Recent Progress in Pb-Free Solders and Soldering Technologies

 

Sung K. Kang

IBM T. J. Watson Research Center

P.O. Box 218, Yorktown Heights, NY 10598, USA

(T) 914-945-3932, (email) kang@us.ibm.com

 

 

 

Abstract

 

Accelerated R&D efforts in industry, universities, national laboratories, and government agencies have recently identified several promising lead-free solders to replace Pb-containing solders in microelectronic applications. The leading candidates are all Sn-rich solder alloys with melting temperature between 210C and 227C. They are recommended for various soldering applications, such as SMT (surface mount technology), PTH (plated-through-hole), BGA (ball grid arrays), flip chip, and others. Despite the R&D progress and the proliferation in published technical findings on Pb-free solders, our knowledge and understanding of these new materials are still at an infancy stage compared to Pb-containing solders. Thus, unanswered questions and unresolved issues still persist such as; can we make and properly test reliable Pb-free solder joints? What are the implications of higher reflow temperatures required for the new solders? Are the new surface finishes needed? What is the Pb-free solder candidate for flip chip applications? Can we maintain the solder hierarchy between the first and second level interconnection? What are the reliability issues of new solder alloys? What are the solidification mechanisms in solder joints? How well are microstructure -property relations understood in Pb-free solder alloys and joints? Have interfacial reactions between Sn-rich solders and new surface finishes been adequately characterized? What are the influences of microstructural evolution during thermomechanical processes? There is much to be learned regarding thermal fatigue mechanisms in solder joints, creep and fatigue interactions, and corrosion behavior, etc. These are but a few of technical questions to be answered and critical issues to be resolved before we can confidently implement Pb-free solder technology.

 

In this presentation, the recent progress in lead-free solders and soldering technologies for microelectronic applications is briefly reviewed in terms of solder alloys, soldering processes, physical/mechanical metallurgy, interfacial reactions, reliability of solder joints, and other related issues.