The advent of technology involving CSPs and other miniature package types, and the demand for solder joints for both mechanical and electrical connections, has challenged the properties of lead/tin solder. Another radical change for the electronics industry, one that will take the industry into a new era of lead-free solder, is now underway. With the indisputable evidence of lead toxicity in multiple forms, the electronics industry is embracing environmental issues that will involve the take-back of products and the move to an appropriate end-of-life treatment for those products. In addition, environmentally aware production techniques and products are required (or soon will be) by makers of electronics to satisfy the consumer's growing insistence that all manufacturing be "green." Lead-free Solders Since the early 1990s, the level of interest in lead-free solders has varied from lukewarm to red-hot. Proposed legislation in Europe calls for a ban on lead in most consumer electronics by January 1, 2004. In Asia, some major Japanese OEMs, such as Hitachi, NEC, Panasonic/ Matshusita, Sony and Toshiba (through their roadmaps), have begun to jointly develop recycling processes for electronic products, with various commitments to eliminate lead in their products. Activity on lead-free solders in the U.S. has been low since the initial attempts in Congress to ban lead in electronics in the early 1990s, although sustained interest in using lead-free solders has persisted within the automotive industry. Even while indicating that the industry's lead use has not proven hazardous, organizations such as the IPC and NEMI (National Electronic Manufacturing Initiative) have been engaging in activities to educate people about alternatives. In addition to the organizations named above, another push has come from the state of California's enactment of Proposition 65, which sets a timetable for eliminating lead that can be dumped in landfills. The very real prospect of electronics assembly-without lead-based solders-is on the horizon in the U.S. Lead/Tin Solder Eutectic lead/tin solder is the foundation of many electronics assembly processes, components and PC boards. In addition, the industry's reliability database-in both laboratory and field-is based on eutectic lead/tin. The mechanical property limitations of lead/tin solder were recognized and compensated for, initially, in a move toward surface-mount technology, or even during package design and lead geometry. There is no one-to-one direct substitute to replace lead in the entire periodic table, and finding an alternative to lead is not a simple task. Over the years, although a lead/tin, eutectic-like solder has been a dream, no drop-in lead-free replacement has been found. Several studies by the National Center for Manufacturing Sciences (NCMS), European-based IDEALS (Improved Design Life and Environmentally Aware Manufacturing of Electronic Assemblies by Lead Free Solder) and the UK's International Tin Research Institute (ITRI) have shown that the most likely substitute will be some combination of Sn (tin), Cu (copper) and/or Ag (silver). Lead-free Solder Alloys There are viable candidates for replacing lead/tin eutectic solders. Some are based on adding small quantities of a third or fourth element to binary alloy systems to lower the melting point and bond strength, and in- crease wettability and reliability. The leading replacement candidates are shown in Table 1. oWhile solders with melting temperatures <183oC may find acceptance in consumer and telecommunications, those with melting temperatures >183oC, will be needed in demanding automotive applications.ITRI has indicated that the main alloy for general-purpose soldering will be SnAgCu, with a melting point of 217oC. NEMI has indicated that it will concentrate on the Sn3.9Ag0.6Cu alloy for surface-mount assembly and reliability work, with Sn0.7Cu as the main alloy to test for wave soldering and with Sn3.5Ag as a second choice for wave soldering.
The wettability of these alloys on thoroughly clean copper coupons at 250oC is shown in Table 2, relative to that of Pb/Sn alloy.
Among the potential problems with lead-free solders are the need for more time to wet, more heat, better solderability and more aggressive flux. Elevated Temperatures Lead in solder affects the physical and mechanical properties, as well as solder's application characteristics. These chracteristics include melting point, surface tension, fluidity, conductivity, CTE, microstructure, metallurgical reactions and mechanical failure mechanisms. The failure mechanisms encountered by solder joints of eutectic lead/tin solder during thermal fatigue correspond to an accumulated degradation, often associated with grain (phase) coarsening. PC Board Finishes The replacement of hot air solder leveling (HASL) has been an area of concern for the PC board industry, which wants to reduce thermal shock to the board during HASL processing and to provide a flatter and more uniform finish for fine-pitch assembly. Options for lead-free surface finishes, compatible with components and circuit boards, include organic solderability preservative (OSP), nickel/gold, palladium, electroless palladium/electroless nickel, palladium/gold, nickel/palladium/ gold, tin, silver, lead-free HASL tin/copper, immersion silver, immersion gold/electroless nickel and tin/bismuth. Many studies of lead-free solders have examined either freshly cleaned copper or copper with OSP. The Printed Wiring Board Manufacturing Technology Center (PWBMTC) is conducting a study of board finishes for lead-free solders. Another issue, that of PC board com-patibility with high soldering temperatures, requires the use of high-temperature laminates, which are more expensive. In this context, low-temperature lead-free solders may be an option. Component Finishes Lead (termination) finishes are most commonly lead/tin based. This use must change for lead-free assembly. Lead-free solders may have to be implemented before component leads with lead-free finishes will become available. Nickel/palladium, nickel/gold, silver/ platinum, silver/palladium, and pure tin and nickel finishes are used on some comp- onents today and appear to be likely contenders in the future. Area-array assembly, on the other hand, employs solder spheres that are typically lead/tin eutectic for attachment. Questions exist as to which lead-free alloy is the clear winner, the one that will be compatible with the other lead-free options. SnAgCu, however, appears to be one of the alloys that will see increased use in lead-free solder spheres. Conclusion With the increased demand on the performance and reliability of interconnections in microelectronics/electronics packaging and assemblies, the task of developing lead-free solders should be taken not as a legislative mandate but as a timely industry opportunity. By Dr. Rao Mahidhara |
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