Recent concerns over the toxicity of Pb have resulted in the development of materials and processes for Pb-free solder alloy solutions. Motorola is a world leader in the development of environmentally preferred packaging solutions to address these concerns. The behaviors of a variety of Pb-free solder alloys are reviewed for Sn-Ag, Sn-Cu, Sn-Ag-Cu, Sn-Ag-Bi, Sn-Ag-X, Sn-Sb, and Sn-Zn for physical (melting, wetting, intermetallic formation, etc.) and mechanical (strength, creep, thermomechanical fatigue, etc.) characteristics. The results of this review show that Sn-4.7Ag-1.7Cu and Sn-0.7Cu have comparable properties to near eutectic Sn-Pb and can be used as substitutes for Pb-bearing solders for many applications. The increased process temperature of Pb-free solder alloys affects the choice of the packaging materials and flux. These restrictions must be considered when selecting the Pb-free alloy. Sn-Ag and Sn-Cu are recommended for lead finish, whereas Sn-Cu is recommended for wave soldering and flip chip bumping and Sn-Ag-Cu is recommended for surface mount assembly.
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