Aging behavior of a Sn-Bi eutectic solder at temperatures between 233 and 373 K

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 15.4Ms, 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 ther e was little change in the hardness up to about 1 Ms after which hardness d ecreased rapidly. At an aging temperature of 373 K the area fractions and w idths of the Sn and Bi layers in the lamellae structure as well as their la ttice constants were determined by microstructural observation, electron pr obe microanalysis, and X-ray diffraction analysis. The decrease in hardness in the initial aging stage may be due to the disappearance of Si particles , followed by dissolution of Bi into the Sn matrix. The rapid increase in h ardness in the transient stage could arise from changes in the Sn content i n 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 frac tion of Sn phase.