You’ve heard of the tin whiskers that can grow on pure-tin solder, now you can fill your nights with nightmares about product failures due to zinc whiskers. The UK-based deisgn and testing company Cobham Technical Services recently worked with a client that suffered from zinc whiskers.
In a case study, Cobham points to a system at a major financial company that experienced zinc whisker problems in its data processing equipment. The company encountered high failure rates with the PCBs and power supplies in its data processing equipment at its center in North East England. The problem was traced to small metallic particles of zinc being deposited on the PCBs and causing inter-track conduction. The source of the zinc was the galvanized coating on the underside of the raised floor tiles.
Examination found that zinc whiskers were present and were being carried in the airflow from the pressurized under floor plenum into the intake vents of the equipment.
Zinc whiskers grow from metal surfaces that have been coated with zinc to help protect them from corrosion. Whisker growth appears to be limited to electroplated surfaces, rather than those that have been hot-dipped or sprayed, and is caused by internal stresses incurred during the plating process. Fine zinc filaments, normally only a few microns in width, but of several hundred microns to a few mm in length, develop from the plated surface. A mass of zinc whiskers removed from a computer installation is shown in image 1.
Although zinc whiskers have been found on wide range of surfaces, access floor tiles are of particular concern, as they have large surface areas, and are often moved during day-to-day maintenance. The zinc whiskers are fragile and can easily be broken off the tile - some whiskers may then get into the supply airflow and be carried to the hardware via the ducting beneath the flooring, as in the above case.
Image 2 is a scanning electron microscope of mixed particulates from a dust sample collected from a computer room. Energy Dispersive X-ray (EDX) analysis can be used for fast identification of zinc within the sample - by selection of a particular EDX energy range, zinc can be imaged as orange within a composite “cameo” image of the same sample area, as shown in image 3.
A quick glance shows that zinc is present in both particle and whisker form. Fibers of glass or mineral composition, are colored blue/green. Using this technique it is possible to see the zinc whiskers at very low magnification, as shown in image 3. Spot checks on individual fibers are made to check the overall analysis.
The consequences of a long zinc whisker getting into the wrong place can be catastrophic. These conductive whiskers have posed a threat to the electronics for many years but older electronic designs with larger gaps between components and pins were less likely to be compromised. As circuitry has evolved it has become more closely packed and operating voltages are lower. The whiskers can be long enough to bridge many gaps between pins of components in current installations and the ever denser geometry of newer technologies means that this problem is likely to increase.
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