LEAD TO USE
14/2/06
10:28 pm
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No Lead in
Your Pencil
Chris Woolf MIBS deals with a potential
embarrassment that might beset many of us
over the next year or two: RoHS.
t’s not really something that one would
talk about in normal conversation – but
living with RoHS (and without lead) is
something that we will have to get used to.
Of course, even the most macho of you
hasn’t really had lead in your pencil for
several centuries because if you had you, er,
wouldn’t... if you see what I mean.
Lead (Atomic No 82) has been known
and smelted for at least 5000 years. The
Romans used it for sophisticated water
pipes and gutters, and church roofs have
relied on it for centuries. It is heavy, soft
and malleable enough that you can shape it
simply and quickly with hand tools, and
since it melts at 327°C it can be moulded,
cast and welded with very primitive heat
sources. Although its surface tarnishes
quickly it is resistant to atmospheric
corrosion – which is what makes it useful
for churches – and alloys easily with many
other metals. It is also an excellent poison!
Lead metal is insoluble and only kills us
if delivered from a gun barrel, but many of
its compounds are soluble enough to reach
our insides via food, water and airborne
particles. The lead (Pb) ion is no less lethal
than the bullet but takes a little longer to
achieve its ends. It is remarkably hard to
I
Lead-free (green wire) and lead (red
wire) solder joints. Note the slightly matt
surface of the lead-free joint, and the
reduced wetting ability which gives a
more humped joint with a trace of fillet
lifting at the front edge. The PVC
insulation of the green wire shows
slightly greater coalescing, and there is
more evidence of the remains of
(Autosol) flux.
remove from enzymes and organs, so it
gradually blocks the function of our
kidneys, wrecks our nerves, dulls our IQs,
and turns us deaf – and impotent – before
eventually killing us. All in all it has been
decreed, very wisely, to be a ‘good thing’ to
minimise the chance of any lead getting
into us, or our environment.
Lead Tetra-lethal?
Lead was declared persona non grata in
paint in the 1960s so that children who
chewed their cots didn’t end up half-witted.
Decorators warned that all woodwork
would rot in 18 months – but it didn’t. In
the 1990s lead tetra-ethyl anti-knock
compounds were phased out of fuel. Petrolheads declared that engine valves would
burn up in a few thousand miles and
pistons would seize forever – but I haven’t
been conscious of seeing any fewer cars on
the roads. There are still a few lead water
pipes around (though they are heavily
discouraged) but plumber’s solder has had
to be lead-free (a contradiction in terms,
one might argue) since the 1980s.
And this is where this article takes on
some relevance for all those child-free,
non-driving, solely beer-drinking readers of
Line Up – as well as everyone else. Over the
next few years the use of lead in electrical
solder is going to be strongly discouraged so
that when we dump our cracked-screen
iPods, beltless Dyson cleaners and defunct
analogue TVs, not to mention all our worn
out XLRs, PC cards and drowned radio mics,
we will avoid polluting our surroundings
and groundwater with the evil stuff.
Of course the eradication of lead will
mean that all solder joints will become
unreliable, the EU will prevent any nonregistered toxin-handler from purchasing a
soldering iron, and possession of two
inches of Ersin Multicore 60/40 will be a
criminal offence…
Except that it won’t. Soldering will be a
little different and will need a little more
understanding and forethought, together
with some new materials and perhaps tools.
But in general, the electronic world will
carry on without too many worries and the
sound of the sucking of teeth will fade into
background noise quite soon.
RoHS
The edict that covers lead-free solder is
‘RoHS’ – the Restriction of Hazardous
Substances in Electrical and Electronic
Equipment Regulations 2004 (where do
these titles and acronyms come from?) –
and it also deals with mercury, cadmium,
hexavalent chromium (which has nothing
to do with shiny six-packs) and
polybrominated biphenyl or diphenyl ether
flame retardants, but most of us can pass on
those.
Batteries are not covered by the
regulations (though you won’t find mercury
in many of them nowadays, and nickel
metal hydride cells were brought in partly
as a counter to the demonising of
cadmium). Physics being what it is,
fluorescent tubes and displays are still
allowed to have mercury vapour to get up a
glow, but most other electrical and
electronic equipment will become that
much less of a risk to landfill and thus
cheaper and easier to dispose of safely. Of
course, disposal is covered by the Waste
Electrical and Electronic Equipment
Directive, which is far from WEEE – but
that monster had better be slain another
time.
The date when all these toxic horrors
should be appearing on the ‘wanted’
posters is 1st July 2006. Some equipment is
exempted for a few more years after that,
and there are a host of possible exceptions
for high voltage gear, some medical and
control equipment, large fixed machinery,
kit for warmongering or supposedly
essential security, and musical greetings
cards (call me a liar?) – but most of these are
rather elastically described and other than
amusing the barrack-room lawyers are of
little importance to us. Simply put, because
it’s a good idea and no manufacturer wants
to risk a futile battle, all new equipment will
be – and probably already is – RoHS
compliant.
LINE UP Feb/Mar 2006
19
LEAD TO USE
14/2/06
10:28 pm
Page 20
The Elderly
Where things become a little trickier is with
elderly designs, spare parts and repairs.
Equipment designed and put onto the
market before the magic date can go on
being sold even if it has lead in it, and non
RoHS-compliant spare parts for such gear
can be produced and sold indefinitely –
although in practice they probably won’t
be. If most products are covered by RoHS
to make them saleable around the world,
few manufacturers are going to want to
continue using ‘contaminated’ component
parts.
This is the area where some care may
be needed. Mixing lead and lead-free solders
is not a good idea, and that means making
some effort to work out what sort of solder
has been used for tinning connections to
make sure that you use the right stuff. The
metals used for pcbs, connector pins and
solder are all able to form alloys very easily,
and lead contamination of a lead-free board
could jeopardise its reliability – which
probably concerns us more immediately
than the technicality of its continued
compliance. Conversely, lead-free solders
may give erratic results when mixed with a
high lead-content solder from an antique
board.
The differences between the two types
of solder are essentially melting point and
ability to ‘wet.’ Lead/tin (Pb/Sn) alloys melt
at around 183°C whereas lead-free ones
liquefy at around 220°C or higher. None of
the lead-free solders is as good at wetting a
metal surface as Pb/Sn alloys, so they need
more heat to be applied for longer along
with rather more aggressive fluxes to keep
the joint surfaces chemically clean while
they are cooking.
Wetting and surface tension are
intimately connected, so lead-free solder
doesn’t form the same nice concave-edged
joints – it tends to spread less far and take
on a more humped joint shape. This
characteristic, together with the greyer,
matt surface texture is often the best guide
to what solder has been used.
The amount of flow and spread is also
determined by how fast the joint cools
below the solder freezing point. With
higher melting points this temperature
profile can be quite steep, particularly on
thermally large areas of ground plane or
hefty plug contacts. Ideally the joint should
stay above the freezing point until it has
wetted as wide an area as it is able to. That
translates into: ‘use a good hot iron.’
Typically, lead-free needs to be heated
for about two and a half to three times as
long – perhaps 4 seconds compared to just
1.5 seconds for lead-based solder. However,
prolonged heating brings additional
20 LINE UP Feb/Mar 2006
“Mixing lead
and lead-free
solders is not a
good idea.”
problems because everything around the
joint also starts to roast. Copper tracks peel
more easily, electrolytics and other
components are more prone to being fried,
and PVC sleeving and insulation can
disappear into a stringy mess. Clean, neat,
reliable soldering is still possible, but you
can’t be as slapdash as you might have been
in the past!
You may also need a less slapdash
soldering iron. The classic ‘electrical
handyman poking stick’ uses a constantly
heated low-powered element and relies on
natural heat loss to keep the maximum
temperature within sensible limits. The tip
temperature can change dramatically during
soldering and may be too hot initially for
delicate plastics and then cool too fast to
prevent premature joint freezing. Older
temperature-controlled irons could
fluctuate pretty wildly too, but the newer
versions – some quite cheap – use much
higher power elements and electronic
control to give very stable tip temperatures
with relatively low thermal mass bits.
More accurate temperature control has
an added bonus, because tip temperature
and bit life are closely connected. A 10°C
increase in temperature roughly matches a
50% drop in bit life so, along with more
aggressive fluxes the shiny, clean needle
point can soon begin to look like the prong
of a long-unloved gardening fork if the peak
temperature is not kept down as far as
possible. And it still has to be wiped clean
on a sponge with obsessive frequency.
Hot Tin
The actual temperatures needed vary a bit
due to the different solder/flux variants. The
main metal used is tin (Sn), alloyed with a
few percent of something to give a lower
melting point and to prevent various
physical reactions, such as leaching of silver
contact materials or the growth of
conductive, crystalline tin whiskers. It is
these sorts of reactions that can make a
solder joint unreliable over time just as
disastrously as a partly crystalline ‘dry’ joint.
Zinc (Zn), bismuth (Bi), antimony (Sb),
copper (Cu) and silver (Ag) are all possible
bedfellows for tin, but the last two are the
most practical for hand-soldering. Sn/Cu,
Sn/Ag and Sn/Cu/Ag solder variants are all
available – the last can have a melting point
as low as 217°C – not too bad for jobbing
work. However, you need a tip temperature
well above that to produce fast melting and
easy flow. The recommendation is to start
at about 350°C and then reduce it in 10°C
steps until the joints start getting sticky.
That gives you a sensible working point, but
wiring to tiny Lemo connectors may need
10-20°C less, and a fat ground connection
with a hefty cable screen might need much
more.
If you have problems getting the solder
to flow cleanly try changing to a different
flux core. Rosin-free fluxes are often
provided because they produce fewer
unpleasant fumes and are less likely to
trigger asthmatic problems, but they are not
as easy to use as the more aggressive
colophony-based ones. Getting rid of lead
from solder doesn’t make soldering entirely
safe, and the need for fume extraction (or at
least an open window) has always been due
to the flux content rather than the heavy
metal.
Polish Up
Lead-based solders will not disappear totally
for a very long while – so there is no need
to panic – but it does make sense to polish
up some of your soldering skills so that you
can start to use the healthier lead-free
option. Polishing is about right too because,
as every plumber knows, soldering a clean
copper joint is very easy but a tarnished
pipe is all but impossible.
Electrical soldering obeys the same
chemical and physical rules. If the surfaces
are clean, kept so during soldering by the
action of a suitable flux, and not
contaminated by a filthy, flaking, oxidised
soldering iron tip, then the joints will be
easy to make. And with lead-free you might
have a few more active brain cells with
which to relish the long-term reliability of
those joints.
Properties of Common Solder Types
Solder
Lead / tin
Tin / copper
Tin / silver
Tin / silver / copper
(Courtesy NPL)
% Composition
Pb 60 / Sn 40
Sn 99.3 / Cu 0.7
Sn 96.5 / Ag 3.5
Sn 95.5 /Ag 3.8 / Cu 0.7
Melting Point °C
183
227
221
217