Fundamentals of solder interconnect wetting

The dynamics of solder wetting on the metallic substrate is investigated based on the integration of broad scientific knowledge and experimental results. Supposing that the target system as a molten solder sphere wetting on the metallic substrate without occurrence of oxide on the surface of the solder and the substrate, the driving force of solder wetting is identified as a surface tension imbalance and the potential physical factors that limit solder wetting dynamics are determined as a viscous dissipation, a molecular kinetic motion, chemical reaction, and diffusion. The experimental setup and experimental methods are specially designed and developed to measure the time dependent behavior of a molten solder wetting in the isothermal condition. The experimental results show the velocity of the triple contact line during wetting clearly depends on temperature, solder materials, and substrate materials but does not depend significantly on the type of flux and the size of a solder sphere. Based on the evaluation of four previous wetting models that consider one of the potential limiting factors as a dominant process with experimental results, a closed form model that predicts the dynamics of a molten solder wetting on the metallic substrate is proposed. The model fits the experimental results very well. Since a solid solder sphere melts and the molten solder simultaneously wets the substrate during the reflow process, the melting model is developed and combined to the wetting dynamics. The experimental observation confirms the importance of a melting process during wetting and verifies the validity of the model. Design guidelines for the reflow time with substrate design parameters and process design parameters are presented based on the verified models.