Transitioning to Pb-free assemblies: convergence on a single lead-free alloy means studying compatibility (backward and forward), and materials and process development.

Five years ago the international microelectronics industry was considering and testing several lead-free alloys that could potentially replace SnPb eutectic for solder reflow and wave solder applications. Today, a narrow range of compositions in the SnAgCu system is the most widely recommended as a general purpose, standard alloy for surface-mount, wave and hand soldering in a variety of product types. There is no drop-in replacement for SnPb, but when considering melting point, toxicology, cost, availability and chemical resistance the SnAgCu system has emerged as the most acceptable compromise, both for solderability and reliability. SnAgBi alloys are sometimes used as low-temperature alloys, especially for surface-mount consumer products. (1) They are more common in the Far East, and rare in the U.S. Although bismuth-containing alloys tend to have good reliability in surface-mount joints, the reliability can deteriorate significantly if there is any lead in the resulting joint and high-tin bismuth solders can show fillet lifting in through-hole joints.

Other alloys considered had drawbacks as well. For example, supplies of bismuth and indium are limited, making them costly; indium in particular may be subject to cracking in the solder joints. (2) Zinc-containing alloys are difficult to use for most applications because it oxidizes too rapidly in processing.

Extensive testing has proven SnAgCu to be at least as reliable as SnPb eutectic. Several industry groups have recommended the SnAgCu alloys, including iNEMI (inemi.org), IPC's Solder Products Value Council, High-Density Packaging User Group, E3 (the top three European semiconductor manufacturers: Infineon, Philips, STMicroelectronics), the IDEALS program in Europe (Improved Design Life and Environmentally Aware Manufacture of Electronics Assemblies by Lead-free Soldering), Soldertec at ITRI (Europe), the Printed Circuit Interconnection Federation (a U.K. trade organization) and the Japan Electronics and Information Technology Industries Association. (1-4)

SnAgCu solders are tin-rich alloys with 3.0 to 4.0% silver and 0.5 to 0.6% copper. Compositions of specific alloys are expressed in weight %. For example, Sn3.0Ag0.5Cu is 3.0% silver, 0.5% copper, with the remainder being tin; in this case, 96.5% is tin. The leading alloy compositions are Sn3.0Ag0.5Cu (JEITA), Sn3.8Ag0.7Cu, (E3/Soldertec) and Sn3.9Ag0.6Cu (iNEMI). With J-STD-006 specifying that an alloying element less than 5 wt% can vary in composition by [+ or -]0.2 wt%, Sn3.9Ag0.6Cu is indistinguishable from Sn3.8Ag0.7Cu.

This column discusses the assembly behavior when alloys and processes are mixed and the effects on the structure and properties of the assolidified solder joints. The conditions under which SnPb pastes, solder baths and processes form reliable joints with lead-free surface finishes and solder balls on components and lead-free surface finishes on boards is referred to as "backward compatibility." Likewise, during the transition to completely lead-free assemblies, the conditions for "forward compatibility" are found when SnAgCu pastes, solder baths and processes form reliable joints when used with SnPb surface finishes on components and boards.

Author's note: This activity is part of the NIST Electronics Packaging and Interconnection Program that also includes modeling and measurements of tin-whisker formation, metrology for on-chip interconnections and public-domain modeling tools for solder joint design.

Dr. Carol Handwerker is chief of the Metallurgy Division at the National Institute of Standards and Technology (nist.gov), and a member of the iNEMI Technical Committee and co-chair of the iNEMI Research Committee; [e-mail].