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Photo: PJ Singh, IBM Corporation, Poughkeepsie, New York.
Zinc whiskers grow on metal stringers or off the bottom and sides of floor tiles that have a zinc electroplated-passivation coating on the sheet-metal pan. During the plating process, compressive stresses are imparted in the zinc coating causing the whiskers to grow. If the zinc is pure enough to create a regular crystalline structure, it will try to relieve this compressive stress by pushing whisker material out of the surface at specific locations in the lattice structure.
These zinc whiskers are about 2 microns in diameter and become a problem after approximately two years of growth when they reach a length of 500 microns. And they can grow to a length of over 2000 microns (2 mm) if left undisturbed for several years. The whiskers are so small that an attempt to use filters fine enough to trap them would dramatically reduce airflow and cooling capacity in the computer room.
As floor tiles are moved or disturbed, the zinc whiskers break off and become airborne in the turbulent airflow under the raised floor. Cooling fans in the computer equipment then draw the whiskers into the internal logic cages and power supplies of the equipment. Once inside the hardware, the whiskers lodge themselves in the electronic components of logic cards and power supplies causing either a voltage or signal perturbation. At this point, the machine posts an error and can shut down.
Several sites have reported failures in the power transistors of power supplies with one site experiencing seventy-seven power-supply failures in one week. This high rate of failure is unusual since the whiskers completely vaporize at only 15 mA to 25 mA (milliamps) which usually is not sufficient current flow to cause shorting within a power transistor. Further investigation indicates these failures were from perturbations in the control circuitry of the power supplies which allowed a short to occur between the voltage and ground planes of the transistor. Another phenomenon observed within power supplies is the ionization of air between two closely spaced high-voltage lands on a printed circuit board. The whisker is vaporized, creating an arc path across adjacent etchings on the board.
Preventing zinc-whisker damage
The first step in preventing damage from zinc whiskers is to examine the data center to see if the condition exists. If wood-core floor tiles are present in the computer room, check to see if the bottom sheet-metal pan is passivated with a coating of hot-dipped galvanized (HDG) or electroplated zinc. If you find the spangle pattern associated with the HDG process, the tiles should be free of whiskers. Electroplated zinc can be identified by a uniformly grey surface rather than a spangled surface.
Very often a bright-grey zinc surface has not yet begun to grow whiskers, while a dull-grey surface indicates the presence of whisker growth. This is not an absolute determination, however, so a closer examination is often required. Tip the suspected tile upright and examine the bottom and sides using a high-intensity light — either a work light or a bright flashlight will do. Position the light at various angles to the tile surfaces and look for tiny sparkles or twinkling. If present, there is a good chance the tiles are contaminated with zinc whiskers. Examine at least twenty-five tiles and the stringers supporting them before concluding that electroplated zinc material is or is not present.
If evidence of zinc whiskers is found, carefully examine the entire raised floor to determine the extent of the contamination exposure. The level of risk will determine the recovery plan of action. If contamination is 10% or less of the raised floor, purchase new HDG tiles and carefully install them in the room while the equipment remains operating. Dispose of contaminated tiles in sealed plastic trash bags, and contract to have the room thoroughly cleaned.
If the raised floor is grossly contaminated, define a more detailed work plan for replacing larger sections of floor or all the tiles. Identify areas of the floor to be replaced, move equipment as needed to isolate the affected areas, and be extraordinarily careful not to disturb any loose whiskers as you proceed with the cleanup.
