ENHANCED SOLDER ALLOY PERFORMANCE BY MAGNETIC DISPERSIONS

New Pb-free solder alloys with improved resistance to deformation and creep have been developed by dispersion hardening with essentially noncoarsening particles. Application of a magnetic field to molten solders containing fine (< 2 mum) ferromagnetic particles led to microstructures with a uniformly distributed, three-dimensional network of the dispersoid particles. Magnetostatic repulsion among columnar chain of spheres and the formation of a network structure overwhelms the gravity effect and prevents the commonly encountered problems of particle agglomeration and segregation caused by nonwetting and density differences between the dispersoids and the molten solder matrix. The presence of the dispersoid particles makes the plastic deformation of the solder material more difficult, thus improving the strength and reducing the creep rate at elevated temperatures. A finer solidification microstructure also results from the dispersion. A magnetically processed Sn-2.5% Fe composite solder exhibited an ultimate tensile strength approximately 60-100% higher than the dispersion-free solder materials and, more importantly, a 20-fold improvement in creep resistance at 100-degrees-C. The presence of magnetically dispersed Fe particles in a Bi-43% Sn eutectic solder under the same high temperature conditions resulted in a five-fold increase in creep resistance.