Zinc whiskers 2.0

White paper from Assurity Consulting

Assurity will be exhibiting at stand E4 at the show.

Whether, until now, you’ve heard of zinc whiskers or not, they are a phenomenon and one which can pose significant risk to electronic equipment, computer rooms and Data Centres.

Having been providing work on zinc whiskers for over 20 years, we have assessed more than a few computer rooms and facilities, with some, but not all identifying the contamination to be present. More recently however, we have seen a trend away from the traditional surfaces prone to zinc whisker growth, to finding it in more unusual places and in relatively new rooms.

So, we thought for this month we’d take a look at the subject.

What are zinc whiskers?

Zinc whiskers are microscopic, metallic filaments that can spontaneously ‘grow’ from surfaces coated with zinc. Because of this fact and their resemblance to hairs they are termed whiskers.

The ‘whiskering’ of some metal surfaces has been know about for many decades and is not unique to zinc, with cadmium, germanium, tin and lead amongst the metals having been documented producing these structures.

It is believed that compressive mechanical stress in the host metal encourages the process, and this can be caused by a range of factors including electroplating.

Whiskers only grow to a few microns (μm) in diameter, but over time can reach several millimetres (mm) in length – typically grow at less than 1mm per year. There also appears to be no consistency in when the process starts, with some surfaces showing signs of whisker growth quickly, and other taking years before they appear. Mechanical strain, thermal cycling, or corrosion on and of the coatings/surfaces are amongst the conditions the that it is believed trigger growth.

Why are zinc whiskers a problem to electrical equipment and how do you know they are there?

Various installations can pose a risk, including, raised access floor tiles, stingers and pedestals and cable trays/baskets. Often these are manufactured in steel and have an anti-corrosion coating applied to them. Where zinc is used as this coating, the process is called galvanisation and the methods of which can include hot dipped, sprayed or electroplated. Zinc whisker growth appears to be limited to electroplated steel surfaces.

In the context of modern computing environments, and other hi-tech areas, the number of components with these surfaces, combined with the density of electronic equipment present, is what leads to the issue.

Zinc whiskers are electrically conductive, so if they contaminate a room and then settle inside servers, switches, and other hardware, they can cause electrical shorting resulting in intermittent or permanent equipment failures.

Following a short/failure, the whiskers tend to ‘atomise’ in the process, so there is usually nothing to identify what caused the malfunction. Regular problems like this can be an indication of zinc whiskers.

What factors could affect zinc whiskers becoming a problem?

There are a number of aspects to consider when assessing the potential for zinc whiskers to cause issues with your work environment. These would include:

• The type of coated metal surfaces within your rooms is primary. If you do not have any electroplated galvanised surfaces (i.e. they are hot dipped, sprayed or other materials), you should not have an issue. Equally, the more electroplated surfaces you have, the greater the potential for whiskers to grow.
• Whiskers take time to grow, age of the room and the structures within it is another factor in that the older the room the more likely and possibly longer the whiskers will be.
• The frequency of activity around surfaces potentially containing zinc whiskers. Especially with floor tiles, the more movement the more likely any grown whiskers will be come dislodged and enter the atmosphere.
• The levels of management. Poorly maintained rooms, poor filtration, clogged floor voids, overcrowding, etc.) can all again contribute to the release of whiskers or the volume of them in the area.
• The levels of cleaning. Poor cleaning with inappropriate equipment can spread more whiskers (and other potential contaminants) around, rather than remove them.
• Circuitry within hardware is another issue where increasingly the size and spacing of components has reduced over time, therefore increasing the likelihood of any entrained zinc whiskers causing a short/damage.

Zinc whisker assessments

A zinc whisker assessment combines a visual and microscopic inspection of galvanized surfaces, sampling for lab confirmation, and a structured evaluation of risk. The outcome guides whether any remediation or ongoing monitoring is necessary.

You will also need, unless you already have them, access to a suitably accredited laboratory to analyse any samples collected to positively determine if zinc whiskers are present. This will involve scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) analysis. We use a UKAS accredited laboratory for our work.

Anyone undertaking a zinc whisker assessment must be competent, as the potential to add to a problem rather than help solve it is a real risk. It also needs to be properly planned and executed.

Unsurprisingly, detailed analysis of room surfaces can identify significant types and amounts of material that could be possible whiskers, much of it however isn’t. Excessively contaminated surfaces are relatively easy to identify, but these are relatively rare fortunately. Mineral fibres, swarf, debris and other artifacts can be less easy to differentiate, as can spotting whiskers on more dirty surfaces. A trained eye is always best.

If zinc whisker contamination is confirmed, the remediation strategy will depend on the nature of the contaminated surfaces, their location and extent of the growth. Mitigation could include:

For grossly contaminated surfaces/rooms:

• The careful and systematic removal of the contaminated components, specialist room clean and conducting a follow-up assessment(s) to confirm success. Specific risk assessments and arrangements with detailed procedures should be produced and followed.
• Coating or removing whiskers and replacing the room components has been recommended in the past, although this does not guarantee the potential re-growth or indeed whiskers growing back through the coated surface in the future.

In other scenarios:

• Limiting the disturbance of floor tiles, increasing filtration quality and optimising cleaning processes;
• Incorporating checks for zinc whiskers into routine inspections of the room/area;
• Training staff to recognise risk areas and avoid unnecessary disturbance of galvanised components; and
• Periodically repeating assessments, especially after facility upgrades or moves (very important if you are introducing equipment from a different area and you don’t know the status of that area/equipment).

Some additional observations:

Recently, several zinc whisker assessments that we have carried out has positively identified their presence on some less ‘traditional’ surfaces such as metal brackets, conduit and connectors. Exacerbating these findings in instances being they were found directly above CRAC units. The other factor was that the Data Centres were less than ten years old.

This means that even in locations considered to be at less risk, proper checks for zinc whiskers or the potential for them to grow, should be considered.