Editorial

Citation

Goosey, M. (2010), "Editorial", Soldering & Surface Mount Technology, Vol. 22 No. 3. https://doi.org/10.1108/ssmt.2010.21922caa.001

Editorial

Article Type: Editorial From: Soldering & Surface Mount Technology, Volume 22, Issue 3

Welcome to this issue of Soldering & Surface Mount Technology. Although the subject of lead-free solders and assembly has been extensively studied for many years and despite the fact that the use of these alloys is now mandatory for many product applications in Europe and indeed the rest of the world, there are still many issues to be addressed. The relatively benign operating environments to which consumer electronics are exposed may not cause too much concern for products assembled with lead-free solder but once their operation moves towards the more severe environments experienced by professional electronics, the potential for reliability issues with lead-free solders continues to be a real cause of serious concern. As a consequence, there remains an unsatisfied demand for the development and assessment of new solder alloys that can provide the long-term reliability performance required in these applications. Fortunately, there seems to no shortage of potential alloy formulations that might provide the answer. Even with the well-established SAC alloy formulations there is the potential to improve their performance by the addition of small quantities of metals such as cerium and other lanthanides. In this issue, the focus is on the heavy metal bismuth and its impact on both tin-zinc and tin-silver alloys. It is important to remember that the use of lead-free solders is not the only way in which connections can be made between components and substrates. Driven, no doubt in part, by the on-going issues with lead-free alloys, there is much interesting work being carried out on the use of conductive adhesives and it seems likely, as these materials are improved and better understood, that they will find an increasing number of applications.

This issue has six papers, with two of them covering work on lead-free solder formulations and three more addressing some of the other important process related areas that have an impact on the reliability of lead-free assembled products. The final paper addresses the use of an alternative to solder in the form of anisotropically conductive adhesives. The issue opens with the first of two papers that investigate the influence of bismuth on solder alloys. The paper by M. He, N. De Leon and V.L. Acoff reports on the effect of bismuth on the microstructure and tensile strength of tin-silver solders cast under different cooling rates. Using conventional tensile testing methods, it was found that the addition of bismuth could increase the tensile strength of the solder, and thus offer potential improvements in solder performance for those seeking better replacements for Sn-Pb solder. Continuing with the theme of bismuth, the paper by K. Bukat, Z. Moser, J. Sitek, W. Gesior, M. Koscielski and J. Pstrus details a study of the influence of bismuth on the surface and interfacial tensions and density of SnZn7Bi alloys. The publication, which is Part 1 of a two part paper, describes how the addition of bismuth can lower these tensions, but also states that the reductions are not sufficient for practical applications. Additional work to verify the influence of bismuth additions on the surface and interfacial tensions, via the contact angles from the interaction with copper on PCBs with different lead-free finishes will be reported in Part 2 of this paper, which will be published in the next issue.

Moving away from solder alloys and their performance, the next two papers address related key factors that impact overall assembly reliability. The first of these is by G. Takyi and N.N. Ekere. The authors detail the results of their work to evaluate the effect of different atmospheres on the solderability of plasma treated hot air solder level finished PCBs using a wetting balance technique. Comparisons were made with conventionally flux treated samples soldered in air and nitrogen atmospheres and non-flux treated samples soldered in air. Auger chemical analysis results were also compared with the solderability test results in order to obtain a complete profile of the plasma treated and non-treated surfaces. The study indicated that the soldering performances of plasma treated PCBs in air and nitrogen atmospheres were comparable and give motivation for the use of plasma assisted dry cleaning for fluxless soldering. In the next paper, O. Nousiainen, T. Kangasvieri, K. Kautio, R. Rautioaho and J. Vähäkangas detail the results of an investigation into the effects of ENIG deposition on the failure mechanisms and characteristic lifetimes of three different non-collapsible lead-free second level interconnections in LTCC/PCB assemblies. It was found that a stable intermetallic compound layer was formed between the nickel deposit and the solder matrix during reflow soldering. The layer thickness did not grow excessively and the interface between the layer and solder was practically free from Kirkendall voids after the thermal cycling testing (TCT). However, the thermal fatigue endurance of the lead-free solder had a major effect on the characteristic lifetime. Depending on the thickness of the substrate and the alloy composition, TCT lifetimes of over 2000 cycles were achieved.

The fifth paper of this issue is by Fang Liu, Guang Meng and Mei Zhao and it addresses another important aspect of reliability and that is the performance of lead-free solder joints in drop tests. In this paper, the focus is on an investigation of the mechanisms of BGA lead-free solder joint failure under board-level drop impact testing. The test results from this work indicated that the combined effect of mechanical shock and PCB bending vibration was the root cause of solder joint failure. The authors also showed that the fracture of BGA lead-free solder joints occurred at the intermetallic compound interface near the package side and that the failure mode was brittle fracture.

Finally for this issue, the sixth paper is by Kati Kokko, Laura Frisk and Pekka Heino and it covers the thermal cycling reliability of flip chips on different types of PCB substrates, with an applied conformal coating. In this work, the die were flip chip bonded using an anisotropically conductive adhesive and the coating was vapour deposited parylene. The results showed differences between the two-substrate materials, with one proving to be reliable enough to withstand the thermal cycling testing. The use of the conformal coating layer did not have any negative impacts on the reliability and it proved to be a reliable choice for protecting and improving the thermal cycling performance of flip chip devices.

I hope that you enjoy reading these papers and, indeed, the rest of the content in this issue of Soldering & Surface Mount Technology. As always, I welcome your comments and feedback and can be contacted at: m.goosey@lboro.ac.uk

Martin Goosey