Depending on whom you ask, the data-center phenomenon of metal whiskering is either a relatively uncommon fluke or a crisis waiting to happen.
Whiskering is caused either by stress from a particular manufacturing technique used by makers of servers, floor tiles and other products, or by a cornucopia of factors. Some say the problem can be avoided by not using old or inexpensive materials, while others say new research is required to eliminate the threat.
Most data-center equipment manufacturers are taking measures to prevent metal whiskers -- troublesome, tiny filaments that can form on their products' zinc and tin coatings
Still, data centers with old or inexpensive materials or equipment run the risk of whiskers forming, breaking off, getting into computers and short-circuiting them. Metal whiskers may cause unusual, sporadic problems, or they may cause a data center's power supplies to short out en masse. Since Network World first covered metal whiskers in 2004, new research and environmental legislation have changed how people approach the issue.
Explaining metal whiskers
The source of metal whiskers is steel that has been electroplated with zinc or tin to prevent rusting, according to Robert Sullivan, senior consultant at the Uptime Institute. When manufacturers deposit zinc or tin through electroplating, he says, the process can introduce stresses that cause whiskering. Zinc- and tin-plated metal has been used in computers, server racks, floor tiles and the like.
Sullivan says that hot-dipped galvanizing -- a process in which metal is dipped into molten zinc or tin -- does not produce whiskers. He adds that if metal is electroplated, a powder coating process can be used to prevent whiskers from forming.
Electroplating involves immersing one metal, such as steel, in water, and running a current through the water to get another metal, such as tin or zinc, to bind to it. Hydrogen atoms from the water also can be deposited on the metal, and that can lead to what Sullivan calls "compressive stress." To relieve some stress, molecules of the metal's outer layer get pushed out in a process that Sullivan says is "like trying to empty the ocean with a kid's sand pail." Stress remains, however, so this process happens over and over, causing a tiny metal whisker to form. Sullivan says that zinc, tin, and cadmium all can produce whiskers through this process.
According to Sullivan, most manufacturers of metal tiles and the like use metals and processes that don't cause whiskering, but old or inexpensive materials may pose a risk. He says that in one data center that was less than five years old, he found metal whiskers on an economy-grade metal bar that was used to support ceiling tiles. Only the bottom of the bar was powder-coated, and zinc whiskers formed on the other sides. Sullivan says metal whiskers generally take about two years to form, although he has seen them crop up in as little as six months.
Mixing lead into tin or zinc prevents metal whiskers from forming, and new, lead-free solder could introduce a new source of whiskers. "I think we're just starting to see the tip of the iceberg on that," says Rich Hill, president of data-center cleaning company Data Clean. Sullivan disagrees: "I don't see that soldering is an exposure to the creation of either zinc or tin whiskers," he says. He adds that lead restrictions should not cause whisker problems for other parts, either. "If you're plating with a lead-zinc alloy . . . it won't grow zinc whiskers," he says, "but there are very few people that are doing that," so restrictions on the use of lead should not pose a problem.