The gold standard for electrical contacts and connectors in electronics has always been gold-plating. While silver actually offers slightly higher conductivity, the noble metal gold offers superior corrosion resistance, as it does not tarnish like silver. Gold does have two significant drawbacks, however, which are interrelated. The first and most obvious one is its high cost. Secondly, gold is also a relatively soft material with limited resistance to mechanical wear. It is softer than silver and wears more easily when subject to sliding friction.

Gold plated connectors typically utilize a substrate made of a copper alloy. This is then plated with nickel layer that acts as both a diffusion and mechanical barrier. Due to the high cost of gold, it is often selectively deposited only where necessary, and in very thin layers, and the thinner the layer, the more susceptible it becomes to sliding friction.

To address these issues, Impact Coatings AB, Linkoping, Sweden, worked with ABB Ltd., Switzerland, and Scandinavian universities to develop a new coating material called MaxPhase. The goal of the development was to provide an alternative to gold-plating for connectors.

MaxPhase has similar corrosion and electrical properties as gold, but exhibits better mechanical wear properties, and has only a fraction of the material cost of gold. In addition, it uses a vacuum deposition production method that eliminates the use of wet chemical plating and its associated environmental issues.

The coating is derived from a group of ternary carbide and nitride materials called MAX phases. These materials gained attention in recent years for their unique properties, due to the work by M.W. Barsoum and co-workers at Drexel University in Pennsylvania.

In its bulk crystalline forms, many of the MAX materials are hard, yet ductile and machinable. They have good corrosion resistance and good thermal and electrical conductivity. The MAX material group consists of about 50 compounds, but the majority of the work has targeted one specific compound of titanium, silicon, carbon: Ti3SiC2.

MAX materials consist of the elements M (early transition metal), A (group A element), and X (carbon and/or nitrogen). The MaxPhase coating material is a nano-composite thin film based on titanium, silicon, and carbon, at the proportions 3-1-2. A transmission electron microscope (TEM) picture shows the film structure with nano-crystalline TiC and amorphous SiC. (See Fig. 1.)

Enlarge this picture Fig. 1. The MaxPhase coating material is a nano-composite thin film based on titanium, silicon, and carbon, at the proportions 3-1-2. A transmission electron microscope (TEM) picture shows the film structure with nano-crystalline TiC and amorphous SiC.

Methods for synthesizing bulk Ti3SiC2 are expensive. However, a method has been developed by the Scandinavian research group to form a low-temperature (less than 300 DegC) nano-composite thin film of titanium, silicon, and carbon with the same proportions: 3-1-2.

The properties of the MaxPhase film resemble the bulk Ti3SiC2. The film is harder and still more ductile than plated hard gold, with similar corrosion and contact properties, and it can be coated in large volume at a low cost. These material characteristics and the process for applying them now provide an attractive alternative to plated hard gold for connectors.

In straight case of substitution, MaxPhase replaces the gold surface material on a contact, while keeping the nickel-plated underlayer that separates it from the copper or brass substrate. The MaxPhase film is typically 0.5 micrometers to 2 micrometers and metal gray in color. Depending on the application, the coating process can be adjusted to meet different specifications. The adhesion of MaxPhase is very good on all metal surfaces.

As noted, selective plating is widely used for gold connectors in order to reduce material costs. Although cost is not an issue for MaxPhase, the coating process can still be made semi-selective to fulfill functional requirements by masking and directed deposition of MaxPhase and secondary films in the deposition system.

Tests

Close up of connectors with gold plating and MaxPhase coating.

The MaxPhase thin film coatings have exhibited good electrical characteristics in tests. In one test, MaxPhase samples with an area of 9 mm x 9 mm were pressed against a silver surface. At a contact force of 800 N, the contact resistance was 15 microohms for MaxPhase against silver and 7 microohms for silver against silver.

Mechanically, the film has a hardness of 5 GPa to 10 GPa, which is lower than other ceramic coatings. For example, TiN has a hardness of 20 GPa to 30 GPa. But 5 GPa (509.8 Hv/5,000 MPa) is still considered fairly hard, comparable with hardened steel.

The mechanical behavior of the MaxPhase film is similar to the behavior of the bulk material. The film exhibits ductile behavior; it does not exhibit brittle fracture under mechanical stress. For example, when a sample with MaxPhase film was scratched, a scanning electron microscope (SEM) image showed that the film deformed in a ductile manner, with material building up around the walls of the scratch in a “knife in butter” pattern. There were no cracks around the scratches.

The film also exhibited corrosion resistance similar to the bulk material. Samples with MaxPhase films have been tested in corrosion chambers with a mixed gas of H2S, Cl2, and NOx at 70 percent humidity for 27 days at 30 DegC. The contact resistance afterward was measured at 15 microohms; the same as before the test.

Initial qualification of MaxPhase has targeted connectors in consumer applications, especially handheld electronics. The material has been qualified by one major mobile phone manufacturer, and work is ongoing with others. Typical connectors for MaxPhase in mobile phones have contact forces below 0.5 N and required contact resistance around 30 milliohms.

Impact Coatings set up a comparative test up to investigate how mechanical wear and corrosion affected contact resistance for two types of connectors, a MaxPhase-coated connector and gold-plated connector (0.8 micrometers gold on nickel-plated bronze) for a handset application. Sets of MaxPhase and gold female connectors were mated against male gold connectors. To simulate a true environment, dust was added to the connectors before the test. In addition, the connectors were kept in an environmental chamber that tests for corrosion. Contact resistance was measured before the mating cycles, after 3,000 mating cycles, and after corrosion. The results indicated better wear resistance for the MaxPhase-coated connectors in environments where the handset would be used.

So far, testing has shown that MaxPhase thin films meet expectations on cost, technical performance, and environment friendliness in connector applications where it has been customized.

The process

MaxPhase coating can be deposited in strip format, as well as on individual contacts.

Another significant difference between a MaxPhase coating and gold plating is the manner of application. The high-volume production equipment being implemented for the MaxPhase coating uses physical vapor deposition (PVD) magnetron sputtering in a vacuum. Multiple processes can be made in sequence in the equipment, including plasma etching for cleaning and oxide removal of the surface, MaxPhase deposition, and multiple secondary coatings depending on the application.

PVD has the advantage, compared to plating, to allow coating of complex metal alloys, as well as ceramics. More importantly, it is a completely dry coating technology with no harmful substances involved, making it an environmentally safe production technology. The MaxPhase coating can be deposited in high volume production, in either strip format or on individual contacts.

The new coating has already been customized and qualified for initial low-voltage connector products for mobile phones and security products. Currently, pilot production is underway as new high-volume production equipment is implemented to coat MaxPhase reel-to-reel on connector strips.

As the reel-to-reel production coating equipment becomes available, Impact Coatings expects that the new technology can be tested for a broader array of applications, and that more manufacturers of electronics and connectors will show an interest in this alternative to gold plating.

For more information, visit: www.impactcoatings.se