2001 Volume 42 Issue 5 Pages 769-775
The hardness of a Sn–Bi eutectic solder was measured as a function of aging time at 233, 298, 323, and 373 K for up to 15.4 Ms, and the time dependence of the hardness could be divided into three stages. At 373 K the hardness decreased up to 259.2 ks (the initial stage) and then increased between 259.2 ks and 1.8 Ms (the transient stage), and beyond 1.8 Ms (the final stage) the hardness tended to decrease gradually. These critical times, 259.2 ks and 1.8 Ms at 373 K, shifted to later at 323 and 298 K, while at 233 K there was little change in the hardness up to about 1 Ms after which hardness decreased rapidly. At an aging temperature of 373 K the area fractions and widths of the Sn and Bi layers in the lamellae structure as well as their lattice constants were determined by microstructural observation, electron probe microanalysis, and X-ray diffraction analysis. The decrease in hardness in the initial aging stage may be due to the disappearance of Bi particles, followed by dissolution of Bi into the Sn matrix. The rapid increase in hardness in the transient stage could arise from changes in the Sn content in the Bi phase. The decrease in hardness in the final stage could be mainly caused by coarsening the lamellar structure and increases in the area fraction of Sn phase.
