M. Mccormack et al., ENHANCED SOLDER ALLOY PERFORMANCE BY MAGNETIC DISPERSIONS, IEEE transactions on components, packaging, and manufacturing technology. Part A, 17(3), 1994, pp. 452-457
New Pb-free solder alloys with improved resistance to deformation and
creep have been developed by dispersion hardening with essentially non
coarsening particles. Application of a magnetic field to molten solder
s containing fine (< 2 mum) ferromagnetic particles led to microstruct
ures with a uniformly distributed, three-dimensional network of the di
spersoid particles. Magnetostatic repulsion among columnar chain of sp
heres and the formation of a network structure overwhelms the gravity
effect and prevents the commonly encountered problems of particle aggl
omeration 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 microstruc
ture also results from the dispersion. A magnetically processed Sn-2.5
% Fe composite solder exhibited an ultimate tensile strength approxima
tely 60-100% higher than the dispersion-free solder materials and, mor
e importantly, a 20-fold improvement in creep resistance at 100-degree
s-C. The presence of magnetically dispersed Fe particles in a Bi-43% S
n eutectic solder under the same high temperature conditions resulted
in a five-fold increase in creep resistance.