Everybody used to talk about the weather, but now the main topic-at least in the electronics industry-is lead. Although we've known for eons that lead is a poisonous element, its value in many fields, particularly electronics, makes it a tough resource to abandon with a snap of the fingers. However, from little more than a scholarly interest a few years ago, the move to get the lead out of electronics, particularly the use of lead in solders, has become a shrill and insistent cry that began in Asia and Europe and has quickly reached our shores. Substantial Impact The impact of abandoning lead-based processes and lead-based materials in electronics will be substantial. It is likely to provoke the greatest brouhaha among lead-free supporters-and those not ready for the conversion-since the switch from chlorofluorocarbons (CFCs) nearly two decades ago. The move to lead-free processing will impact two key areas in the industry, device packaging and circuit board materials, notes Anthony Primavera, manager of Universal Instruments' Advanced Surface Mount Laboratory (Figures 1 and 2), Binghamton, N.Y.
In small or sparsely populated assemblies, the temperature gradient across the PC board may be small enough to allow standard SMT reflow profiles (230oC or lower peak temperatures), he says. "In many cases, however, the peak reflow temperature will have to reach 240-250oC. This will ensure that on a densely populated board, every solder joint, regardless of its location on the board, forms properly." Another factor that may plague board assemblers, says Primavera, is glass transition temperature. "Current assembly materials, such as FR4 derivatives, which possess a GTT between 117o and 175oC, may not survive exposure to excessive reflow temperatures for extended dwell times." Board makers, adds Primavera, should be considering alternative materials such as cyanate esters, BT, ceramics or polyimides, that are better suited to higher-temperature processing. Universal Instruments believes that the migration to lead-free materials "is one of the critical trends for the electronics assembly industry in the new millennium." Primavera says. Consortium The company initiated a large industrial research consortium in January. The consortium, a partnership between Universal and about 30 of the world's leading board and component makers, OEMs, CEMs and material suppliers, will investigate the lead-free process options available. There is almost total industry agreement that the higher assembly temperatures required with most lead-free alloys will approach the acceptable limits for some packages. Moisture in the package can cause the rapid delamination and cracking phenomenon known as "popcorning," observes Alan Rae, director of technology for Cookson Electronics, whose products include the Accel, Camalot, MPM and Speedline equipment brands. The severity of popcorning is determined by pre-existing voids, thermal expansion or shrinkage stresses, the adhesion of polymer to die and the thermal profile-as well as the moisture content, Rae says. High-lead solders are the materials of choice for some packaging applications. The alternatives, Rae adds, are tin-antimony with a lower melting point, or high-cost precious metal joining alloys. "With greater circuit density, diffusion distances in the IC are reduced and a long thermal cycle may cause degradation." Solder-coated package leads, too, will need to employ lead-free solderable finishes. The solutions, says Rae, are robust quality control and the right material choices, combined with exact thermal profile control, which will minimize popcorning. "With attachment alloys, the best solution currently may actually be to lobby to have them exempted until acceptable alternative technologies are available." Packages, adds Rae, normally contribute less than 0.5% of the 7-8% solder on a circuit board. The use of insulating packaging materials or a separate attachment system are options to prevent ICs from overheating. Rae says Cookson Electronics' recent "Greenline" initiative combines Cookson's matching equipment, materials and technology to address the lead-free issue. "From our running of a complete lead-free demonstration line at Productronica, last November, to ongoing systems development and active work with industry bodies, Cookson is committed to ensuring that customers will be able to make the lead-free transition as smoothly as possible," Rae says. New Opportunities
"It appears that the material of choice for a solder substitute from a number of material suppliers is an Sn/Ag/Cu eutectic alloy." The benefits, in addition to its eutectic nature, are that its processing temperature is only 40 degrees higher than Pb/Sn alloys. Additionally, there are adequate supplies of Sn/Ag/Cu to meet demand for the foreseeable future." However, Adamson believes there will be a cost factor "of at least 2x" over Pb/Sn solders, as well as greater thermal stress, in opting for this substitute. "The largest unknown, however, is how these new alloys will behave in the field. That is potentially the biggest problem." It took more than 20 years of manufacturing with the present lead-based alloys to understand all of the problems and design them out, Adamson recalls. The problems include tin whiskers, joint fretting with impact shock, thermal fatigues and several other issues. "Before we move these new alloys into production, we need to rigorously test if lead-free is safe in crucial applications. While the elimination of lead can be a great benefit to society, it may create opportunities for personal injury lawyers as well as for engineers." Asymtek, in conjunction with solder paste makers, developed criteria for dispensing solder paste alloys of varying compositions years ago. This requires a balance between the flux and the metal content. Pastes with an alloy volume fraction greater than 40% tend to be too dry for dispensing, according to their research.* Dr. Luu Nguyen, engineer at National Semiconductor Corp., Santa Clara, Calif., notes that "Lead-free solder compositions with melting points of 215-240oC are proliferating." A higher no-lead reflow temperature of 260oC has often been mentioned as "the upper target," Dr. Nguyen says. At 260oC, all components on a fully populated board will reach the melting temperature of the lead-free solder. "With the current industry package qualification spec of reflowing three times, followed by an examination of internal damage, a majority of plastic packages will have difficulty meeting such a constraint at JEDEC levels 1 and 2," he adds. Thermal Strain Currently, certain small packages, such as SOs and TSSOPs, can withstand the thermal strain without suffering from interfacial delamination, popcorning or warping. Larger packages, however, will not meet this requirement. "More often, their JEDEC classification will be downgraded by one or two levels," Dr. Nguyen says. The lack of a lead-free industry standard "has not helped" National to implement lead-free processing, he adds. NSC has taken a three-tiered approach, Dr. Nguyen says. First, for leaded packages, NSC is re-evaluating alternatives to eutectic Sn/Pb. Next, for array packages, which include wafer-level CSPs, alternative lead-free alloys are under evaluation, and qualification is planned during the year. Finally, involvement with a few industry consortia "will also help with benchmarking information." Despite the obstacles remaining to a lead-free electronics assembly industry, there will be positive results, in addition to environmental protection, according to Dr. Elke Zakel, president of Pac Tech GmbH, Nauen, Germany. "One positive aspect of lead-free solder materials on semiconductor packaging is the potential for improved reliability." Better Reliability Data gathered by Pac Tech indicates that certain lead-free solders improve the reliability of flip-chip and CSP contacts, she claims. Since lead-free solders have a higher melting point, what was a negative in reflow, becomes a positive in the field. With their higher melting points, lead-free soldered devices are better suited to harsh environments, particularly in the automotive area, she observes. The problems in converting from lead-based assembly materials to lead-free materials will be manifold. Still, after decades of lethargy, the domestic industry has finally jumped on what is likely to become a lead-free, unstoppable bandwagon. *For details, request a copy of "Is your Solder Paste Dispensable?" at leadfreeinfo@asymtek.com. By Ron Iscoff |
