From Military & Aerospace Electronics online 091406
Electronics designers grapple with lead-free solder guidelines
The European Union WEEE/RoHS directives cause concern in the military
and aerospace market as to the availability and reliability of
lead-free electronic components.
By Courtney E. Howard
The European Union (EU) has issued two directives that will have a
significant influence on the global military and aerospace market.
First, the EU Restriction of Hazardous Substances (RoHS) in electrical
and electronic equipment directive (Directive 2002/95/EC of the
European Parliament and of the Council), which took effect July 1,
prohibits the sale of new electronic equipment containing certain
hazardous substances, including lead, cadmium, mercury, hexavalent
chromium, polybrominated biphenyls, and polybrominated diphenylethers
in the EU.
Second, and expected to take effect on January 1, is the EU Waste
Electrical and Electronic Equipment (WEEE; Directive 2002/96/EC)
policy, which calls for organizations to take responsibility for
recovering and recycling products containing hazardous materials at the
end of their useful lives.
The EU is not alone in its environmental legislation with regard to the
manufacture and recycle of electronics; it is a global trend. Japan,
one of the world’s leading producers of electronic components and
printed circuit boards, has been aggressive in its efforts to remove
lead from its manufacturing processes and is expected to institute
lead-free legislation this year. China is following suit. In the U.S.,
meanwhile, California legislature approved SB-20, with a compliance
deadline of January 2007 that prohibits the sale of electronic products
in the state that do not meet EU RoHS standards.
“The component industry is moving all of its parts slowly but surely to
be lead-free,” recognizes Andy MacCaig, operations director for
Radstone Digital Processing in the United Kingdom. “Even for those
countries where there is no lead-free legislation and no requirement to
manufacture lead free, while it’s not directly applicable to them, it
is going to affect them. The electronic components they need to
purchase to make their products are moving to a lead-free finish. All
the evidence at the moment certainly suggests that that’s the way the
world is going, and that we will be seeing lead-free solder
everywhere.”
Getting the lead out
The biggest, although not the only, influence of the RoHS legislation
for the electronics industry is the requirement to reduce
significantly-nearly to zero-the amount of lead in products, including
component finishes and solder. Lead has been used in solder, which is
predominantly made of tin, for several decades with good reason. For
starters, the addition of lead makes solder softer and more pliable,
which is of particular benefit to military and aerospace applications.
“Electronic components and the circuit board that they are soldered to
expand and contract at different rates as they heat and cool,” MacCaig
explains. “The solder joint connecting them has to be able to
accommodate those items expanding and contracting at different rates-it
has to link and move without fracturing so that, over lots of thermal
cycles, the joint stays intact and doesn’t break.”
A primary concern in moving away from leaded solder is compromising the
integrity of the solder joint, which is of keen concern to military
systems designers where reliability is paramount. A lead-free solder
joint is likely to be far more brittle and intolerant of extreme
temperature changes, which are common in military and aerospace
environments.
“If you take the lead out, you increase the risk of that solder joint
failing due to the expansion and contraction of the parts it is
connecting together,” MacCaig says. “That is certainly one of the
challenges of moving to lead-free solder-working out exactly what
substances and what processes to use to make sure we can get the sort
of reliability that we’re used to from a lead-based solder.”
The use of lead-free solder further calls into question the reliability
of the overall printed circuit board and its components, not just the
solder joints. The move to lead-free products affects the whole
manufacturing process, says Doug Patterson, vice president of worldwide
sales and marketing at board manufacturer AiTech Defense Systems Inc.
in Chatsworth, Calif.
“Lead-free solders require higher processing temperatures, which stress
the board during manufacture quite a bit more,” remarks Patterson.
“Military customers care about and won’t accept the higher temperature,
which puts greater stresses on the boards’ components. Some of our
customers are saying, ‘We will not accept anything but tin-lead.’”
Another effect of moving to lead-free solder and components-tin
whiskers-is garnering much attention among defense and aerospace
systems designers, and rightly so. Tin whiskers have caused documented
failures to various satellites, radars, and missiles, including the
Galaxy-3 satellite, F-15 jet fighter radar, and Patriot missile.
More recently, officials of the Aerospace Corp. in El Segundo, Calif.,
debated with NASA leaders about whether NASA should launch the space
shuttle Discovery STS-121. Aerospace Corp. executives fear that tin
whiskers on the shuttle’s flight-control-system (FCS) avionics boxes
will fall onto circuit boards, which would create a failure of the
orbiter’s electrical components and the subsequent loss of crew and
shuttle. NASA launched STS-121 on July 4.
Tin tactics
In moving to lead-free electronics, component manufacturers must
abandon their use of a tin-lead finish on the component legs, designed
to ease the process of soldering parts onto the board. Gold, silver,
zinc, and other metals can grow whiskers, but tin is perhaps the most
susceptible.
“Certain lead-free finishes are very prone to tin-whisker growth,”
MacCaig says. “The exact reasons for that are becoming better known,
but the phenomenon of tin whiskers is still something of a mystery.”
Tin whiskers represent a crystalline metallurgical phenomenon by which
tin grows tiny electroconductive filaments that resemble microscopic
hairs. The cause of tin whiskers is not completely understood, even
after decades of study, yet experts do know that thermal or compressive
stresses encourage their growth. Able to carry a current, whiskers are
known to cause shorts in high-voltage circuits and intermittent
failures in low-power ones.
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