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Purpose -Historically, tin-lead solder has been a commonly used joining material in electronics manufacturing. Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from tin-lead solder to a lead-free solder composition is imminent. Several alternative solder alloys (and their fluxes) have been researched for electronics assembly in the last few years. The objective of this research was to develop a systematic selection process for choosing a "preferred" lead-free solder paste, based on its print and reflow performance. Design/methodology/approach -After a detailed study of industry preferences, published experimental data, and recommendations of various industrial consortia, a near eutectic tin-silver-copper (SAC) composition was selected as the preferred alloy for evaluation. Commercially available SAC solder pastes with a no-clean chemistry were extensively investigated in a simulated manufacturing environment. A total of nine SAC pastes from seven manufacturers were evaluated in this investigation. A eutectic Sn/Pb solder paste was used as a baseline for comparison. While selecting the best leadfree paste, it was noted that the selected paste has to perform as good as, if not better than, the current tin-lead paste configuration used in electronics manufacturing for a particular application. The quality of the solder pastes was characterized by a series of analytical and assembly process tests consisting of, but not limited to, a printability test, a solder ball test, a slump test, and post reflow characteristics such as the tendency to form voids, self-centring and wetting ability. Findings -Each paste was evaluated for desirable and undesirable properties. The pastes were then scored relative to each other in each individual test. An aggregate of individual test scores determined the best paste. Originality/value -This paper summarizes a systematic approach adopted to evaluate lead-free solder pastes for extreme reflow profiles expected to be observed in reflow soldering lead-free boards.

A team of NEMI companies collaborated for three years to develop Pb-free assembly and rework processes for doublesided, 14-layer, printed circuit boards (PCB) in two thicknesses (0.093" and 0.135") with electrolytic NiAu and Immersion Ag surface finishes. This work followed the initial SMT manufacturing feasibility effort carried out by the first NEMI Pb-free development team (1999)(2000)(2001)(2002). All SMT assembly, PTH wave assembly and component rework processes were carried out on production equipment. Various test vehicles including the reliability test board were used in a multiphase development project to develop Pb-free assembly and rework parameters and temperature profiles prior to a 100-board process technology verification build. Following the double-sided SMT and wave assembly build, half of the printed circuits assemblies were passed through a series of representative component rework protocols. Each build group was then subjected to a series of mechanical and thermal reliability stress tests, including 5700 cycles of 0 to 100°C, followed by failure analysis. A special test board was designed utilizing a high temperature laminate designed for Pb-free soldering. Approximately 30% of the assemblies were SnPb control samples. This paper will present the Pbfree SMT assembly and rework development process using the NEMI Sn3.9Ag0.6Cu solder, and results of the reliability stress tests. The rework of large, thick PCB's with Pb-free solder poses a significant challenge to the industry. The lesson's learned and recommendations for future work will be discussed.

Flip chip circuit assembly is an emerging technology providing faster, smaller, and less costly electrical circuitry.

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