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SMT Perspectives and Prospects
By Dr. Jennie Hwang
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Lead-free Symposium at APEX
Amid a variety of programs, lead-free technology was "a conference in a conference" at APEX 2002, in San Diego. The program was designed with six focal themes covering components, printed wiring board (PWB) surface finishes, materials, assembly processes, test and rework, and reliability. Additionally, SMT Magazine developed an illuminative panel forum and NEMI discussed a summary report by NEMI.In the component session, FCI Electronics' S. Lal, Ph.D., provided an overview of state-of-the-art lead-free connector technology, including plating processes and associated solderability and reliability issues. P. Kinner of Concoat in the United Kingdom examined the surface insulation resistance (SIR) testing methodology to characterize a lead-free electronic production process. With respect to PWB surface finishes, P. Snugovski, Ph.D., of Celestica reviewed their studies on the "black pad solder joint failure" phenomenon. While this issue generally is associated with the corroded Ni layer in the electroless Ni/immersion Au surface finish as well as board design, the understanding of a full failure mechanism calls for in-depth study. Microstructural examinations at the interfaces for both Pb-bearing and Pb-free solder joints were reported in this presentation. It was found that solder joint formation on black pad-affected interconnects is identical for both Sn/Pb and Pb-free solders. As expected, significant microstructural changes were detected in aged Sn/Pb and Pb-free solder joints under elevated temperatures. One interesting find is Au does not concentrate around the interface area in the case of Pb-free solder joints, contrasting with Sn/Pb solder joints in which the Au-rich phase was observed when Au thickness on a PWB pad is more than 5 μ". P. Roubaud, Ph.D., of Hewlett Parkard reviewed their solder joint reliability study on dual inline memory module (DIMM) assemblies, which consist of thin small outline package (TSOP) components with Pb-bearing lead coating soldered with Pb-free no-clean solder paste on organic solderability preservative (OSP) and Ni/Au PWB finishes. Their results indicated sound solder joint reliability for this assembly.
In assembly processes, E. Smelik, Ph.D., of Cookson and D. Barbini of Vitronic Soltec in The Netherlands reported materials compatibility studies, machine modifications and selective soldering processes in the manufacturing environment. Additionally, C. Hunt, Ph.D., of National Physical Laboratory of the United Kingdom presented the study results on the solderability comparison of lead-free solders in nitrogen. Various experiences in rework and test also were shared in the rework and test session by K. Tellefsen, Ph.D., of Alpha-Fry, R. Szymanoski of Celestica and A. Gowda of State University of New York. The reliability session included a discussion of Sn whisker growth, and thermal cycling and aging effects on lead-free solder joints.
The lead-free panel forum, moderated by Gail Stout of SMT Magazine, featured a broad view of lead-free technology and implementation. The panel collectively shared knowledge, experiences and views concerning the lead-free arena.
Impressive advancements have been achieved in all six focal areas. While interacting with the industry, one frequently asked question appears to be shifting lately from "Do you think lead-free is viable?" to "What is the next step for lead-free?". A logical answer to the latter is to select the proper alloy composition(s) by applying fundamentals and manufacturing know-how, evaluate the process by using solid knowledge and validate the system with an open mind.
As was well said in the program brochure, the conference was a global gathering of the best of the best researchers, manufacturers and users. Overall, the conference provided the timely information for optimizing today's technology and investing in tomorrow's. Certainly, this prepares the industry for the economic upturn that is right around the corner and the global environmental thrust that is sizzling.
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