Investigation On Coalescence And Rheology Of Sn-Ag-Cu Solder Powders

Sn-Ag-Cu solder is considered to be the most prospective replacement of Sn-Pb solder for its excellent mechanical properties and solderability. However, the surface oxidation of Sn-Ag-Cu lead free solder powders is not only directly related to soldering properties and quality but also leads to tin loss. As the most important soldering properties, coalescance and rheology of solder paste, soldering quality directly depends on , are closely associated with such soldering imperfections as solder beads, bringing and needle clogging.In the present paper, four kinds of Sn-3.5Ag-0.5Cu lead-free solder samples are investigated in order to improve the coalescence and rheology of solder paste. The effects of particle size distribution, thickness of oxide film on the surface of solder powders on the coalescence and rheology of solder paste are studied. A unique oxide thinning process is proposed in order to investigate the effect of oxide thickness on soldering properties. Chemical removal approach is utilized and the oxide thickness is controlled by controlling the oxygen pressure and temperature during solder drying after organic acid removal.In order to investigate the oxide thickness on the surface of solder powders on the coalescence of solder paste, oxide thinning approach is utilized to varify the surface oxide thickness. When there is little change on morphology and surface oxide composition after surface oxide film treatment, the thickness of oxide film decreases from 5.6nm to 2.7nm, decreasing approximately 50%. The oxide thinning process could not only reduce the coalescent resistance of solder alloys during melting but also make the complete removal of oxide with flux easier. Results of standard reflow process test and solder-ball defects test demonstrate that the surface oxide film treatment could obviously improve the coalescence of solder powders, avoiding the appearance of solder beads. As for particle size distribution, the sieving of small powders fails to improve the coalescence on Electroless Nickel / Immersion Gold pads.The effect of particle size distributions on rheological properties is investigated. When the weight fraction of small particles is fixed, the viscosities of solder paste decrease as the increase of size ratios between small particles and large particles, especially at low shear rates. For solder paste with more small particles introduced, shear-thickening appears before the onset of shear thinning. At low shear rate, thermal diffusion dominates over the hydrodynamic interactions, resulting in the equilibrium structure. As the shear rates rise, the hydrodynamic forces begin to dominate and the particles are forced into layers, rendering the shear thinning behaviors. After surface oxide film treatment, the increase of viscosities at low shear rates results from the decrease of surface roughness on the surface of solder powders.