Replacing Nickel with Tri-Metal in Electronics Plating

Q. I am using nickel in an electronics plating application, but what can you tell me about replacing it with tri-metal technology? Does it deliver the required corrosion resistance for other applications, too?

A. Tri-metal—or white bronze—is a hot topic, and here’s why:

Increasing electronic frequency demands and the tightening of boundary bandwidths require components that are corrosion-resistant and non-magnetic, with higher hardness properties.
Precious metal costs have risen sharply, with silver consistently over $20/oz.
Tri-metal process controls have improved.
New enhancements, including development of a high speed version, have expanded product applications.

Single-metal deposit properties can only be enhanced slightly. But by depositing two or more metals simultaneously as an alloy coating, properties not possible with single metal systems become possible. Examples include tin-alloys, copper-tin alloys and copper-tin-zinc alloys. Copper-tin alloys are now being used for jewelry, architectural components, medical parts and electronic connectors. Copper-tin alloys are usually plated over acid copper or standard cyanide copper, which levels the underlying deposit and increases the alloy’s adhesion.

White bronze is actually an alloy of copper, tin, and zinc, commonly referred to as tri-metal. Tri-metal alloys are white, similar to bright nickel, silver or rhodium and are extremely resistant to tarnishing and corrosion. Although tri-metal deposits are not as corrosion resistant as nickel or nickel phosphorus alloys, they do offer significantly improved protection over silver, copper and zinc. As such, tri-metal has found applications in outdoor connector and decorative applications. The target alloy composition is 55-percent copper, 30-percent tin and 15-percent zinc.

The impetus for this alloy came from the “nickel-free” legislation of 15 years ago involving costume jewelry. Nickel-free requirements now encompass clothing fasteners, car interior trim and electronic applications. With nickel banned from skin contact applications throughout the European Union, tri-metal became the preferred alternative.

Coupling tri-metal’s corrosion and wear resistance with the metal’s solderability and non-porous properties, the result is a highly-versatile deposit. This makes tri-metal an ideal replacement for nickel and silver in decorative and technical applications including high-frequency RF connectors. The bright white finish of tri-metal plating can also be used as an undercoat and diffusion barrier for palladium, palladium/nickel, silver and often gold.

The deposit has low porosity and a low coefficient of friction. Lead-free tri-metal is ideal for component leads and all soldering and welding applications.

Tri-Metal Chemistry

Tri-metal plating delivers excellent brightness and some leveling, even with thicknesses of only 2-3 microns. It operates in rack or barrel mode and is compatible with steel, brass, copper and zinc die-cast material. A key feature of tri-metal chemistry is its throwing power. The process can plate the same thickness inside a zip fastener slider as the outside of the fastener, even in a barrel application. Throwing power, combined with electrical wear resistance, non-tarnishing properties and low cost have made white bronze popular with companies who previously would have used silver with an anti-tarnish coating. The process is straightforward, and baths have a long life: Many are more than three years old and still performing well.

Processing and Control

The goal is to tightly control the plated alloy ratio, so it is critical to keep the corresponding ratio of individual metal concentrations in the bath consistent. Pretreatment includes a soak cleaner and standard reverse electro-clean step, then acid activation before a standard strike using cyanide copper, or acid copper. Tri-metal’s plating rate at 0.9 microinches per minute requires a 15-minute cycle at 5.0 ASF to deposit the preferred 120 microinches of alloy. Three or four tri-metal plating cells per bath normally balance a full production line. Alloy ratio is dependent upon current density; it favors a copper-rich alloy at high current densities and a tin-rich alloy at low current densities, so it’s essential to control plating current density. Part design, rectifier sensitivity, number and spacing of anodes, and size and distance from the plated part all impact localized part current density.

The ability of the additive system to compensate for these variations is critical in holding the target alloy and the part’s color and brightness. The newest formulations have dramatically widened the process operating window. Thickness uniformity ranges of less than 20 percent are typical of well-engineered tri-metal processes. Single-metal system thickness variation on the same connector plated with bright nickel resulted in thickness variations five times the tri-metal level. Functionally, thickness uniformity prevents over-plating, saving materials. Additionally, the process holds tolerances required for threaded and tight fitting connectors.

Deposit Properties

White deposits retain the character of the underlying base metal (usually electrodeposited copper). Brighter deposits are achievable with bright acid copper, which has better leveling than cyanide copper. Tri-metal copper-rich alloys are yellow in color and are bright and level with lower porosity. Some applications combine the leveling advantages of a yellow deposit with a white bronze overplate.

Copper-Tin-Zinc and Solderability: Tri-metal coatings are easy to plate over, and in combination with activated fluxes can be soldered and welded. The excellent anti-tarnishing properties and corrosion resistance of the tri-metal layers more than compensate for the slightly higher contact resistance of tri-metal as compared to silver.

RF Connectors and Passive Intermodulation Distortion: With increasing frequency demands, increasing transmitter power levels and more sensitive receivers, Passive Intermodulation has surfaced as a serious problem for GSM, DCS, PCS and other wireless devices.

Unlike nickel, tri-metal layers are diamagnetic (resulting in lower signal distortion) and particularly suitable for high-frequency signal transmission of connectors in RF telecommunications, high-power wireless telecommunications and applications such as antennas, base stations and satellites.

Richard DePoto is a business development manager with Uyemura International Corp. Visit uyemura.com.

Originally published in the October 2016 issue.