No blame,
no shame?
Techno Talk
Mark Nelson
Some folk have fabulous luck: every one of their electronic projects works perfectly, every time. The rest of
us can feel proud if we are successful most of the time. But what if you really are an expert electronicist and
still find yourself completely stuck. What on earth is the cause? Maybe it’s not of this earth at all. Confused?
Just read on...
P
icture this scene. You are
building a Practical Electronics
project, using fresh components
bought from reliable suppliers and
inserted correctly in the PCB, your soldering is impeccable and after all this
effort, the gizmo then refuses point blank
to work. You check and re-check your
workmanship, then you use a magnifying lamp to seek out minute solder
bridges and splashes – all to no avail.
You definitely haven’t botched anything,
so why won’t it function properly?
Consult the experts
Have courage – you may be entirely
blameless! Even experts can suffer this
syndrome. The late Gerry Wells, former
curator of the Vintage Wireless Museum
in London often encountered problems
of this kind. He put them down to what
he called ‘dirty electricity’, and as he always had plenty of repair jobs on hand,
he just set aside refusenik projects and got
on with something else. But that won’t
solve the problem of your present project.
Maybe you should look further afield
for inspiration. Sherlock Holmes had
the right idea, saying: ‘When you have
eliminated the impossible, whatever
remains, however improbable, must
be the truth.’ ‘Fairy nuff’, as they say,
where do wo go next? Remember when I
asked earlier, what on earth is the cause?
Maybe the cause is not on earth at all.
Cosmic chaos
A serious contender for causing electronics to malfunction is the effect of
cosmic rays from space. These little
devils are so devious that they are
not really rays at all. Instead, they
are high-energy particles that move
through space at close to the speed of
light, made up of nuclei of elements
like hydrogen and helium, along with
other subatomic particles such as neutrons, electrons and neutrinos. They
originate in stars and the supernova
effects produced at the close of life of
truly massive stars. Although most cosmic rays and their effects are absorbed
by the atmosphere, eight cosmic ray
10
showers reach each square metre of
the Earth’s surface each second.
They can carry positive or negative
charges and although human life at
ground level is not affected significantly
(space travellers are at greater risk), cosmic rays certainly represent a real risk to
electronic devices made of silicon. The
most prevalent effect is from bit-flips in
memory chips (known as ‘soft errors’).
It is stated that if you have 1Gb of memory per chip and 1,000 chips, then you
can expect 0.4 flips every hour. There
is also a real but rare effect known as
single event latch-up (SEL), which can
cause a transistor to conduct continually until the power is removed.
Vulnerability defined
Earlier investigations into the effect
of cosmic rays implicated them in a
number of mishaps, such as when the
accelerators of Toyota cars throttles became stuck in the open position. This
defect, which was said to be possibly
the result of cosmic rays, led to the
recall of a substantial number of vehicles in 2009 and 2010. Wikipedia also
cites data corruption in a flight control
system that in 2008 caused an Airbus
A330 airliner twice to plunge hundreds
of feet, giving rise to injuries to multiple passengers and crew members.
Although cosmic rays were investigated initially as a possible cause, this
was later ruled out as very unlikely.
Since then, our understanding of
the effect of cosmic rays has improved
considerably and responsible manufacturers design safety into products that
are potentially vulnerable. Statistics
play an important role here, as the
Silicon Valley PCB manufacturer Royal
Circuit Solutions explains on its website at: http://bit.ly/pe-mar21-cosmic
Since it is impossible to know exactly when or where a cosmic ray impact
will occur, engineers must rely on statistics to determine the probability of
impact. Manufacturers provide cosmic-ray data separate from terrestrial
radiation sources, quantified in terms
of Failure-in-Time, or FIT units. One
FIT unit is equal to one failure in onebillion device hours.
That sounds quite impressive until
you do the maths. If, for example, a
circuit that you designed was installed
on 500,000 products and had a single
transistor with a 14 FIT rating, you
could anticipate a failure every six
days. This might be acceptable for a
fan on an industrial air-sensor unit.
But it would be a wholly unacceptable
risk for a device that controls, say, the
accelerator on a car.
Please understand that all of the foregoing applies only to electronic devices that
are deployed at ground level. In satellites
and spacecraft the case is altered entirely,
with electronic circuitry requiring special radiation-hardening protection or
sophisticated error checking.
Back in the real world
It must be made clear that the likelihood of cosmic rays affecting a hobby
electronics project is pretty minimal for
now. That said, the situation could increase in times to come. As electronic
devices continue to shrink physically
and operate at lower voltages, active devices will include more bits per square
millimetre, with less electrical charge
required to define a bit as being in the
1 or 0 state. On one hand, as electronic
packages shrink, the chances of a direct
impact by a charge particle decrease. On
the other, as operating voltages decrease,
the degree of threshold between logichigh and logic-low voltages is reduced,
meaning the likelihood of a failure
caused by a cosmic ray will increase.
Lessons learnt
So, what have we learnt? At least two
things, I would say. First, the effect of
cosmic rays is real but only transient,
so they are unlikely to have kyboshed
circuits that refuse to work. Apologies
for raising your hopes; random charged
particles from space are not a viable
get-out-of-jail card! Second, you probably now know more about cosmic rays
than you did five minutes ago. Now
start looking for dry joints…
Practical Electronics | March | 2021
