White Bronze, Copper-Tin-Zinc Tri-metal: Expanding Applications and New Developments in a Changing Landscape
by Richard E DePoto and Al Gruenwald C., Uyemura & Co. Ltd., & Joerg Weber and Klaus Leyendecker, Umicore Galvanotechnik GmbH
This paper deals with the renewed interest in applications for white bronze tri-metal (Cu-Sn-Zn alloy). The increased interest is driven by several factors. Continually increasing electronic frequency demands and tightening of boundary bandwidths requires components which are corrosion resistant, non-magnetic and have higher hardness properties. The cost of precious metals has risen sharply, with silver consistently selling at over $25 per ounce. Recent research and applications developments have improved process control, ease of analytical measurement and resulting performance of tri-metal alloy electroplating. New proprietary formulation-based enhancements including a high speed version, have been introduced to expand the product usage and applications. Tri-metal “white bronze” chemical processes have historically been difficult to control and too often produced a less desirable and less capable alloy than was required. Recent applications development has improved these chemical systems, determined the most critical control parameters and control points, and standardized the preferred analytical techniques for precise alloy control. Specifically, these improved techniques allow for a preferred alloy to be plated consistently, resulting in higher performance and expanded applications in the electronic industry.
Keywords: tri-metal, copper-tin-zinc, alloy plating, electronics, white bronze
The plating of copper-tin alloys has been done for many years and is widely used for a variety of applications. The most common processes are the plating of brass for decorative purposes and the plating of copper-tin deposits for electronic components. Recently there has been an increasing requirement for high performance and specialty electrolytic deposited layers. For example, decorative layers and electronics applications now must fulfill higher technical requirements for corrosion resistance, deposit hardness and wear resistance.
The properties of single-metal deposits are fairly stable and can only be slightly enhanced. Process enhancement emphasis is restricted to improved brightener and leveling additives which improve metallurgical properties such as ductility, elongation and overall process stability. By depositing two or more metals simultaneously to form an alloy coating, we can produce properties that are not possible with single metal systems. The properties and performance can be varied by considering an unlimited number of alloying elements and alloy compositions. Therefore the properties of the deposits can be tailored to fulfill specific requirements and customized applications.
A perfect example of this approach is the plating of tin alloys, copper-tin alloys and specifically copper-tin-zinc alloys. Copper-tin alloys invented over 40 years ago are now being refined and used in a wide variety of applications from jewelry and architecture to medical and electronic connector parts. In most manufacturing process sequences, copper-tin alloys are plated over acid copper deposits, which tend to level the underlying deposit and increase the alloy coating adhesion
White bronze is not actually bronze. It is an alloy consisting of a combination of copper, tin and zinc. Tri-metal alloys are white in color, similar to bright nickel, silver or rhodium and are extremely resistant to tarnish and corrosion. The alloy range is centered around 55% copper, 30% tin and 15% zinc.
The impetus to develop processes for this deposit alloy initially came from the "nickel-free" legislation first introduced about 15 years ago and was intended to address costume jewelry. Nickel-free requirements have now crossed over into clothing fasteners, car interior trim and numerous commercial and consumer electronic applications. With nickel being banned from any potential skin contact application in the European Union, tri-metal is considered the preferred, practical and safe alternative for items that can come into contact with the skin. Staggeringly, 15% of the population today is estimated to have an allergic reaction to nickel as opposed to only 10% in the 1980s suggesting nickel-related allergies appear to be on the rise. Tri-metal white bronze eliminates this problem and has been proven to be a cost effective and safe alternative.
Because of its appearance and chemical properties - being highly resistant to corrosion and wear, being solderable, non-magnetic, smooth and non-porous - tri-metal is an ideal substitute for nickel and silver for high frequency RF connector and other electronic applications. The bright white finish of tri-metal plating can also be used as an undercoat with palladium, palladium-nickel, silver or gold products or as a topcoat with those finishes. Tri-metal plating creates a non-toxic, non-magnetic deposit that is highly resistant to corrosion. This metal finishing application and deposit has low porosity and a low coefficient of friction. Lead-free tri-metal plating is outstanding for solder applications.
This paper focuses on our work on a recent advance in tri-metal plating, a proprietary copper-tin-zinc alloy** which meets the requirements for a robust nickel substitute, among many other advantages. We discuss here both operational properties as well as several of the performance properties required in the many applications noted above.
The plating process is a cyanide-based plating solution, with organic additives that give excellent brightness and some degree of leveling, even with thicknesses of only 2 or 3 µm. The chemistry operates in either rack or barrel mode and is suited to deposition on steel, brass, copper or zinc diecast basis material. An important feature of tri-metal chemistry is the throwing power of the process. The process is sufficiently capable to plate the same thickness inside a zip fastener slider as the outside of the fastener. Plating can be accomplished even in a barrel application. This throwing power feature, combined with electrical wear resistance, non-tarnishing properties and low cost has made white bronze popular with companies who previously would probably have used silver with an anti-tarnish coating. The process and the chemistry is relatively straightforward, well understood and uses standard analytical capabilities. Baths routinely have a very long life. In fact, depending on the manufacturer, many bath solutions in use today are seven or eight years old and still performing well.
One of the few procedural adjustments recommended when running a tri-metal production process is the frequency of analytical measurement as compared to single metal systems. Since our goal is to control the plated alloy ratio tightly, keeping the corresponding ratio of the individual metal concentrations consistent is a critical characteristic. As one plates the target alloy, the metal concentrations will drop accordingly and keep the metal ratio in balance. Although this is a distinct advantage, slight adjustments are always part of well-run processes. Metals are added on a prescribed schedule based on ampere-hours of plating and are easy to calculate and very predictable. Specific metal chelaters and complexers such as hydroxide, cyanide and organic additives, are also consumed according to ampere-hours and time-related decomposition and have to be analyzed more frequently. Typical chemical additions and analyses are made on a shift basis and at startup but soon become predictable and well-understood methods.
The basic process sequence is depicted in Fig. 1 below and is very similar to single metal plating processes that exist in any plating facility. Pre-treatment steps are comprised of a soak cleaner followed by a standard reverse electrocleaning step. This step is followed by an acid activation before a standard cyanide copper strike. In some processes, acid copper is used because of its higher leveling capability and the fact that tri-metal plating tends to duplicate the underlying plated surfaces more closely.