By Beverly A. Graves, Editor

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Air bags, car telephones, cruise control, actuators, anti-lock brakes. Nearly all the systems in your automobile require connectors, the unseen components that you would miss if they weren't in place. Connector sealing systems protect the electrical connection from the influences of gases and fluids in the engine compartment. New systems such as intelligent cruise control and near-obstacle and blind-spot detection have generated a need for more sophisticated coordination of electronic components. Along with the need for a more sophisticated connector system, is an improved finish, palladium-cobalt.

AMP (Aero-Marine Products), Harrisburg, PA, a leading supplier of electrical and electronic connection devices, worked with Lucent Technologies to study and develop connector finishes. "We didn't want to develop this in a vacuum," noted Dr. Joseph Abys of Lucent. "So we talked to one of our larger customers, AMP. It had the capability to run more connectors than Lucent ever could."

AMP and Lucent's studies indicated that contacts finished with palladium-cobalt provide better performance than palladium-nickel plated connectors at about the same cost. According to Dr. James Sykes, director of AMP Global Technology, the company sees palladium-cobalt as the next generation of connector contact finishes. "Our research data indicate that when properly applied, palladium-cobalt exhibits a surface hardness in the range of 500-600 Knoop hardness, which is about two times greater than that of palladium-nickel alloys and three times greater than gold. We have also seen a reduction in coefficients of friction when compared to other coating systems," he stated.

In connector applications where high durability is a requirement, the separable contact interface must be constructed with a specially designed finish. Coatings that are extremely hard and resistant to sliding wear are one element of the solution. Other factors contribute to the coating's high durability such as thickness, plated surface finish, etc. Reductions in coefficients of friction result in lower insertion forces for a given contact design; a growing concern as connectors are miniaturized in overall size while increasing in actual pincount.

Figure 1: Palladium-cobalt thermal stability.

Compared to hard gold and pure palladium, palladium alloys demonstrate superior material properties. Palladium-nickel deposits are harder, brighter and more ductile than hard gold. They also have lower porosity and greater wear resistance than hard gold.

There are disadvantages to using the palladium-nickel as a contact finish, however, because it is electroplated on a nickel sublayer, making it difficult to accurately measure composition and thickness using non-destructive x-ray fluorescence analysis since there is no differentiation between the nickel and the palladium-nickel layer. This is a quality control issue.

Because of this, Lucent Technologies developed an alloy that would have the positive attributes of palladium-nickel without the deficiencies. In testing, AMP and Lucent found that palladium-cobalt eliminates the problems encountered with palladium-nickel. It also reduces the potential for precious metal losses, and composition and thickness can be maintained.

AMP noted that pore corrosion was responsible for most connector corrosion failures. Because of this, the porosity of electroplated metals and alloys was studied. Connector pins and coupons were plated with hard gold, palladium-nickel and palladium-cobalt both with and without a gold flash. The sulfurous acid vapor method (ASTM B799-88) was used to measure porosity. The number and size of pore sites visible at 10 times magnification were measured. Palladium-cobalt had the lowest porosity.

Figure 2: Palladium-cobalt sliding-wear contact resistance.

Another factor determining long-term performance is sliding wear. This is important for systems requiring high durability and long life, such as connectors. Sliding wear performance of the hard gold, palladium-nickel and palladium-cobalt plated coupons was measured using frictional force and contact resistance.

Coefficients of friction increased for all finishes during the first 1,000 cycles. It then remained steady for approximately 10,000 cycles. Contact resistance increased to more than 10 megaohms for hard gold after 20,000 cycles. It did the same for palladium nickel after 35,000 cycles. Palladium-cobalt remained at less than 10 megaohms after 80,000 cycles of wear.

AMP and Lucent Technologies used an aging study to monitor temperature stability. Copper panels were nickel plated (2.5µm) and then coated with hard gold, palladium-nickel or palladium-cobalt (1.25µm). Half of the palladium-based finishes received a flash of gold (0.1µm). The panels were put in a 150C furnace for 40 days. Then contact resistance was measured. Hard gold had higher resistance values; however, the palladium-cobalt finishes, both gold flash and not, were more stable than the other finishes.

AMP tests demonstrated that palladium-cobalt offers both performance and processing advantages over palladium-nickel, which has stood as the cost-effective finish of choice for about 20 years. "Palladium-cobalt has greater durability," Dr. Sykes said. "Hardness has a significant impact on durability; its hardness is greater than palladium-nickel or hardened gold. Durability is especially important for connectors that undergo numerous connect-disconnect cycles during their life span."

AMP plates the copper contacts of connectors to minimize corrosion and optimize electrical contact. Existing high-performance plating systems, in order of decreasing costs, are gold, palladium and palladium-alloys. In addition, a plated nickel layer always acts as a barrier between the copper contacts and the finish layer, preventing diffusion of the copper. A thin, 1- to 5-microinches final layer of gold flash is generally recommended with all palladium systems.

Because palladium can absorb hydrogen from the plating baths, localized brittle deposits can form, leading to a tendency for cracks to develop in the contact surface. Palladium also reacts with carbon from the air to form an insulating polymer. This works well for an automotive exhaust system, but must be avoided for a connector contact.

Alloying palladium with nickel minimizes these problems and reduces plating costs even further. Nickel makes the contact more prone to corrosion and complicates thickness measurements during processing. To offset thickness uncertainty, companies often plate more metal on the contacts than necessary. Because of these conditions, AMP and Lucent Technologies developed palladium-cobalt alloy plating.