result of the nascent oxygen
produced at the anode. These
negative aspects can significantly
influence the decision to implement
insoluble anode technology or not.
OnBoard Technology June 2006 - page 22
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Developing a viable alternative
The research and development
work reported in this article was
designed to develop insoluble anodes
that could overcome these
negative aspects and provide users
with a technical and economical
alternative that would enable them
to benefit from the advantages that
this technology offers.
The first objective was to focus on
additive consumption at the anodes
and work begun with the investigation
of the acid Copper electrolytes
for PCB applications. The following
reaction took place at the insoluble
anode:
2H 2
O => O 2
↑
+ 4e - + 4H +
This reaction, which generates Oxygen
(anodic oxidation), is largely
responsible for the “overconsumption”
of organic additives at the
anode.
A large number of consumption
tests were carried out using different
types of insoluble anode
designs, with additive analysis performed
using the Titraplate CP CVS
instrument. The basic approach was
to create a physical barrier between
the anode and the working electrolyte,
essentially achieved by placing
a mesh on top of the anode. The
mesh acts as a membrane, limiting
the access of the organic substances
to the active anode surface, and
therefore also their destruction.
proved to be much more efficient,
reducing consumption by a factor
of 3. It is therefore clear that an
electrochemical phenomenon is
taking place, meaning that positively
charged compounds can be
repulsed before they reach the active
surface of the anode.
The graph in Figure 1 shows the
additive consumption data according
to the type of insoluble anodes
used. The very encouraging results
produced with acid Copper led us to
consider these “modified” insoluble
anodes (SIA) for other applications
such as Tin electroplating.
As mentioned previously, with
standard insoluble anodes we observed
a strong oxidation of the Tin
ion at the anode:
Sn 2+ => Sn 4+ + 2 e -
Table 1 compares the formation of
stannic Tin within a MSA matrix of
a pure Tin bath when using standard
insoluble anodes or the modified
insoluble anodes. The same results
were confirmed in a sulphuric
acid matrix. The principal reason
for this behaviour is the significant
modification of the Oxygen evolution
we observed at the anode using
the SIA insoluble anode. Actually, it
appears that the size and the generation
of the Oxygen bubbles at the
anode are very different from those
observed with the standard insoluble
anode.
Although the phenomenon is difficult
to quantify, it is visibly evident
during plating using the 2 different
types of anodes. Other significant
advantages of using the SIA insoluble
anodes in production have also
been documented. These include
an increase of approximately 10%
in cathode efficiency on the various
installations of the SIA anodes in
operation today, as well as a much
lower oxidative degradation effect
overall and therefore an improvement
in plating bath stability over
time. There are also fewer “bubble
entrapment” problems on applications
requiring the horizontal positioning
of the anode, above or below
the cathode (semiconductor/wafer
or roller-gravure applications), and
an actual increase in anode lifetime
thanks to the SIA concept.
Figure 2 - Example of gassing evolution using insoluble anodes in an acid
Copper electrolyte
PCBFABRICATION
Standard vs. modified insoluble
anodes
Using this approach, jointly patented
by Metakem and MPC (along
with materials used), a specific
mesh/anode configuration was developed
which significantly reduced
additive consumption as compared
with the use of “standard” insoluble
anodes. The work also showed that
this barrier is not only physical. An
electrochemical effect was also uncovered
by comparing plastic and
metallic mesh designs. Although
both configurations reduced additive
consumption, the metallic grid
Table 1 – A comparison of the formation of stannic Tin within a MSA
matrix of a pure Tin bath when using standard insoluble anodes and
modified insoluble anodes
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