Impact of the ROHS Directive on high-performance electronic systems. Part II key reliability issues preventing the implementation of lead-free solders

There are important logistical and technological issues that confront the electronics industry as a result of implementing lead (Pb)-free technology to comply with the European Union's (EU) ROHS Directive effective July 1, 2006. This paper focuses on the technological matters that pose the greatest potential risk to the reliability of so-called high-performance (H-P) systems (i.e., servers, storage, network infrastructure/telecommunication systems). The European Commission (EC) specifically granted a special use of lead (Pb) in solder exemptions, independent of concentration, for applications in H-P systems. The intent was to preserve the reliability of solder joints, particularly flip-chip solder joints. H-P systems perform mission-critical operations, so it is imperative that they maintain continuous and flawless operation over their lifetime. H-P systems are expected to experience virtually no downtime due to system failures. This paper discusses several major technological issues that impede the implementation of Pb-free solders in H-P systems. The topics discussed include solder compatibility and the reliability risks of mixed solder joints due to component availability problems. The potential effects of microstructural factors, such as the presence of Ag 3 Sn platelets, and ways to eliminate them are described. Also discussed are the effects that stress-test parameters have on the thermal-fatigue life of several Pb-free solders compared to eutectic Sn-Pb. Another issue discussed involves the effect that tin's body-center tetragonal (BCT) crystal structure has on solder-joint reliability, and the complete loss of structural integrity associated with an allotropic phase transformation at 13.2°C, referred to as tin pest. Yet another important consideration discussed is the tendency of pure tin or tin-rich finishes to grow "whiskers" that can cause electrical shorts and other problems. Finally, the paper notes the potential for electromigration failures in Pb-free, flip-chip solder joints. Based on the current status of the issues discussed, it appears likely that the exemption allowing the use of lead (Pb) in solder will need to be extended by the EC when scheduled for review in 2008, and perhaps well into the future.