Emerging Technologies Conference Proceedings


METALLURGICAL AND RELIABILITY ASPECTS OF LEAD-FREE MIXED TECHNOLOGY ELECTRONIC ASSEMBLY FOR MOBILE COMMUNICATION PRODUCTS

Authors: Steven O. Dunford & Dr. Puligandla Viswanadham
Company: Nokia Mobile Phones
Date Published: 11/1/2000   Conference: Emerging Technologies


Seika Machinery, Inc.

Abstract: Pb-free electronics processing fits the definition of a "social technology", a technology that could benefit large populations and is relatively easy to develop, but one that requires the involvement of numerous institutions in government and industry in order to commercialize. Examples of other social technologies are alternative energy sources (ex: solar energy), low-cost prefabricated housing, and anti-pollution technologies (ex: electric vehicles). The common point between them is that politics, expressed through government regulations, is often the strong influence behind adoption and commercialization. The implementation of social technologies can be cost prohibitive in competitive markets and when market demand is insufficient to drive implementation, governments often step in to provide additional incentive to change. The legislative incentives provide the impetus to overcome the technical barriers to adoption and build the necessary industry infrastructure.

Most of the electronic industry manufacturing infrastructure is in place, leaving technical infrastructure issues to be solved, in order to successfully implement a change to Pb-free electronic packaging technology. For Pb-free electronics, one of the current technical issues that could impede progress is the lack of a common component termination metallurgy. Although it may never be entirely consistent, the vast number of options today complicates processing and reliability predictions. To speed the introduction of Pb-free processing, one must reduce the number of component termination metallurgies offered. Current offerings, combined with today's board surface finishes, result in significant variation in board level solder joint metallurgy. Standardization should contribute to greater consistency and predictability of solder joint reliability but may require some who now have Pb-free terminations to consider modifications to enhance the solder joint reliability of mixed technology assemblies.

Metallurgical aspects of mixed technology circuit board assemblies are discussed in this paper. Multiple variations of solder joint metallurgy possible in mixed technology assemblies are demonstrated. Many of these components meet the criteria for "Pb-free" but raise reliability concerns due to the intermetallic formations when combining the component termination with Sn/Cu/Ag/Sb solder paste and Ni/Au surface finishes. Moisture induced "popcorn" failures of one component indicate an increased sensitivity to humidity exposure. Post-assembly cross sections of the solder are compared to solder joint cross sections made after drop and thermal shock tests. Unique intermetallic combinations that are possible in mixed technology assemblies and changes in the failure locations are highlighted. Despite of the multitude of metallurgical combinations, the robustness of Pb-free solder joints in comparison to eutectic solder joints in thermal shock tests, was demonstrated.



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