Experimental evaluation of wetting dynamics models for Sn63Pb37 and SnAg4.0Cu0.5 solder materials

Abstract

The paper evaluates four of the well-known wetting dynamics models for their applicability to eutectic solder (Sn63Pb37) and lead-free solder (SnAg4.0Cu0.5) that are being used widely as interconnect media in electronics manufacturing. The wetting dynamics models evaluated are the viscous dissipation-limited model, the molecular kinetic model, the chemical reaction-limited model, and the diffusion-limited model. Each of the wetting dynamics models is assessed by the ability to fit the experimental measurements of wetting angle over time of solder ball with several different settings of wetting temperatures and substrate surface finish. The viscous dissipation-limited model fits the experimental measurements relatively well and clearly shows the dependency of the wetting speed on the contact angle. However, the fitted parameters are beyond the physically meaningful range. The molecular kinetic model is very sensitive to contact angle and deviates from the experimental measurements. However, the effects of surface tension imbalance and temperature on the solder wetting dynamics are clearly explained by this model. The chemical reaction-limited model does not match the experimental wetting dynamics well. However, it has an ability to explain the temperature dependence of solder wetting. The more recent version of the chemical reaction-limited model fits the experimental measurements better by the new feature added to explain nonlinearity in wetting speed at high contact angles. The diffusion-limited model predicts the solder wetting dynamics relatively better than the other models. However, the model dictates zero equilibrium angle, contrary to nonzero equilibrium contact angles in reality. It creates deviations from the experimental results, in particular, in the “strict linear spreading” zones where the contact angle approaches the equilibrium contact angle with a constant low speed. The paper discusses the limitations that the wetting dynamics models would have when applied to the eutectic and lead-free solder materials and considerations to be taken into for modeling solder wetting dynamics.