An effect of dispersoids in β-Sn matrix on creep properties of chip scale packages joined by Sn-xAg-0.5 mass%Cu (x = 1, 2, 3 and 4 mass%) solder alloys were investigated by stress relaxation tests performed at 298, 348 and 398 K. Creep behavior observed can be described by the power law at all temperatures tested, and solders with higher silver contents had better creep properties. It is considered that lattice diffusion of β-Sn affected the creep deformation of the solders because activation energy of the observed creep properties was about 100 kJ/mol, which is close to that for lattice diffusion of β-Sn. Stress exponents estimated from Norton’s law were larger than seven for all the solders tested, and the solders which showed better creep properties had more submicron-size dispersoids. Therefore, submicron-size dispersoids are considered to be useful to strengthen the β-Sn matrix and increase the creep properties of the solders tested.