The electronics industry is learning to do without: it is having
to abandon one of its long-time staples, lead-tin solder.
For decades lead-tin solder has been used to attach electronic components to
printed wiring boards. However, with the body of evidence
pointing to serious adverse health effects of lead, the search for a replacement
has spawned intense effort in the electronics industry
and in universities.
Now scientists think they may have found some promising
leads: solders made of alternative alloys and polymer formulations known as electrically
conductive adhesives (ECAs).
The Linchpin of Electronics
Solder is the “linchpin of electronics manufacturing,” says
Jack Geibig, acting director of the Center for Clean Products and
Clean Technologies at the University of Tennessee. “Without
it, it’s difficult to achieve a proper electronic connection
that is durable and reliable. ”
Lead has been ideal for solder. In fact, says Carol Handwerker,
chief of the metallurgy division at the National Institute of Standards
and Technology, “The whole electronics infrastructure was designed
around the melting point and physical properties of [lead].” Lead
is malleable and thus easy to work with, and it doesn’t fracture,
she says. When lead is combined with tin in the correct proportion
(63% tin to 37% lead), the resulting alloy has a low melting point
of 183°C, which is another advantage, Geibig says: “If
you’re not operating at really high temperatures, you have
more control over processes, so that the processes aren’t sensitive
to slight temperature variations, which are costly to control.” Low
temperatures also mean less strain on the equipment and materials
(such as printed circuit board and components) that must be heated
as part of the assembly process.
The main impetus for the industry to leave lead behind is a ban
on lead in electronics imposed by the European Union. Under the Restriction of
Hazardous Substances directive, as of 1 July 2006 lead must be replaced
by other substances in electronic equipment. (The directive also
bans mercury, cadmium, and hexavalent chromium.) Any electronic components
bound for Europe are subject to the ban.
Lead is not a problem when contained in electronic equipment, says
Robert Donkers, an environmental counselor for the European Commission
who is based in Washington, DC. However, when electronic components
are deposited in landfills, he says, people may scavenge for equipment
and break it open, or the lead may leach out of landfills and into
drinking water. The risk is compounded in countries that receive
massive imports of electronic waste.
In China, for example, unprotected workers, including many children,
strip recyclables out of electronic components in a cottage industry
of sorts [see “e-Junk Explosion” in
the April 2002 issue of EHP].
Lead exposure, even at low levels, is well known for its harmful
effects on children, resulting in lowered IQ. Lead also affects the
ability to pay attention. Children exposed to low levels may appear
hyperactive and irritable, according to the American Academy of Child
and Adolescent Psychiatry. The current maximum allowable level for
blood lead in the United States is 10 micrograms per deciliter (µg/dL).
Alternative Alloys
The main approach to replacing lead in solder has been to look
for other metals as substitutes. Electronics manufacturers began
to look for alternative metals in the 1990s, notes Handwerker, when
now-abandoned proposals were being discussed in the United States
to ban lead in electronics.
Ronald Gedney, a consultant for the International Electronics Manufacturing
Initiative (iNEMI), a technology consortium, has been intimately
involved in the search for alternatives. He says that a search by
industry experts for possible replacements for lead-tin solder winnowed
down 75 metal alloy alternatives to about half a dozen. “We
decided the biggest benefit for the industry would be to pick one
solder, concentrating our development and research efforts on one
alloy and making it work,” he says.
The industry eventually selected a tin-silver-copper combination
as offering the most reliability and ease to work with as a replacement.
The formulation--95.5% tin, 3.9% silver, 0.6% copper--is also known
as SAC solder, for the first letters of the chemical symbols of each
of the elements (Sn, Ag, Cu). “Tin-silver-copper appears to
have at least as good reliability if not higher reliability than
tin-lead,” says Handwerker.
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The alloy alternative. Tin-silver-copper
solder offers a safer solder than the lead-tin
alloy, and research is continuing to address limitations
on its use.
image: Bernzomatic |
Furthermore, according to a 2005 draft report issued by the U.S.
Environmental Protection Agency titled Solders in Electronics:
A Life-Cycle Assessment, silver was “rarely encountered
above the detection limit” in synthetic landfill leachate created
to test the stability of electronics components. Silver--which is
regulated as a hazardous material--is toxic to aquatic life.
With a melting point of 217°C, SAC solder also is closest in
melting point to the conventional lead-tin solder. This does mean,
however, a yet-unquantified increase in energy use. Furthermore,
the higher temperature may pose problems for the electronics industry.
Higher temperatures mean more stress on components and the entire
manufacturing process, notes Geibig. Higher temperatures also mean
increases in the time it takes to make products, because more time
is required to heat and cool the products during the course of their
manufacture.
SAC solder is used widely in the industry today. However, many
of the components being made could not withstand the higher temperatures,
says C. Michael Garner, director of materials technology operations
at Intel: “That required re-engineering and getting new
materials, not only for newer products but for older products. All
the older products that had been in production for ten or fifteen
years had to be converted over to high temperatures.” He says
it has taken a massive effort to integrate the new solder into production
processes.
There are also short-term consequences of using the new solder.
Anytime there is a change in materials, there is a learning curve
in using the new materials, says Karl J. Puttlitz, who managed IBM’s
efforts to reduce lead in its products before he retired last year.
He anticipates the occurrence of more manufacturing defects as a
result of the change-over. “We can expect that at least initially
the failure rates [of products] will increase,” he says. In
fact, he notes the industry has asked for exemptions to the EU lead
ban in certain critical electronic components where lives and security
might be involved, such as equipment used in hospitals, until a track
record is established with consumer goods such as cell phones and
digital cameras. (The EU directive does permit exemptions to the
lead ban if replacing lead is technically or scientifically impractical
or if negative health, environmental, or safety consequences of replacing
lead outweigh the benefits of the ban.)
A Stickier Approach
A more experimental alternative to lead-tin solder is the use of
ECAs. These are polymers, such as silicone or polyamide, containing
tiny flakes of metals such as silver. The polymers adhere to the
printed circuit boards, and the metal flakes conduct electricity.
ECAs offer a range of advantages, notes C.P. Wong, a professor
in the School of Materials Science and Engineering at the Georgia
Institute of Technology who is regarded by many in the field as the
leading researcher in this new technology. Silver’s electrical
conductivity is very high, and its electrical resistance is very
low, he points out. “If the current-carrying capability [can
be boosted], ECAs can replace solder,” he says.
And there is another benefit. The temperature required to apply
ECAs to circuit boards is far lower than that required for lead-based
solder--150°C compared to 183°C. “You save energy,
number one,” says Wong. “Number two, you subject all
the components to lower temperatures and thus less thermomechanical
stress. That enhances their reliability. ”
Preliminary studies comparing parts using ECAs instead of solder,
such as a Finnish study presented in 2000 at the 4th International
Conference on Adhesive Joining and Coating Technology in Electronics
Manufacturing, suggest that ECAs boast a much tighter bond than solders--perhaps
an order of magnitude better, says James Morris, a professor of electrical
and computer engineering at Portland State University. But he adds
this research has to be replicated before it is regarded as valid.
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Current advances. Researcher
Grace Yi Li holds samples of electrically conductive
adhesives being
studied at Georgia Tech’s School of Materials
Science and Engineering. Such adhesives may one day
replace lead-based solders.
image: Gary Meek/Georgia Tech |
ECAs are available for a small number of applications requiring
low power--for instance, liquid crystal displays--though they are
not ready for the marketplace in general, where greater amounts of
current are needed. Wong is working to enhance their ability to carry
current. He is adding molecules of dicarboxylic acid to the silver
flakes, which provides a link between the flakes, allowing for efficient
and quick conduction of electric current. “It looks like it
can be as good as or even better than lead-tin solder. We demonstrated
that it works [in a presentation at the March 2005 national meeting
of the American Chemical Society], but we still need further research
and development,” says Wong.
Wong and his collaborators are also using another means to boost
the capacity to carry current--self-assembled monolayers. These are
single layers of sulfur-containing molecules known as thiols that
are attached to gold pads in the electronic device. At less than
10 angstroms (10 ten-billionths of a meter) in length, the molecules
chemically bind to the gold pads in the device and the board, providing
a direct electrical connection.
Still more work is needed on these structures, however, because
they begin to fail structurally if the component heats up above 150°C.
And there are other concerns about ECAs. With time, notes Wong, the
ability of ECAs to conduct electricity drops, and resistance to electricity
increases. Another concern is moisture. “Water is absorbed
by polymers, in general,” says Morris. That can encourage corrosion,
he says, and may cause other as yet unknown problems, he says.
Wong also points to the need for ECAs to become tougher so they
can withstand the force of being dropped. One way to do this, says
Wong, is to develop polymers that are rubberized and made more elastic,
so they won’t break. Finally, Garner reiterates that these
materials have not been reliable for carrying moderate to high amounts
of current under normal operating conditions.
Wong and Morris are optimistic that with more research and development,
ECAs can be successful alternatives to lead-tin solder. And Puttlitz
does see a place for them in consumer electronics such as cell phones
and digital cameras, which are not “mission critical” applications
where reliability is a matter of life and death as in medical monitoring
equipment or aircraft electronics.
Solder Replacement Soldiers On
Even as efforts to replace lead in solder move ahead, there still
appear to be concerns about the impact that newly implemented metals
will have on human and environmental health. “The alternatives
to lead have not been researched as well as lead in terms of potential
health and environmental impacts,” says Oladele A. Ogunseitan,
a professor of environmental health, science, and policy at the University
of California, Irvine. “When the Europeans said industry must
get rid of lead, they didn’t say you must replace lead with
something that is obviously safer,” he notes wryly. It is important,
he adds, to keep looking for lead alternatives that are environmentally
benign.
Indeed, the draft Solders in Electronics report indicates
that no metallic alternative to lead is free from environmental concerns.
For instance, whereas lead may pose a greater public health problem
than SAC solder, the latter uses noticeably more energy than lead-tin
solder.
But the presence of today’s substitutes is good enough for
Donkers. “Since there are alternatives, we have chosen not
to have lead in the products anymore,” he says. And while he
does acknowledge that there are relatively few data on the impact
of the current lead solder alternatives, he asserts that “in
terms of active policy, you cannot always wait till you have complete
certainty, because in the meantime a lot of people get exposed [to
lead].”