Environment, submicron design prompt lead concerns
By Terry Costlow
EE Times
September 3, 1999 (1:15 p.m. EST)
MINNEAPOLIS — Lead's in trouble. Government and industry groups are getting concerned about the effects of lead exposure in nature and the workplace. There's also a growing awareness that lead-based solder is hurting reliability in circuits as design rules shrink into the ultra-deep-submicron realm.
A recent study by the Institute for Interconnecting and Packaging Electronic Circuits (IPC; Northbrook, Ill.), a trade association for board fabricators and assemblers, showed that a whopping 97 percent of respondents were aware that cooking up a lead-free solder is of prime concern. The IPC vigorously fought U.S. legislation to ban solder in 1992, but now it's advising members to watch lead-free solder closely. Indeed, the institute is hosting an international summit on lead-free electronics here Oct. 23 to 28.
"What's happening is that a couple of Japanese companies are pushing green electronics, and in Europe there's a proposal that could ban electronic assemblies with lead solder from going into landfills," said Ron Gedney, director of operations for the National Electronics Manufacturing Initiative (NEMI; Herndon, Va.). "The two biggest users are batteries and leaded glass in displays, but the European directive lets those off [the hook] because there is no alternative." Tin-lead solder, he said, represents "reportedly around 0.6 percent of the world's lead usage."
There's also plenty of activity in the United States. NEMI, a large consortium whose membership roster includes most of the big names in electronics manufacturing, formed a task force on lead early this year. It has also set up a lead-free assembly project to examine the many issues surrounding a shift to lead-free solder. One aim is to make sure American manufacturers aren't caught by surprise in case legislation banning products that contain lead should pass.
As in many segments of the device packaging world, Japanese suppliers are moving quickly. "Panasonic has indicated that all its audiovisual products will be lead-free by 2001," said David Bergman, at the IPC. "They have 60 lead-free products in Japan, so they are already getting reliability data and working on their process issues." He said Panasonic "has seen an increase in market share with a Mini-Disc player that has a green sticker saying it's lead-free. They seem to be concerned about the environment [there]."
Aqueduct seals
Although 60,000 metric tons of solder are used annually worldwide, electronics is not the largest consumer. Solder has been around for 5,000 years, and soft solders such as tin and lead were first developed in Roman times to seal the joints of aqueducts. The lead-tin combination was especially attractive at the dawn of the electronics age, when miniaturization and volume production pushed mechanical joining techniques aside.
But the hyperminiaturization seen today is straining this venerable technology at the seams, as it were, stretching the tin-lead solder's ability to form sound and reliable joints.
A paper prepared by Mulugeta Abtew of SCSI Systems Inc. (San Jose, Calif.) and Guna Selvaduray of San Jose State University indicates that the radius of curvature of molten solder is already larger than the sizes of the solder joints of surface-mount devices with less than 0.5-mm pitch.
"This forces the solder to form joints with a smaller radius of curvature, which may cause it to flow away from the desired locations, due to high liquid pressure," the two write.
Further, ultra-deep-submicron designs have other problems with lead-based solder, as the interconnects move closer and closer to the silicon device. Lead and tin-lead alloys have radioactive daughter elements of uranium and thorium that decay to form alpha particles. Those particles, in some cases, have caused electron-hole formation in small-gate-dimension CMOS, prompting soft errors.
To date, however, no drop-in replacement for tin-lead solder has emerged that is as cost-effective, flexible and "designable" as the standard.
The most likely alternatives so far seem to be tin-copper or tin-silver. Other mixes don't react well to the lead solder on the contacts of many devices and components, and can't be used until the lead-free movement prompts component vendors to give up lead solders, too.
A defection from lead will have a subtle yet critical impact on the manufacturing process. For starters, lead is abundant and costs about 50 cents a pound. Alternatives like gallium or bismuth are hard to come by or, in the case of precious-metal alternatives such as silver, subject to price fluctuations. Another option, zinc, can have poor resistance to atmospheric corrosion.
"There are alternatives, but they are going to require changes in the manufacturing infrastructure," said Carol Handwerker, chief metallurgist at the National Institute of Standards and Technology (Gaithersburg, Md.). "For many alternatives, processing temperatures are 30° to 40° higher [than for lead], at 250° to 260°C. For component designers and suppliers, that's an issue, since some molded materials [used for packaging] may not survive those temperatures."
There are also reliability questions, since new materials will have to be tested over the long haul. At the same time, however, the higher melting temperature actually provides benefits in some cases.
"I expect to see applications where the auto industry will switch early on, especially in areas where they need high temperature, like under the hood," said IPC's Bergman. "For them, [the shift from lead] has some advantages."
Though lead-free processing is being treated as a new trend, it's old hat for some manufacturers.
"Ford has been manufacturing with tin-silver for seven years, and IBM did mainframes without lead for several years," said Handwerker of NIST. "There are already manufacturing lines in Europe that use tin-silver solder."
Just when all this activity might have an impact on the market remains an open question. Proposed legislation in Europe and Japan now sets deadlines of 2001 and 2004, respectively, for getting rid of lead, though those could change as the political wrangling evolves. If manufacturers don't have governmental prodding, as in the United States so far, the timetable is less clear.
"It's difficult to say how quickly the change will take place, since it's driven more by the market than by legislation, but I do think we'll see end products in the United States next year," Bergman said. "If the U.S. had legislative requirements like Europe and Japan are discussing, we would know what date had to be met. A lot of OEMs are now asking questions like, Can the components we use work at higher processing temperatures?"
— Additional reporting by Brian Fuller and Ron Wilson
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