Coating of high-efficiency lead-free soldr particles with antioxidation film and its evaluation

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Spherical copper particles were used as model particles of lead-free solder particles. The copper particles were coated with wax powder by means of a dry mechanical method using a ball mill and the antioxidation film was formed on the surface of copper particles. The mechanical energy applied to the copper particles during the film formation processing was evaluated using the relationship between the compressive energy and the strain of a copper particle, which was measured by the unconfined compression test for a single particle. The optimal operating conditions of the ball mill in the processing were determined based on the applied energy in which the copper particles hardly deformed. The elution test in which the wax-coated copper particles were soaked in a dilute nitric acid solution proved that the wax film on the particles prevented the elution of copper to the nitric acid solution. The alloy particles of tin and zinc which were one of the representative lead-free solder particle … More s and were softer than the copper particles were coated with the wax powder using the same method. The solder paste was prepared by kneading the flux with the wax-coated alloy particles and stored at low and room temperatures for a given period of time. The pastes were melted on a copper plate to evaluate the wettability. The results showed that the wax film prevented the oxidation of the alloy particles and the paste including the wax-coated alloy particles had the satisfactory wettability.The flowability of the solder pastes was evaluated with a capillary rheometer. In order to investigate the influence of the particle content in the paste, the wax coating and the particle shape (the strain of particles) on the flowability, the exit velocity of the paste from the capillary was measured under various conditions and then the viscosity coefficient was determined. The results showed that the flowability was maintained even though the particle content was 89.5 wt% (corresponding to 50 vol%) and decreased with increasing in the particle deformation (the strain). The viscosity coefficients of the pastes including the particle having the strain less than 0.1 and the wax-coated particles in which the film thickness was less than 1μm were almost equal to that of untreated spherical particles. It has been proved that the mechanical processing method developed in this study enables the powder design of high-efficiency lead-free solder particles. Less