One-Step Fabrication Of Al@Sn-Bi Core-Shell Spheres Via Phase Separation

To meet the trend of cheaper, portable, and multi-featured electronic consumer products, the advantaged electronic packaging technology moves towards the package with miniaturization, high-density and fine pitch. Thus, it becomes challenging for monolithic tin alloy solder balls to manage effectively the ever-increasing electrical and thermal burdens, as well as the bridging and reliability problems. An ideal solution to this is to employ core-shell solder balls which are comprised of a solder shell with a core having higher melting-point temperature,better electrical and heat conductivities and higher mechanical properties than tin alloys. However, since they are made mainly through electroplating, the application of core-type solders has been seriously restricted. Therefore, in the present thesis, after detailed investigations on the phase separation and solidification of Al-Bi-(Sn) monotectic alloys, the formation and influencing mechanism of core-type morphologies were obtained, and a novel one-step fabrication process of Al@Sn-Bi core-shell spheres via phase separation was successfully developed, accordingly. This research will facilitate the development of fabrication of core-shell spheres and potentially lead to the commercial application of core-type solder balls in advanced electronic packaging.First, for Al-Bi binary alloys, the effects of alloy compositions, temperatures of melt and silicon oil,free fall distances and sizes of droplets were studied by a free jet breakup process. Irrespective of compositions, a Bi-rich-phase always formed the shell due to surface segregation. Two types of concentric and eccentric double-layer core-shell morphologies were observed. Based on the ANSYS simulation of temperature field, the cooling rate and temperature gradient of droplets solidified under different conditions were estimated, and then the velocities of Marangoni and Stokes motions were calculated. It was concluded that only if the interplay between the Marangoni and Stokes effects, in other words, between the temperature gradient and cooling rate of droplet is achieved, can the regular ring-type particles be obtained;Otherwise, an eclipse-type would be formed instead. At a superheat degree of 100 K, silicon oil of room temperature and free fall distance of 30 mm, a perfect core-shell microstructure in Al-65.5 wt. % Bi particles with a diameter of 0.9 mm was successfully obtained. The solidification paths and formation of the core-type morphology were then analyzed and illustrated. Additionally, for Al-65.5 wt. %Bi alloys, the dimensional relationship between the core and whole particle by a linear function of Dcore=0.9137 Dparticle-0.0312 with a high regression coefficient of R2 (=0.96) was observed.Second, for Al-Bi-Sn ternary alloys, after studying the macroscopic morphologies in 7 compositions, it was found that the core-type morphology is easily formed when the composition is located in the miscibility gap or the boundary. For (Al34.5Bi65.5)67.8Sn32.2 alloy, the spheres changed the morphology from three-layer to concentric or eccentric double-layer core-shell types with decreasing the melt superheat and increasing the oil temperature. As the melt is superheated to 140 K above its critical temperature and then ejected into the silicon oil with a temperature of 283-473 K, the regular ring-type Al@Sn-Bi morphology was formed. The Sn-Bi-rich shell has a low melting-point around 407?431 K which is very similar to that of Sn-58Bi eutectic solder, satisfying the temperature requirement of soldering. Meanwhile, the Al-rich core can be melted around 823?844 K, so it will remain solid during reflowing to ensure coplanarity. This kind of core also can help to improve thermal conductivity, electrical conductivity and mechanical properties of the alloy balls. Phase transformation in Al-Bi-Sn alloys was studied through differential scanning calorimetry analysis, together with composition analysis and scanning electronic microscopy observation, the solidification paths and formation of the core-type microstructure were concluded and illustrated.