Raised access floors

In data centres, the unlikely sounding phenomenon of zinc whiskers can be a serious problem, leading to regular and apparently unexplained systems failures in power and data supplies.

Zinc whiskers are essentially tiny conductive filaments that 'grow' on the underside of floor tiles and floor pedestals. Although the mechanism behind metal whisker growth is not well understood, the filaments are actually crystals of zinc that form as a result of compressive stresses imparted to a zinc passivating layer added as part of a corrosion protection process. It is thought that if the zinc layer is pure enough to have a regular crystalline structure, it will relieve these stresses by pushing filaments outwards. Growth can also be encouraged by mechanically induced stresses, thermal stress and stresses induced by the diffusion of different metals. Growth on zinc was reported by Lindborg in 1975, who suggested that to initiate zinc whisker growth, a minimum stress must be present in the zinc coating. Unlike other metallurgical problems, whisker formation does not require either dissolution of the metal or the presence of an electromagnetic field.

The filaments can be dislodged during routine maintenance work and carried via air currents (particularly where the floor void is used as a plenum) into electrical equipment, switchgear, etc. to cause a short circuit.

The whiskers are typically no more than a few millimetres long (2mm or so) and a few microns in diameter, so they may not be visible to the naked eye and almost certainly not without proper lighting. There is uncertainty as to why they occur, how they grow and why not all zinc surfaces are affected. A similar problem is the formation of tin, gold, cadmium and antimony whiskers, but since they are unlikely to be used on floor tile components the features of these metals are not discussed here.

Numerous studies have been undertaken into the propagation of zinc whiskers but no reliable conclusions have been reached as to when and to what extent growth will occur.

Zinc whiskers and raised floors

To provide strength, floor tile encasement as well as the supporting pedestals and stringers are often constructed of steel. To provide corrosion protection, the steel surfaces are usually coated with zinc - hence hot dip galvanising (HDG) (where the metal is immersed in a bath of molten zinc) or zinc electroplating (where the metal is immersed in an electrolyte). Research has shown that electroplated products are more inclined to display evidence of whisker growth and some reports suggest that HDG surfaces (which can be recognised by the lozenge pattern of zinc on the surface) are immune.

However, research (by Brusse and Sampson, and Lahtinen and Gustafsson) has identified that HDG can propagate whisker growth, but for this to happen the zinc must be exposed to chlorine and sulphur, small and slow temperature fluctuations (which would cause the fretting of grains in the metal) and compressive stresses in the coating.

The whiskers tend to grow in length - but they do not necessarily start to grow immediately after the installation of the flooring component. Incubation times can vary from months to years, but once growth commences the filaments can continue to grow at up to 1mm a year for a period of time. However, growth is not predictable and some zinc surfaces may never develop the filaments. Raised floors can pose a particular problem because of the large surface area and the possibility that growth can continue undisturbed for a number of years. With some raised floor installations now approaching 40 years since installation, the risk of disturbance could be high.

Zinc whiskers growing on the surface of a zinc sample. (Photograph courtesy of the NASA Electronic Parts and Packaging (NEPP) Program)

Visual identification is not easy, but close examination of white powder-like or fuzzy deposits may reveal the existence of the whiskers, particularly on the underside of floor tiles or other components. Visual examination under bright lighting may reveal a bright-grey zinc surface that has not yet begun to grow whiskers, but the discovery of a dull-grey surface may indicate the presence of whisker growth. Sparkling or twinkling under strong directional lighting may also reveal contamination.

X-ray elemental analysis together with scanning electron microscopy (SEM) is the most common method used to confirm the presence of zinc whiskers in dust collected from suspect areas. A less expensive but also less reliable method is to collect a wipe sample of suspect surfaces. The presence of zinc can be identified in a laboratory but it is not possible to see individual whiskers or to confirm the source.

The fibres are easily dislodged but removal by cleaning does not necessarily deal with the problem as there is a risk that they could grow again. While the deposition of fibres in an underfloor void may never cause a problem, there is a risk that sensitive electrical or electronic equipment could be affected if the fibres are allowed to work their way into system casings where they could bridge electrical contacts - even more likely now considering the gradual drift towards miniaturisation and lower working voltages. The fact that electrical components are smaller and closer together means that they are more likely to suffer from an electrical short circuit even by short fibres.

Zinc whiskers will vaporise at currents of about 15-25mA, but in so doing, the arcing patterns that result can damage fine printed circuit boards.

Diagnosis of the problem may follow reports from data managers of unexpected equipment failure, particularly following the installation of new equipment or alterations to cabling - actions that may lead to the disturbance of the whiskers. Because they are so small, the filaments are easily carried by air distribution systems, not only within the floor void, but within the rooms. Given their size, the filaments are likely to be respirable and capable of entering the lower parts of the lungs. So far, studies as to the health effects of filament inhalation have not identified any serious or long-term effects, but there is some doubt as to the reliability of investigations and a possible lack of suitable medical evidence. Available material regarding the inhalation or ingestion of zinc in oxide or dust forms suggests that it is generally harmless.

Remedial works

Cleaning may not provide a permanent solution to the problem, but it could be a valuable first action to reduce the effects. Vacuum cleaning with HEPA (high efficiency particulate air) filters may enable large concentrations to be removed, but there is a risk of further filament release when the floors are opened up and so specialist cleaning will be necessary to carefully controlled standards.

In severe cases, where the consequences of failure could be very severe, replacement of the raised floor may be a preferable alternative to cleaning or the application of barrier coatings. The selection of HDG components is thought to be a safe option, but alternative protective coatings such as epoxy coatings may be a better choice in high risk environments.