Canadian Finishing & Coatings Manufacturing September/October Issue - 34

PLATING AND ANODIZING: COMPONENT FINISHING ALTERNATIVE
SHINING
S
THROUGH
Dupont launches cyanide-free silver for
industrial and electronic component
finishing
BY DAVID WAYNESS
ilver has unique properties that are well matched
to many performance requirements of electronic or
industrial components. High thermal and electrical
conductivity, brightness, high corrosion resistance and
low contact resistance fit well with the emerging and growing
markets. The unprecedented changes ongoing in the component
markets-driven by harsher operating environments, higher
speed electronics, and a sustainability agenda that limits the
use of certain substances that have been commonplace in our
industry for decades-bring demands for new process solutions.
DuPont's SILVERON GT cyanide-free electrolytes are key
additions to the company's portfolio of innovative interconnect
materials created to address these parameters. This article
provides an overview of DuPont's SILVERON GT-101 Bright
Silver and a comparison to conventional cyanide silver
electrolytes.
The alternative
Electroplated silver is a popular finish due to its attractive
appearance, excellent electrical and thermal conductivity, high
resistance to oxidation, and suitability for consumer applications
involving contact with skin. It's also much less costly than gold or
palladium.
While cyanide-based electrolytes have traditionally been
used for plating silver, cyanide-free silver-plating processes have
become available. However, because cyanide is a unique Ag(I)
ligand, developing a cyanide-free silver electrolyte that could
offer performance comparable to that of traditional cyanidebased
offerings presents unique challenges. As a result, many
cyanide-free alternatives have drawbacks that have limited their
acceptance. These include unsatisfactory deposit appearance,
electrolyte instability, difficulties with bath maintenance and
limited plating rate.
The GT-101 silver electrolyte is an improved cyanide-free
silver-plating process that has undergone production testing for
barrel, rack, and reel-to-reel plating processes. These industrial
tests demonstrate that the cyanide-free silver electrolyte is
chemically stable, provides consistent plating performance, and
results in a silver deposit that is highly suitable for numerous
low-speed and high-speed applications.
34
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The GT-101 alkaline silver electrolyte integrates the following
chemistries:
*
Silver ions
* Cyanide-free complexing agent for Ag(I)
* Organic grain refiners
* pH buffering agents
* Bath additives to extend plating capability at higher current
densities
It's important to note that the GT-101 silver maximum usable
current density is still significantly lower than what a cyanidebased
electrolyte can achieve. In electrochemical studies, the
cyanide-free complexing agents exhibited substantially increased
polarization during silver deposition compared to cyanide.
When plating at the same rate, the cyanide-free electrolyte has a
substantially higher plating potential and a pronounced negative
influence of hydrogen evolution-which ultimately reduces the
material's high-speed plating capability.
The maximum plating rate for the cyanide-free agent is 10 A/
dm2 in reel-to-reel plating applications and up to 15 A/dm2 in jet
or wire plating. Thus, a compromise in plating rate is required
when selecting a cyanide-free silver-plating process, or when
converting a cyanide-based process to a cyanide-free process.
On the other hand, one well-known technical drawback
of cyanide-based silver electrolytes is that they accumulate
carbonate during production. An electrolytic decomposition
product of cyanide, the carbonate must be removed from cyanidebased
silver electrolytes to enable longer bath life. Unlike cyanide,
the complexing agents used in GT-101 are chemically stable, so
no decomposition product formation was observed either during
development or during yearlong production testing.
Generally, soluble silver anodes (i.e., those consumed during
the plating process) are recommended for production so that
metal salts don't need to be replenished during the process. In the
few cases where using silver anode is either not possible or not
desirable, platinized titanium insoluble anodes can be used. In
this case, using a silver replenishment concentrate is necessary
to maintain the metal concentration in the electrolyte. As a
result, bath additive consumption is expected to be higher due to
oxidation reactions that occur on insoluble anodes.
Deposit characteristics vital to the successful use of SILVERON
GT-101 Silver include the following:
Appearance
The silver deposit obtained from the GT-101 silver electrolyte
is white and semi-bright to mirror bright, depending on the
substrate's initial surface condition. The appearance is very
similar to that of deposits from conventional cyanide silver
baths. For decorative purposes, where a mirror bright deposit is
required, a smooth substrate surface is highly beneficial.
Grain structure
The details of the material's grain structure can be only observed
at high magnifications, due to the extremely fine deposit structure.
The average grain size estimated from a scanning electron
microscope (SEM) analysis of the surface is approximately 50
nm.
Ductility
Defined by the degree to which a material can sustain plastic
deformation under tensile
stress before failure (cracking),
ductility is a highly desirable property for coatings in several
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