Thermal diffusivity of Sn–Ag–Cu-based, Pb-free, micro- and nano-sized solder balls

In order to determine the effects of ball size and porosity on the thermophysical properties of solder materials, several Sn–3.0Ag–0.5Cu solder balls with average ball diameters of 170 nm, 10 μm, 29 μm, and 140 μm were prepared, and disk-type samples were formed under compaction pressures of 100, 200, and 300 psi. The thermal diffusivity of each sample was then measured using a laser flash apparatus over a temperature range of room temperature to 150 °C. The results showed that the thermal diffusivity increased as both the diameter of the solder ball and the compaction pressure increased. On the other hand, the thermal diffusivity decreased by as much as 28% for the same ball sizes and pressures at higher temperatures. Overall, the sample with a ball diameter of 140 μm prepared under a compaction pressure of 300 psi exhibited the highest thermal diffusivity (about 30 × 10−6 m2/s). Thus, it was found that the thermal diffusivity of a sample composed of solder balls is strongly dependent on the ball size, porosity, and preparation temperature.