Network Communications News (NCN) September 2016 | Page 31

cable management & labelling

Intermetallic compound formation
– the diffusions between the plating
and substrate material will form
intermetallic compounds, thus
generating compressive stress within
the plating.
Diffusion – atomic substrate species
may have high diffusivity when they
diffuse into the plating film.
External stress – or applied stress,
which can be introduced by turning a
nut or screw, bending or stretching of
the surface etc.
Coefficient of thermal expansion
mismatches – between the plating
materials and substrate.
Surface oxide – it is possible to
form zinc oxide and tin oxide on the
tin surface and cause stress due to
specific volume change.
Although the fundamental
mechanism of whisker growth is still
not fully understood, the consensus
of opinion is that whisker growth
will proceed either via a dislocation
mechanism or through a grain
growth/recrystallisation mechanism.
Furthermore, many of the theories agree
that stress provides the driving force for
whisker formation. Eight out of 10 data
centre operators don’t prefer them.
So how big is the risk? Unitrunk
commissioned a report on zinc
whiskers from Dr Colin Gagg, a
leading expert on the topic with a
PhD in Forensic Engineering, who has
studied contamination phenomenon in
electronic assemblies.
In general, Gagg reported that zinc
whiskers will not be a problem until they
break off and begin circulating freely in
the air. This is exactly why handling older
electro-plate galvanised fixtures, such

as cable trays and baskets; computer
floor tiles and supports, etc, will raise the
potential for problems.
Even the lightest touch can dislodge
zinc whiskers off their growing surfaces,
allowing them to be circulated by air
conditioning systems – and thus drawn
into sensitive computer equipment. Even
if the electronic equipment is equipped
with dust filters, zinc whiskers can still
filter through – only High Efficiency
Particulate Air (HEPA) filters are fine
enough to stop them being drawn into
computer equipment.
Once inside any electronic or electrical
equipment, electrically conductive zinc
whiskers can settle onto exposed circuit
boards, where they can and do cause short
circuits. Symptoms of zinc whisker related
failure range from minor data corruption
or anomalies to catastrophic hardware
failures. A high incidence of power supply
failure is a commonly reported problem.
Furthermore, equipment malfunctions
can be ongoing and intermittent, so
diagnosis is often difficult.

How should I prevent
zinc whiskers?
Unfortunately, there is a lot of
misinformation in the marketplace
about how best to tackle zinc whiskers.
Unscrupulous suppliers may well
offer hot-dipped galvanised products
as a solution. Although hot-dipped
galvanised products may not form
zinc whiskers in such a short time as
electro-galvanised systems, the evidence
suggests that it will only be a matter
of time before whiskers initiate and
grow. Therefore in terms of zinc whisker
elimination, hot dipped galvanising may,
at best, only be a delaying tactic.

S P E C I A L F E AT U R E

The presence of whiskers can cause short circuits
and arcing in electrical and electronic equipment.

Powder coating is another option put
forward. The reality in a site environment
is that once powder coated solutions are
cut or formed, the open end is unlikely
to be treated to provide complete
integrity of the coating. Another issue of
powder coating is in achieving continuity
throughout the system. This is very often
only achieved by scraping away the
coating from an area and applying an
earth lead across joints.
As our report expressed, if you want
to eliminate the risk you have to specify
stainless steel cable management
systems and apply the same rigour to
other parts of the installation. The reality
is that this is not happening. Very few,
if any, data centres are opting for this
approach to risk.
The message is this: specify stainless
steel or accept there is a level of risk in
the installation. What you can’t do is opt
for anything less than stainless steel and
believe that you are fully protected, as
some suppliers may well argue. These
whiskers belong to something with a
nasty bite.

With more than 20
years’ experience within
the cable management
sector, Tim Brown
joined Unitrunk as
specification manager
in 2011 and was made
national sales manager
in 2014. Originally
trained as an electrical
apprentice, Tim began
his career in the
electrical contracting
sector, later training as
an electrical engineer
before moving into
estimating and
surveying roles. He
left contracting in 1993 to take on a sales role and has
specialised in cable management ever since.

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