Editorial Free

Article Type: Editorial From: Soldering & Surface Mount Technology, Volume 24, Issue 4

The conversion of electronics assembly to lead-free has been underway for many years and, in terms of products impacted by the European RoHS Directive,that change became mandatory over six years ago. Nevertheless, the introduction of alternative alloys to replace conventional tin-lead has highlighted a number of significant issues that continue to cause real concerns for producers,particularly in terms of providing the requisite long term reliability performance required for complex products that have to operate in harsher environments. I have highlighted these issues in previous editorials, but the research that is reported in several of the papers contained in this issue also confirms their continuing significance and the levels of ongoing international activity to address them.

Of the six papers included in this issue, four of them are directly concerned with tin-silver copper (SAC) alloys, the solders that have been the preferred replacement for tin-lead in many applications. The first paper reports on work to assess the influence of the solderable finish used on ceramic chip resistor reliability. In particular, the work reported investigated the use of three different lead-free surface finishes: organic solderability preservative (OSP),immersion silver and electroless nickel-immersion gold. The results indicated that, under the test regime utilised, boards assembled using an OSP had the best reliability of the three finishes tested.

One of the concerns about using some SAC alloy formulations is the relatively high levels of silver they contain and the impact on alloy cost. There has thus been much work undertaken to develop low silver SAC alloys. In the second paper,work is reported on the evaluation of the properties of a new low-silver solder SAC alloy that had been modified by the addition of bismuth and erbium. The authors report that the addition of these elements significantly improved the tensile properties of solder joints and enhanced resistance to high temperature aging. The results suggest that the addition of Bi and Er could complement effectively the effects of silver, thereby reducing the overall cost of solder and helping to make low-silver SACBE solder a potential alloy for electronic packaging production.

The third paper also has a focus on low silver SAC alloys and, in this case,the effect of minor additions of iron on the bulk alloy microstructure and tensile properties of the solder alloy are reported. The authors report that the addition of iron to the alloy resulted in the formation of large circular shaped FeSn₂ IMC particles. This had a significant effect in reducing the elastic modulus and yield strength and maintaining the elongation. Moreover, the additions of iron also resulted in the inclusion of Fe in the Ag₃Sn and Cu₆Sn₅ IMC particles. The additions of Fe did not have any significant effect on the melting behaviour.

Carbon nanotubes can be produced in a wide range of forms that have interesting and unusual properties and this has encouraged investigation of their use in many electronics applications. The fourth paper is part one of an investigation of the influence of carbon nanotubes on the properties of SAC solder pastes. In this initial paper, the authors describe a method for preparing a “nano” solder paste that employs the two-stage dispersion of carbon nanotubes in a SAC 305 solder paste. The results of a comparative study of the properties of “nano” pastes with the basic SAC solder paste are reported and it was found that the carbon nanotubes had a positive effect in reducing the IMC thickness. Further studies of the mechanical properties and the reliability of solder joints were deemed to be necessary prior to practical implementation and more information will be reported in part two of the paper.

The fifth paper continues with the theme of solder alloys and the improvement of their properties by the use of additional elements. In this case the authors report on work to address another problem that has become associated with tin-rich, lead-free alloys; the paper details the results of a study of tin whisker growth in tin-zinc solder and the influence of the addition of the rare earth praseodymium. The results indicated that the addition of Pr could improve the wettability and mechanical properties of Sn-9Zn solder while also avoiding tin whisker growth.

With the relentless demand for greater levels of performance and functionality, assembly approaches such as die stacking provide an increasingly important solution for some device applications. In the final paper of this issue the subject matter is related to aspects of chip stacking and it covers an evaluation of microbump bonding processes for 3D integration using wafer-level underfill film. Current multi-chip stacking approaches can have significant limitations in terms of cost and processability and, to avoid such issues, work was undertaken to develop a flip-chip bonding technology with a wafer-level underfill process on ultra-fine pitch chip-on-chip packages for advanced high performance devices.

I hope that you find the work reported in this issue to be both interesting and useful and, as always, I welcome your feedback and comments. I would also be pleased to hear from readers who would be interested in joining our international team of expert peer reviewers for submitted papers. I can be contacted at: m.goosey@lboro.ac.uk

Martin Goosey