White Bronze, Copper-Tin-Zinc Tri-metal: Expanding Applications and New Developments in a Changing Landscape

Each alloy composition is achieved by using its distinct plating process. The reason for the varying parameters is to achieve the largest operating window and avoid large deviations of the alloy composition in the daily operation. The next generation formulations of tri-metal copper-tin Miralloy^® alloys are denoted by 2850 and 2851 and can be electroplated at higher current densities and at higher maximum thicknesses.

Deposit properties

With regard to the brightness and leveling of the coatings, different degrees of leveling can be achieved. White deposits typically have brightness and retain the character of the underlying basis metal, which is usually electrodeposited copper. Brighter deposits are seen when using bright acid copper, which has better leveling than cyanide copper. Yellow layers are bright and level with an even lower porosity, and certain applications combine the leveling advantages of a yellow deposit with a white bronze overplate.

Applicable thicknesses up to 3.0 µm (120 µ-in.) for white deposits and 5.0 µm (200 µ-in.) for yellow high copper-bearing deposits are typical. Both deposits can be used as intermediate layers, e.g., in combination with precious metals over tri-metal deposits. However, the mirror white copper-tin-zinc layers, with their higher contents of tin, are predominantly used as final layers. This is based on their appealing bright white color and excellent tarnish/corrosion resistance. A long-standing application is the plating of zippers, snaps and buttons with a white tin-alloy as the final finish. These deposits replace the potentially allergenic nickel deposits that were previously in use.

Hardness and wear resistance

The copper-tin-zinc layers have a finely crystalline and homogeneous structure. The coating hardness of the white alloy at 550 VHN is between the hardness values of matte and bright nickel. Compared to tin and zinc, the Cu-Sn-Zn deposits show an improved hardness, as seen in Fig. 6. At 550 VHN, white Cu-Sn-Zn deposits exhibit a hardness close to that of a bright nickel deposit and significantly higher than copper, brass or tin or silver deposits. Increasing the copper content decreases the hardness proportionally with 350 VHN being typical at 80% copper. Improved hardness is one of the most important advantages of the alloy and has expanded its substitution for silver in both electronics and jewelry applications.

Figure 6 - Hardness of deposits of copper-tin alloys compared to single metals.

Tri-metal deposits have excellent abrasion resistance as compared to tin, zinc, silver and copper. The wear resistance of Cu-Sn-Zn deposits was measured and compared as shown in Fig. 7. The wear resistance testing was done using the Bosch-Weinmann Emery paper method (Swiss standard 6/0) / 300 g wt. The method measures the mass (mg) removal which occurs when a 300-g block laminated with Emery paper is slid across the plated deposit for 1000 strokes. A comparison to nickel, brass and silver is shown for reference, with white tri-metal positioned between nickel and brass and well ahead of silver.

Figure 7 - Wear resistance of copper-tin alloys compared to single metals (Bosch-Weinmann emery paper (Swiss standard 6/0), 300 g wt).

The test was repeated using a Taber abrasion test technique, which is very similar in concept to the Bosch-Weimann test. The results showed very similar wear indexes for the deposits, as seen in Fig. 8. Tri-metal wear resistance was far superior to silver but not as good as bright nickel.

Figure 8 - Abrasion tests of copper-tin alloys compared to bright nickel and silver deposits (Taber abrader test wt. loss in mg/1000 revolutions).

Corrosion and oxidation resistance

As with casted copper-tin (bronze) alloys, plated copper-tin-(zinc) deposits are generally known to withstand corrosion quite well. By varying the ratio between tin and copper, the contact resistance, corrosion resistance and volume resistivity can be influenced and optimized. The results of a comprehensive battery of aging tests are shown in Fig. 9 comparing changes in the volume resistance of tri-metal compared to bright nickel. The environments included exposure to a quick temperature change, salt spray, mixed gas, high temperature, damp heat and a sulfur-containing atmosphere. The data shows that tri-metal equals or outperforms bright nickel coatings in every test and environment that was evaluated. In the sulfur-containing atmosphere, the tri-metal coating was particularly effective versus bright nickel deposits.

Figure 9 - Volume resistance of copper-tin-zinc versus bright nickel.

Figure 10 shows the result of a corrosion test under sulfur-containing atmosphere (Kesternich Test, according to DIN 50018-KFW 0.2S). The test results document the superior corrosion resistance of copper-tin-alloys as compared to nickel deposits on different basis metals including iron, brass and copper. By comparing results from different basis materials (copper, brass and iron), the test also evaluates the preferred basis metal application for the plating of tin-alloys, namely the plating on copper and copper-alloys.

In addition, the deposit thickness was also varied from 0.5 to 3.0 µm (20 to 120 µ-in.) and showed good correlation of corrosion protection and deposit thickness. This test is helpful in specifying minimum thicknesses for acceptable corrosion protection, whereby the tri-metal deposit has been appropriately standardized at 3.0 μm (120 μ-in.).

The higher electrochemical potential of the tin-alloy compared to the one of iron and the large gap between both potentials can cause anodic attack of iron. If iron is to be plated, the alloy composition should be changed to a copper-tin deposit (Fig. 10).

Figure 10 - Corrosion test results for tin alloy and nickel plated coatings on different basis materials in a sulfur-containing atmosphere (Kesternich Test/DIN 50018-KFW 0.2S). Variation of basis material and thickness: 0.5-3.0 µm (20-120 µ-in.).

Copper-tin-zinc and solderability

Tri-metal coatings are easy to plate over and, in combination with activated fluxes, they show good solderability and weldability. The excellent anti-tarnishing properties and good corrosion resistance of the Cu-Sn-Zn layers more than compensate for the slightly higher contact resistance versus silver. Most important, the tri-metal coatings ensure an acceptable volume resistivity, even in an atmosphere containing high humidity, high heat and sulfur, thus reducing component failure rate and making Cu-Sn-Zn the preferred overplate alloy.

Functional applications of Cu-Sn and Cu-Sn-Zn deposits

Tri-metal alloy finishes represent a cost effective and dramatic improvement over previous finishes of nickel, silver and stainless steel in RF connectors, filters and components applications. These applications frequently are exposed to highly corrosive environments where oxidation and tarnishing can be a detriment. Silver, which is the Standard Process of Record (POR) does have excellent electrical properties but tends to tarnish and breaks down, potentially allowing copper-tin-zinc to surpass silver as the POR for RF connector applications. Tri-metal copper-tin zinc is able to provide better throwing power and improved electrical performance versus both bright and matte nickel, based on its diamagnetic properties, lower permeability, better screening effects and lower intermodulation distortion (RF-applications). In addition, these electrical performance advantages are gained without a significant difference in hardness, wear and abrasion characteristics as compared to nickel.

The higher mechanical properties of tri-metal layers enable Cu-Sn-Zn to better handle the mechanical torque, dynamic stress and the duty cycle required of these components. This significantly reduces the number of component failures and costly field repairs. Based on the multitude of advantages and competitive cost, Cu-Sn-Zn can be used in a variety of commercial and consumer industries, including:
• Radio frequency (RF)
• Coaxial connectors
• Microwave components
• Electrical contact applications
• High Wear Applications
• Medical
• Automotive connectors
• Consumer electronics.

Additionally, the excellent corrosion resistance of Cu-Sn layers on brass substrates is also used in the automotive industry. Examples are connectors used for the car-trailer interface (12 volt). These parts have to perform under severe environmental attack, such as chlorides used in the winter for de-icing. In this regard, copper-tin layers are superior to commonly used coatings of nickel and pure tin.

RF connector industry and passive intermodulation distortion

Passive intermodulation distortion (PIM) was of little concern to the RF connector designer some years ago. Today however, with the ever increasing frequency demands, increasing transmitter power levels and more sensitive receivers, PIM has surfaced as a serious problem for GSM, DCS, PCS and other wireless services. PIM is unwanted signal or signals generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. If a circuit has non-linear characteristics, then the frequency components will be distorted and generate a higher order of harmonic frequency components. If the frequency distortion falls within the receiving band, they can effectively block a channel, when in fact, a carrier is not actually present. Ever tightening frequency boundaries will only exacerbate this problem.

As noted earlier, tri-metal layers are non-magnetic (i.e., diamagnetic) and therefore particularly suitable for high-frequency applications. Unlike nickel, tin-alloys are diamagnetic, which is a critical feature for high frequency signal transmission of connectors in the RF telecommunications field, high power wireless telecommunications applications such as antennas, base stations and satellite communications. Figure 11 shows four examples of acid copper-plated brass RF connectors overplated with 3.0 µm (120 μ-in.) of tri-metal. Figure 12 shows an RF filter module application where 3.0 µm (120 μ-in.) of tri-metal replaces silver plating.

Figure 11 - High frequency RF connectors.

Figure 12 - High frequency RF filter module.

There are three major causes of PIM in passive devices:
1. Poor contact junctions, which fail under dynamic flexing and vibration.
2. Components made with materials that exhibit hysteresis.
3. Contamination or oxidation.
The most common and serious nonlinearities occur where the current carrying area becomes microscopically separated. This can occur because of insufficient contact pressure, irregular contact pressure and oxidation causing a metal oxide junction, and contact impurities or corrosion.

Based on their intrinsic non magnetic properties, copper-tin alloys simply avoid interference with (RF) signals as compared to nickel and stainless steel, both of which can increase distortion by 50 dBs or more. Figure 13 compares the dBm distortion (increase) that is seen when using nickel versus copper-tin-zinc or silver. The distortion f_IM3 is measured on a log scale and is ~10E²⁰ higher for nickel (~log -110 to log -130). This increased distortion is enough to overlap with nearby receiving bandwidths. By overlapping these receiving bandwidths, carrier positions become blocked or viewed as present when they actually do not exist.

Figure 13 - Intermodulation at 935/960 MHz (GSM)
f₁ = 935 MHz at 43 dBm (20 W) – 2×f₁-f₂ → f_IM3 = 910 MHz
f₂ = 960 MHz at 43 dBm (20 W) – 2×f₁-f₂ → f_IM3 = 910 MHz

At high frequencies and as a result of the “skin effect,” where the majority of electrons are conducted at the outer surface (80 µ-in.), a stable, low porosity and highly conductive metal is critical. The improved hardness and wear resistance of Cu-Sn-Zn allows for higher compression crimping, allowing Cu-Sn-Zn to outperform silver consistently in long term contact resistance reliability. Reliability under stressful vibration and dynamic flexing applications has proven Cu-Sn-Zn’s superiority as the best performing alloy for this application. Best of all, these performance features are obtained using a standard electroplating process and at a highly competitive cost.

Decorative applications of Cu-Sn and Cu-Sn-Zn deposits

A requirement for the plating industry during the last few years has been to develop an alternative to electroplated nickel for decorative applications such as fashion jewelry, accessories and watches. These articles are typically plated with a nickel deposit to avoid diffusion of the basis material through the top layer of gold. In other cases, nickel is also often used as the final coating.

If nickel comes into intense contact with the human skin, approximately 15 to 20% of women and approximately 5% of men will show an allergic reaction to nickel. The actual illness “nickel allergy” is preceded by a so-called sensitization phase. After the first contact of the allergen nickel with the immune system of the human body, e.g., during ear piercing, the metal salts combine with the protein components of the blood to form so-called haptens. The immune system identifies them as foreign and develops a counter strategy. A sensitization takes place. After repeated contact with the allergen, the actual allergic illness breaks out in the form of reddening of the skin, small blisters or inflammation.

To avoid such exposure to potentially allergenic nickel, a number of countries have taken measures to establish consumer protection against the nickel allergy. An example is the European community, which introduced the EU directive 94/27/EG. This directive with the technical standards EN 1810, EN 1811, EN 12472, was converted into national law within all countries of the European community in January 2000. Whether there are legal requirements or not, the nickel allergy issue also affects the United States and other countries.

Summary

The deposits of single-metal processes cannot be varied to a large extent. However, the plating of alloys offers the possibility of specifically tailoring the deposit by selecting the alloy metals and the alloy composition. The plating of copper-tin-zinc alloys is a perfect example of this approach. By using tin-alloys, plating requirements such as the plating of corrosion-resistant, diamagnetic or antiallergenic deposits can be fulfilled with a single alloy process. Due to the development progress of the plating of tin-alloys that has been made within the last few years, these processes are now being used for a variety of applications. These range widely from decorative parts such as zippers and fashion jewelry to functional deposits, for example the plating of automotive parts and RF-connectors.

Fields of application for tri-metal Cu-Sn-Zn coatings offer a wealth of possible applications. Copper-tin-zinc layers do not have the allergenic effect of nickel coatings. Tri-metal copper-tin-zinc therefore is an excellent alternative to nickel. It even offers possible solutions to complicated coating tasks and can also be used in combination with other metals, e.g., copper as an undercoat, or with precious metals as final layers.

Copper-tin-alloys are diamagnetic and, therefore, are applicable as deposits for high-frequency RF connector applications. Tin-alloys show a lower passive intermodulation (PIM) at high transmission frequencies versus nickel. The use of Cu-Sn-Zn alloys on brass substrates is superior to nickel in electrical interference and to silver in corrosion. Furthermore, Cu-Sn-Zn layers offer improved wear resistance and low porosity. The thickness distribution and alloy consistency of the deposit is excellent. This enables the total thickness to be reduced whereby typically consistent 2.0-3.0 µm (80-120 μ-in.) of alloy can be easily deposited.

Copper-tin alloys white bronze processes will deposit over most substrates and maybe overplated with almost every metal. We look for this alloy and process to find expanded applications and present significant cost-saving opportunities for many electronic applications. Since white bronze processes were introduced, research has continued and other alloys of bronze may now be electroplated for special jewelry applications. Yellow bronze is an 18 karat color bronze alloy containing typically 80% copper and 20% tin. This process is intended as an undercoat to very thin gold deposits. When the gold wears through, the yellow bronze deposits will exhibit the original color, whereas if white bronze or nickel were the undercoat, the lack of gold would be obvious.

Table 3 gives an overview of the performance features of Cu-Sn-Zn compared to the single metal systems which are most commonly used. Although slightly more expensive than standard commodity metals such as nickel, copper and tin, tri-metal provides key advantages in corrosion protection, mechanical and electrical properties, non-allergenic and diffusion underplate capability. Its versatility and competitive cost will clearly find expanded roles as a high performance coating.

Table 3 - Summary of tri-metal Cu-Sn-Zn properties.