September/October 2022

Dupot launches Cyanide-free silver for industrial and electronic component finishing

By David Wayness

Silver 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 cyanide-based 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.

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 cyanide-based 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 cyanide-based 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 applications, especially for connector applications. Tests demonstrate that the bright silver deposit can meet industrial requests regarding ductility.

Adhesion

In most applications, silver is electroplated over either copper and copper alloys, or nickel undercoats. To ensure good adhesion of silver over nickel undercoats, a thin silver strike layer is necessary prior to silver plating. For plating over nickel substrates, GT-101 Silver Strike, is employed.

Purity

Silver deposit purity is important to many end users. Unlike most cyanide-based silver electrolytes, the cyanide-free electrolyte does not contain any metallic brightening agents. Any impurities are mainly associated with co-deposited carbon originating from the organic additives used in the electrolyte. Tests confirm > 99.9 per cent silver purity in deposits from GT-101 Bright Silver.

Corrosion and tarnish resistance

An accelerated tarnish test shows that the tarnish tendency of silver from SILVERON GT-101 Silver is comparable to that of deposits from a conventional cyanide-based electrolyte. Several commercially available Cr^6+- free, organic, and inorganic anti-tarnish agents have been found to prevent tarnishing of the SILVERON GT-101 silver deposit, even under severe test conditions.

Contact resistance

Electroplated silver is an excellent contact material and has been widely used for electrical and electronic connector applications, where low contact resistance of the silver deposit is essential. A very low contact resistance (below 1 mOhm) can be achieved when contact loading is above 40 cN. This result is comparable to that obtained on silver deposits from conventional cyanide silver.

Solderability

Solder wetting performance is another important parameter. Tests performed using a wetting balance technique showed that none of the aging conditions tested (deposition on copper, dry heat aging, steam aging) have any significant impact on the solderability of the silver. Zero-crossing time (ZCT) observed at all testing conditions was well below one second, indicating that the new silver deposit has excellent solderability (Fig. 2).

Wire bonding capability

In wire bonding applications, several different types of silver finishes provide protection of copper substrates and allow formation of a strong bond between gold wire and silver-plated substrates. Several gold wire bonding and pull tests performed on wire bonding samples (prepared by plating GT-101 directly over a copper substrate) confirm that cyanide-free silver can provide a good wire-bondable surface.

Reflectance

To date, silver finishes from previously available cyanide-free electrolytes have not gained wide acceptance for LED applications – due mainly to their limited reflectance, especially after aging at high temperatures. Reflectance of GT-101 deposits was evaluated at wavelengths between 360nm and 740 nm and compared to reflectance of a silver deposit from a cyanide-based electrolyte currently used in manufacturing LED packages. Its performance at 400-450nm, the key wavelength range for LED, has initiated further evaluation of the SILVERON™ GT-101 Silver deposits for LED applications (Fig. 3).

Microhardness

Silver is a relatively soft metal, and electroplated silver will become progressively softer with time, even at room temperature. This effect is more pronounced when the deposit is exposed to elevated temperatures. After heating at 150°C for 1 hour, the microhardness of most silver deposits from cyanide-based electrolytes dropped up to 50 per cent from their original values. This characteristic is considered one of the main barriers to the use of a silver finish in many applications. The new silver deposit does not show this softening effect and has been found to be significantly harder than deposits from a conventional cyanide silver bath, especially after heat treatment.

Friction and wear

Generally, electrodeposited silver deposits have high coefficients of friction (COF) and poor wear resistance. Improving these parameters is desirable for electroplated silver finishes. Intensive investigations of silver finish wear behaviours were carried out during the development of the new silver-plating process to aid in the understanding of the failure mechanisms behind silver wear and to help improve the silver finish wear resistance. All wear tests were conducted without lubrication. No significant change in COF was observed for the new silver deposit when compared to a conventional cyanide silver deposit. COFs on both silver finishes were found to be around 1.5, which is relatively high. In comparative ball-on-plate wear tests, using silver or hard gold plated on nickel coated substrates against a stainless-steel ball with a hardness of between 600-800 HV served to reduce wear rate for the new silver deposit compared to a conventional cyanide silver deposit.

To eliminate the use of cyanide in silver electroplating solutions due to growing concerns about cyanide handling and material transport in many regions—as well as a desire to deliver more environmentally friendly materials—DuPont has developed an improved cyanide-free silver electroplating process, GT-101 Bright Silver, following several successful industrial scale evaluation tests.

 David Wayness is Finishing Marketing Leader for the DuPont Electronics and Industrial business.