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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