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OEM Report: Automotive
Tuesday | 11 January, 2011 | 8:34 am

Coating with tin-silver

By Meghan Boyer

December 2010- Precious-metal coatings in the automotive industry perform well in different applications, but the cost of using them can be prohibitive. This has led some companies to use tin-silver, a replacement coating for pure-silver-plated automotive terminals and an alternative to gold for certain uses. Using it can help companies reduce costs while continuing to meet industry requirements.

FCI began researching tin-silver as a possibility for its products because it has a higher melting point than pure tin, says Glen Sparks, a manager in FCI’s Product-Evaluation Laboratory. FCI is a manufacturer of harness connectors for the automotive industry with U.S. Motorized Vehicle Division headquarters in Novi, Mich. Its global headquarters location is in Guyancourt, France.

It’s important not to exceed 125 degrees Celsius in applications using tin because it will soften at contact areas and the asperity points will weld together and then tear apart in high-vibration scenarios. Tin-silver can gain companies an additional 15 to 20 degrees Celsius versus this phenomenon, says Sparks.

Cost considerations
The tin-silver option for high-temperature, high-vibration applications is significant "because normally if you go past tin, the next step up is silver and the step up after that is gold," says Sparks. With the precious-metal options, "you are talking significant differences in cost for the plating alone just to get up that extra degrees that tin alone won’t give you that tin-silver will give you."

Marjan Inc., Waterbury, Conn., specializes in tin-silver coatings along with 100 percent pure tin and tin-lead. The family-owned company works with major mills, service centers and OEMs. Its tin-silver product is 95 percent tin and 5 percent silver. Marjan has been testing tin-silver to automotive specifications USCAR-20 and USCAR-2, which are key life tests performed on connectors, says Richard Strobel, Marjan marketing manager. Tin-silver works on all standard sizes of connectors, including 0.64 millimeters, 1.2 millimeters, 1.5 millimeters, 2.8 millimeters, 6.3 millimeters and 9.4 millimeters.

"We have successfully changed several precious-metal coatings to our tin-silver coating, which saved our customers a tremendous amount of money without any field failures," says Strobel. Many of the company’s automotive customers purchased connectors with tin plating, but "they wanted an upgrade, and they also wanted to see if there could be a reduction in costs from precious metals of pure silver and pure gold because they are so expensive," he says.

The cost of a terminal includes the tooling and die to make it, the raw material, the engineering and press time--which means the coating material is only a fraction of the overall price, says Jim Roberts, an automotive consultant. "The coating that you put on a terminal is not meaningful [cost-wise] if you are talking about tin. If you are talking about gold, obviously, it gets really expensive," he says, noting using gold can double the price of a terminal. Using tin-silver at a 95/5 ratio only affects the cost modestly, "almost to the point where it is no cost change for adding the advantages of silver to it," says Roberts. A lot of silver is used for electrical-contact coatings in Europe, very little in Japan and almost none in the United States, where tin is the predominate contact coating, says Roberts. In the United States, some gold coatings are used in select applications. "What happened was all of a sudden we started looking at silver applications to replace some of the gold applications used in the [United States]. Gold electrical contacts are much better suited for high-temperature and dry-circuit applications than tin-coated contacts, but gold is so prohibitively expensive that we started investigating other types of materials," he says.

Tin-silver benefits
Companies can see temperature performance improvement over tin contacts as well as terminal-to-terminal insertion-force improvements by working with tin-silver, says Roberts. Adding silver to tin hardens the coating surface, which creates lower insertion forces when mating multiple-contact connectors. "It would be like the equivalent of your sled in snow or your sled on ice. The two parts will slide together easier because it’s harder when you add a little bit of silver into the tin," he says.

Lower insertion force is better for operators who install the wiring in vehicle assembly plants. Operators are susceptible to repetitive stress injuries because they may assemble 60 or 70 vehicles per hour, says Roberts. "Imagine if you held something like an extension cord between the tips of your fingers and plugged it into the wall 60 or 70 times an hour. Reducing assembly efforts reduces the chance of operator injury and improves vehicle quality because of reduced operator fatigue."

The lower insertion force can mean the difference between meeting the Newton-based criteria for mating force or not meeting it on a multiway, agrees Sparks. "Tin-silver gives you a slight advantage over a hot-air-leveled tin and especially over electro tin," for example, because the male/female contacts tend to have lower mating friction because of the surface hardness of the SnAg plating, he says.

Tin-silver also is an improvement from tin in conductivity, but pure silver and gold remain better conductors, says Strobel. Indeed, silver is the best conductor of electricity among all the metals used in electrical contacts, notes Roberts, but pure silver is expensive, too.

Tin-silver also is environmentally friendly because it doesn’t contain lead, says Strobel. "For many, many years, tin-lead was used," he says. "We have done a lot of things to get the lead out of coating products and plated products."

Being lead-free is an advantage in coating electrical contacts. Until the early 1990s, most of the terminals made for automotive wiring used a tin-lead alloy coating, says Roberts. The automotive industry, responding to environmental concerns, issued a directive to remove lead from vehicles. Removing the lead from tin electrical-contact coatings also can cause other problems.

Pure tin coatings have a tendency to grow whiskers in the grain boundaries that can cause unintended electrical paths resulting in functional failures, says Roberts. Adding a little silver to pure tin reduces the likelihood of growing these whiskers without having to add lead. "Tin-silver doesn’t grow whiskers," he says.

Companies not only domestically but globally have integrated tin-silver, says Strobel. In addition to the automotive industry, tin-silver has been used a lot in the solar industry for photovoltaic interconnect ribbons and replacing tin as a soldering application, he says.

Ultimately, tin-silver’s benefits can help companies succeed. "Any competitive advantage you can get in automotive--or anywhere for that matter--is a good thing," says Sparks. MM

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