Issue link: http://cp.revolio.com/i/611691
22 | THE DATA CENTER JOURNAL www.datacenterjournal.com consumer suffered little impact, aside from some automobiles whose accelerator-con- trol modules were later found to contain tin whiskers. e electronics users that did suffer an impact were the operators of nuclear power plants—among them, the Millstone nuclear reactor in Millstone, Connecticut, which unexpectedly shut down because of a tin whisker. Also concerned were owners and operators of satellites that failed prematurely because of tin whis- kers. e average person noticed one of these failures when all the pagers in the country suddenly went dead. A history of these tin-whisker-caused failures is at www.nepp.nasa.gov. In addition, defense companies and the U. S. Department of Defense suffered; they needed absolutely reliable radios, vehicles, ships, planes and weapons of all sorts. Holding the great- est responsibility was the Missile Defense Agency (MDA), because failure of one of their missile-intercepting missiles could have led to the loss of a city. Pure-tin-finished parts were abso- lutely forbidden from manufacture of any of MDA products, as well as just about anything used by any defense agency, since the tendency of the tin-plated electronic components to grow tin whiskers had become very well known in the preceding 12 years. But the pure-tin-finished parts had become so pervasive, and defense use had become such a minuscule part of the entire worldwide manufacturing activity, that it was impossible to guarantee they would not slip in here and there—which they did, and in an entirely unacceptable amount. e situation came to a head in about 2003. Electronic circuit boards can use conformal coatings, which are so called because they are thin and conform to the shape of the components across the board. Normally, they protect against condensa- tion, dirt and dust that might cause electri- cal short circuits. e combined defense industry initiated testing of many kinds of conformal coatings for their ability to con- tain the tin whiskers that inevitably and unpredictably grew on electronic assem- blies. All the coatings failed; the whiskers grew right through them. So, the industry, in conjunction with its aerospace partners, funded research to develop a coating that could not be punctured. One of the first discoveries was that the science of how thin, flexible elastic films could contain tin whiskers, or how they were punctured by them, was actually poorly understood. We all know a pin punctures a balloon. But the physics of failure as a function of the properties of the elastic materials in the balloon was not a mature science in 2008. A huge knowledge base existed for the thin, hard coatings used in making microcircuits, but these coatings did not apply to designing a new conformal coating that would contain tin whiskers. at was the beginning of understanding the problem. A team of scientists received funding from the MDA to work this problem out and develop a circuit-board coating that would contain the stronger-than-steel tin whiskers. Aer nine years and some millions of dollars, they succeeded, and the tin whisker risk is now being addressed by a specialized coat- ing designed for electronic circuit boards. As oen happens, an invention designed to solve a bizarre problem in a highly technical field has found other applications in fields that more directly affect the consumer. e mathematical equations that govern a tin whisker trying to puncture an elastomer film on an elec- tronic circuit board also explain how a nail punctures a tire and then its inner tube. Most tires don't have inner tubes anymore; if they did, a very puncture-resistant tire would now be available. Other areas have metal-whisker problems besides tin-plated parts on circuit boards. Although these problems didn't affect the earliest computers, they became problematic with the advent of large computer arrays that contained many racks. e air-cooled cards in these devices, stacked side by side in layer aer layer, were covered with microcircuits having fiy leads to the inch. ey appear in data centers, which store and handle the information and records of every large company, of state and local governments, of Internet companies and of telecommu- nications companies. In the last 10 years or so, even these devices began to malfunc- tion, and entirely too oen. e culprit turned out to be not so much tin whiskers on the electronic com- ponents of the air-cooled circuit boards, but zinc whiskers. ey actually grew in the air-distribution system in between the floor and subfloor, a foot or two below the computers. Many data center floors have a metal structure of steel frames on which the floor tiles rest, and vertical supports carry the weight loads of the equipment racks to the subfloor. e undersides of all these floor tiles are covered with steel sheeting, and the supporting structure of the floor similarly comprises steel frames and posts. To prevent the structure from rusting, all the steel is galvanized. at means zinc plating, and as Bell Labs discovered three generations ago, zinc also grows whiskers that look much like tin whiskers. ey are very light when they break off and can travel through the air. ey collect just like dust, except they are conductive and can cause short circuits. Not only can air flow break off these whis- kers, but swarms of them can detach when floor tiles are removed, whether for main- tenance work, running additional cabling or to access last year's Christmas tree that the office staff stored there. (You didn't know? Ask around.) e metal whiskers can be blown into the electronics, shorting out power supplies and electrical signals. Systems then quit working, intermittently or permanently—sometimes with a flash of light and a cloud of smoke. e random computer-card failures are frequent enough that these systems are oen designed with multiple redundan- cies so that even if many parts fail, the whole system still works. And each plug-in card that fails can be removed, even while everything is running, and a new one plugged in. is solution is expensive, but it's the only practical option when