Whiskers are back and
killing your electronics!
Techno Talk
Mark Nelson
No, this is not an April Fools’ joke, and has nothing to do with hipster beards. A ticklish problem that
plagued electronics in the past has made an unwelcome return. It’s a ‘disease’ that is devious and
creeps up undetected. Not every product is affected, and the remedy is not simple. The name of this
pestilence is ‘tin whiskers’, and it’s one of the causes of sudden death syndrome in PCBs using SMDs.
N
ASA summarises these
whiskers very succinctly: ‘First
recognised as a problem during
the 1940s by Bell Labs in the USA, tin
whiskers are electrically conductive,
crystalline structures of tin that can
grow from surfaces where tin (especially electroplated tin) is used as a
final finish. Tin whiskers have been
observed to grow to lengths of several millimetres and in rare instances to
lengths in excess of 10mm. Tin is only
one of several metals known to be capable of growing whiskers.’
Destructive and unpredictable
The deadliness of this seemingly bizarre
growth is considerable. When it occurs on
the pins of semiconductor devices, it can
short-circuit adjacent leads. Sometimes
the short is only momentary (if the resulting current flow is strong enough to melt
or ‘fuse’ the whisker), but if the short is
maintained, it can result in total device
failure. In application environments involving high levels of current and voltage,
a highly destructive ‘metal vapour arc’
can occur. Here the solid metal whiskers
are vapourised into a plasma of metal
ions that are more conductive than the
solid whiskers themselves.
The growth of these whiskers is also
unpredictable, beginning after an incubation period that varies from seconds
to years. However, recent research (published at www.doEEEt.com) sets out key
characteristics of whisker growth:
n The filaments grow most often from
tin-plated copper
n The most likely driving force for
whisker formation is compressive
stress in the tin layer. Stress on
tin-finishes may also be induced
mechanically, and consequently
bending, scratches or nicks in the
tin may favour tin whisker growth
n Whiskers grow spontaneously without
any applied electric field or moisture
n Tin whiskers can grow either under
ambient conditions, under reduced
pressure or even in a vacuum
n The spontaneous growth of whiskers
is an irreversible process.
10
What is not disputed is that whiskers
have been proven to have caused the
failure of satellites and heart pacemakers, and even caused the shutdown of
nuclear power stations.
Public safety
The whisker problem was not a major
issue until a decade or so ago, mainly
because the 40% lead content of solder
was effective at preventing whisker eruptions from the other 60% – tin. From
around 2008, however, tin whiskers
became reported as problematic again,
triggered by the phasing out of lead in
electronic component leads, with many
parts using pure tin instead of leadtin alloys. Lead is now banned by the
Restriction of Hazardous Substances
(RoHS) directive, and although RoHS
originated in Europe, this directive now
affects virtually every piece of electronics gear manufactured today or planned
for the near future. Connectors, passive
and active components, switches, and
relays must now all be lead-free.
As semiconductor maker Maxim
states, lead is recognised as a neurotoxin and is known to affect brain
development, with children at greater
risk than adults. The ban on lead in
electronic products followed on from
its removal from petrol, and paint, with
the public no longer breathing in lead
fumes and children no longer able to
lick lead-based paint surfaces. This
was a well-intentioned but ill-informed
move. As Maxim points out, banning
lead in electronics didn’t make sense.
the public do not ingest it and lead is
not a contaminant of groundwater because it is not soluble in water, nor does
it break down in the environment (it
usually sticks to soil particles).
Blame the EU?
The leaded solder ban began in Europe,
and according to Joe Smetana, now
a principal engineer with Nokia in
Finland but then a principal engineer
and tin whisker expert with French telecoms equipment maker Alcatel-Lucent,
‘The EU’s decision was irresponsible
and not based on sound science. We
are solving a problem that isn’t one and
creating a bunch of new ones.’
It’s hard to fault Smetana’s logic, but
despite this, EU officials retaliated that
the regulations banning lead and other
hazardous substances are needed to
protect people and the environment regardless (huh?). In any case, the RoHS
directive contains exemptions from
the lead ban for companies in certain
industries, such as aerospace, defence
and, ironically, medical equipment.
Back with a vengeance
Moving rapidly forward to present
times, the use of SMD integrated circuits with their little legs packed more
closely together than a millipede’s is
near-universal. This means they are
at high risk of tin whiskers growing
directly on the leads themselves; alternatively, well-nigh invisible whiskers
can become fractured and migrate elsewhere on the PCB, causing faults there.
So, what can practical electronicists
like us do to avoid the tin whisker
plague? For commercial products, with
PCBs hidden out of sight and populated
with densely packed SMDs, the answer
is not a lot. But if you build your own
projects that employ ‘electronic millipedes’, the remedy is to protect the
components immediately after soldering to mitigate the subsequent growth
of whiskers. One remedy mentioned in
an online forum suggests that a good
scrub with a nail brush might do the
trick, followed by a ‘conformal’ coat of
epoxy or polyurethane varnish. ‘Might’
is the operative word, because Trizo Ltd
warns that tin whiskers have managed
to penetrate polymer coatings, causing
an even greater risk of critical electronic
system failures.
Alternatively, simply don’t use leadfree solder on your projects! Stick to the
good old 60/40 tin-lead formula; it’s far
less prone to tin whiskers, perfectly legal for home construction and melts at a
lower temperature, meaning your soldering iron’s tip will last longer. Result! (Do
remember to wash your hands properly!)
Practical Electronics | April | 2021
