Studies On Syntheses And Properties Of Sn-based Pb-free Solder Powders By Mechanical Alloying Technique

With blooming electronic industry and increasing consciousness of environment-protection and health concerns, Pb-free materials and technologies have attracted more attention and gradually applied in the electronic industry since 1990s. Pb-free Sn-based alloy solders are key materials of manufacturing the Pb-free electronic products. As one type of the alloy solders, the solder pastes mainly consisting of lead-free alloy powders, are widely used in joining the electronic devices. It is known that mechanical alloying (MA), a non-equilibrium powder forming technology, is usually utilized to synthesize the binary and/or multi-element metals powders. The advantages of the MA technology are that the varieties and contents of raw materials are not restricted, and the composition segregation can be effectively avoided. It is believed that the MA technology can also be used to synthesize the Sn based alloy powders.In the present study, the MA technology was taken to synthesize the Sn-Ag, Sn-Cu and Sn-Ag-Cu powders at room temperature. Structural evolutions, morphologies and alloying mechanism of the Sn-based alloy powders during MA, melting temperatures of the milled powders, and microstructures of the melted Sn-based alloy solders were investigated using XRD, TEM, SEM, DSC and laser particle size analyzer. Moreover, the effects of the Ag, Cu compositions on the microstructures of the Sn-based alloy powders were also discussed, and the soldering characteristics of the milled powders was preliminarily evacuated.The Sn-Ag binary alloy powders are of large lamellar composite particles in the initial stage of MA. As increasing the milling time, they are gradually fractured into finer lamellar particles, and finally the ultrafine spherical particles are formed. The particle sizes of the 60 h milled Sn-Ag alloy powders (2-5 wt%Ag) increased with increasing the Ag content, but all situated in the range of 0.1-20μm. The milled Sn-Ag alloy powders was composed of nanocrystalline Sn(Ag) and Ag3Sn particles. The melting point of the 60 h milled Sn-3.5Ag powders was tested to be 224°C. After the 60 h milled Sn-3.5Ag powders melted on the Al2O3 substrate, the Sn-3.5Ag alloy solder was composed of the primaryβ-Sn and Ag3Sn particles and the (Ag3Sn+β-Sn) eutectic structure. Long Ag3Sn laths are located at the Sn-3.5Ag solder/Cu substrate interface and Ag3Sn needles are embedded in the solder.The Sn-Cu binary alloy powders are also made up of a lamellar composite structure in the initial stage of MA. As increasing the milling time, they are gradually fractured into irregular particles. Finally the superfine Sn-Cu alloy powders are formed. The particle sizes of the 60 h milled Sn-Cu alloy powders (0.7-10 wt%Cu) decrease with increasing the Cu content, but all situated in the range of 0.1-20μm. The milled Sn-Cu alloy powders was composed of nanocrystalline Sn(Cu) and Cu6Sn5. The melting points of the 60 h milled Sn-Cu alloy powders decreased from 231 oC to 228 oC with increasing the Cu content from 0.7 wt% to 10 wt%. After the 60 h milled Sn-0.7Cu alloy powders melted on the Al2O3 substrate, the Sn-0.7Cu solder was composed of the primaryβ-Sn and Cu6Sn5 particles and the (Cu6Sn5+β-Sn) eutectic structure. Except those phases, the hexagonal coarse Cu6Sn5 particles are also detected in the Sn-0.7Cu solder, after the 60 h milled Sn-0.7Cu alloy powders melted on the Cu substrate.The structural evolutions and the morphologies of the Sn-3.5Ag-0.7Cu ternary alloy powders during mechanical alloying are similar to the Sn-Ag and Sn-Cu binary powders. The untrafine spherical Sn-3.5Ag-0.7Cu powders are finally obtained. The 60 h milled Sn-3.5Ag-0.7Cu powders was composed of nanocrystalline Sn(Ag, Cu), Ag3Sn, and Cu6Sn5 particles. The average particle size (D4, 3) and the melting point of the powders were tested to be 19.9μm, and 219 oC, respectively. After the 60 h milled Sn-3.5Ag-0.7Cu powders melted on the Al2O3 substrate, the solder consists of the hypereutectic structure of the primaryβ-Sn particles and the (β-Sn+Cu6Sn5+Ag3Sn) eutectic structure. Soldering of the milled Sn-3.5Ag-0.7Cu solder powders on the Cu substrate was well performed with a wetting angle around 20°, showing a good wettability. The continuous scallop-like Cu6Sn5 and Cu3Sn layers are formed along the Sn-3.5Ag-0.7Cu solders/Cu interface. Meanwhile, the Ag3Sn laths at the solder/Cu6Sn5 layer interface, the hexagonal coarse Cu6Sn5 plates in the solder are also detected.