IPC/JEDEC Lead-free Conference
By Julia Goldstein, Ph.D., Advanced Packaging contributing editor

A combination of logistical and technical information, including an update on the European Union’s RoHS directive; news on similar legislation pending in other countries; and the issue of backward compatibility was the focus of the IPC/JEDEC 12th International Conference on Lead Free Electronic Components and Assemblies, held March 7–9.

Steve Andrews of the UK Department of Trade and Industry reported on the EU’s directive, including the ever-expanding list of exemptions granted and exemption requests. German Avila of Synapsis and Michael Kirschner of Design Chain Associates discussed lead-free legislation from China, Japan, and the U.S. Kirschner described differences between RoHS and a Chinese version &#8212 whose title is unclear because all original documentation is in Chinese with several possible English translations &#8212 noting that the Chinese legislation will provide a catalog of covered products that is narrower in scope than RoHS, rather than a wide scope followed by exemptions. The concept of exemptions does not apply. Because Chinese legislation specifies “materials, technologies, and processes which are energy-efficient, easily recyclable, and environmentally friendly,” products manufactured using restricted substances that do not appear in the final product &#8212 lead in PCB sacrificial layers, for example &#8212 may be acceptable for sale in the EU, but not in China. Also, China will require both product marking and testing to achieve certification, and will issue Industrial Standards documentation (in Chinese) in the next few months. An issue of contention is that only testing done by Chinese labs will be accepted.

Addressing the issue of backward compatibility was John Pan of Solectron, who presented research into the optimum reflow profile. As more suppliers move to lead-free manufacturing, companies whose products are exempt are having a harder time finding non-RoHS-compliant components and are faced with the prospect of attaching BGAs with tin-silver-copper (SAC) solder balls onto PCBs printed with SnPb eutectic solder paste, for example. Using a SnPb reflow profile will result in partial mixing of lead into the solder ball, incomplete melting, and no self-alignment, while using a SAC reflow profile will allow the solder ball to melt and self-align. However, fluxes in the SnPb solder paste are not designed for such high temperatures.

Solectron used a method developed by the National Institute of Standards and Technology (NIST) to calculate the liquidus of the mixed SnPb/SAC solder based on volume of the solder paste and ball and compositions of each. Reflow profiles are then tailored to the melting point of the mixed solder. Increasing the volume of solder paste will lower the liquidus, allowing a lower reflow profile, but the higher lead content weakens the solder and may reduce reliability. Pan showed temperature cycling data that demonstrated reduced reliability of SnPb/SAC assemblies as compared to SAC/SAC assemblies, regardless of reflow profile.

“If you’re forced into it [mixing SnPb boards and lead-free components], don’t do it,” was the response of one audience member with reliability concerns. Avoid mixing SnPb and SAC if at all possible was Pan’s recommendation. If it must be done, increase peak reflow temperature to above the liquidus of the mixed alloy for at least 30 seconds, and check that all components on the board can withstand the elevated temperature. It is also important to evaluate the microstructure in cross-section to verify complete mixing and self-alignment.

–Julia Goldstein, Ph.D., contributing editor


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