Zinc whiskers were first discovered in 1948 by Bell Labs when a "whisker" on a zinc-plated bracket caused increased losses in quartz filters used in a telephone transmission system superscript [1]. Originating from zinc electroplated products commonly used in today's computer rooms, these whiskers constitute a serious anomaly that wreaks havoc on your sensitive electronic equipment.
For the past several years, the electronics industry, as well as the metal and plating industry, has been aware of this phenomenon. IBM's AS/400 Physical Planning Reference V4R1 (SA41-5109-00) states: "Raised floor tiles that have a zinc-electroplated passivation coating have the potential to grow zinc whiskers. IBM believes zinc whiskers cause intermittent AS/400 operational problems. The AS/400 may either post an error or power down" superscript [2].
Any zinc-electroplated surface may experience this anomaly, but the products of most concern are certain types of floor tiles used in computer rooms' raised floor applications. Suspect floor tiles are wood, or wood composite cores, with a flat sheet steel bottom or underside. The steel surface, if passivated using electroplated zinc, can be a major source of contamination. (To our knowledge, no zinc electroplated floor tiles have been manufactured for several years.)
Implications The main concern is that zinc is a conductive material. A whisker can be considered a low-capacity fuse with DC resistance of 10W to 40W, depending on the whisker geometry, with a DC fusing current of 10mA to 30mA superscript [3].
Although the whiskers are small in size, they are large enough to cause problems in today's microcircuits such as short circuits, voltage variances, and other signal disturbances. When sensitive electronic equipment becomes contaminated with zinc whiskers, equipment failures and system resets can occur. In most cases, the same short circuit caused by the whisker will either vaporize the contaminating whisker by the current flow or else it will become dislodged when the board or card is removed - leaving definite fault analysis impossible. Data suggests that a large percentage of no trouble found (NTF) statistics may be due to intermittent failures caused by zinc whisker contamination.
The effects of this contaminant could turn out to be the most failure-causing anomaly of electronic and computer equipment in data centers. These data centers located deep inside banks, stock exchanges, government facilities, and hundreds of other businesses are all susceptible to this unique and possibly catastrophic contaminant.
How Zinc Whisker Contamination Occurs The most common introduction of zinc whiskers is by floor tile removal. In today's data centers, the air space under the raised floor is used as an air plenum, pressurized to supply adequate cooling and ventilation to the data center. Floor tiles used in computer rooms are the largest source of whiskers. Zinc whiskers grow downward from the floor tiles like stalactites in caves, and the whiskers are dislodged when the tiles are removed to gain access to the under-floor area. The removed tile is placed on top of other tiles and scuffed around on the floor. This process strips off thousands of whiskers from the underside of the tile, and the normal airflow in the data center causes the contamination to spread quickly. Power supply fans and cooling blowers easily ingest the contaminant and distribute it within the equipment. Movement of cables and equipment under the floor can also dislodge the whiskers.
How to Determine the Existence of the Contamination If a data center experiences a high failure rate of equipment (i.e. power supplies and logic boards), and if the failures are fairly well distributed around different equipment vendors, you should suspect zinc contamination. Another indication of zinc contamination is if failures often occur in the aftermath of work in the data center, such as equipment being rearranged, construction, pulling of cables, etc. This unusual amount of activity can cause whiskers to be released into the facility, causing a rash of failures.
Inspecting for evidence of zinc whiskers is relatively easy but must be done with caution. Pick an area of the data center farthest away from sensitive equipment and air handling equipment and carefully lift a floor tile. If the bottom of the tile has a metal pan bottom and the color is a uniform dull or slightly shiny gray color, you should suspect the tile. Examine the tile using a high-intensity work light or good flashlight and shine the light on the metal surface at an angle. Look for a reflective sparkling or twinkling on the tile surface. If you find a reflection, it is very likely that zinc whisker contamination is present.
Abatement The abatement process, although not inexpensive, is effective and can safely remove whiskers and other contaminants. The only way to successfully deal with zinc whisker contamination is to remove the source. This process involves the redirection and reduction of airflow, removal of the contaminated floor tiles (individually bagged), cleaning of the air plenum (HEPA vacuums), cleaning and sealing unmovable tiles, and installing new tiles.
Zinc whiskers are a serious problem, but the contamination is manageable without system interruption. If you're experiencing unexplained computer problems and think you may have a zinc whisker contamination issue, you should consider zinc whisker containment. After all, the heart and soul of your business is in your computer room. For more information on zinc contamination or abatement, visit www.pqsconsultants.com.
References 1. K.G. Compton, A. Mendizza, and S.M. Arnold, "Filamentary Growth on Metal Surfaces-Whiskers," Corrosion, 7 (1951) 327-334.
2. IBM AS400 Hardware Support Manual.
3. P.L. Key, "Whisker Growth on Electrodeposited Zinc Finishes," Compaq Computer Corporation, 2000.
Zinc whiskers grow from an electroplated surface without any external stimuli. They have been observed to grow in a vacuum superscript [1], which is known as "spontaneous" growth. This growth differs from a growth known as "compression" whiskers, which are formed by applying an external compressive stress to the electroplated film superscript [2]. Spontaneous whiskers are observed on electroplating of tin, zinc, and cadmium - and all appear to be similar superscript [3]. Electroplated zinc, when viewed under an electron microscope, resembles fibers in a basket weave or lattice pattern. This pattern is created by the configuration of zinc atoms in lines or rows across the film surface. The zinc atom structure begins to separate from the steel and pushes the zinc coating away from the steel surface. The whisker growth occurs from the base and not the tip. The result of this separation is tiny zinc columns that are pushed, or grow, away from the surface. The growth of the whisker does not leave a depression or thinning of the zinc film, which indicates that the atom transport occurs over a long distance superscript [3]. This breakdown process is known as "atom migration."
Whiskers grow at a rate of about 250 microns per year and have a uniform diameter of about 2 microns They can reach lengths of up to 1cm (0.4 in). When they reach a length of 500 microns (0.5mm or approximately 1/64 in.), the whiskers can become a potential problem for electronics.
References 1. S. M. Arnold, "Metal Whiskers - A Factor in Design," Electronic Components Conference (1954).
2. R. M. Fisher, L. S. Darken, and K. G. Carroll, "Accelerated Growth of Tin Whiskers," Acta Metallurgica, 2 (1954) 368-373.
3. P. L. Key, "Surface Morphology of Whisker Crystals of Tin, Zinc and Cadmium," Electronic Components Conference (1970).
Zinc (Zn) is an element used to prevent rusting or oxidation of steel. Steel pieces are coated (passivated) with zinc. This coating can either be zinc hot dipped galvanized (HDG) or electroplated. The HDG coating is not associated with the zinc whisker contaminant and can be easily recognized by the irregular triangular (spangled) pattern on the metal surface. Electroplating provides a uniform gray surface, but in some cases brighteners may be added to the zinc plating process.