Experimental wetting dynamics study of eutectic and lead-free solders with various fluxes, isothermal conditions, and bond pad metallizations

Demands on solder bump interconnects have increased in modern electronics. This is characterized by high density, small size, and fine-pitch devices. In solder bump interconnects, solder wetting onto bond pads is the key factor that determines the interconnect process yield and the solder joint reliability. Solder wetting involves various physical phenomena such as surface tension imbalance, viscous dissipation, molecular kinetic motion, chemical reaction, and diffusion. In this paper, an experimental study on solder wetting dynamics will be presented. The effects of solder reflow process parameters and bonding materials will be discussed, as they relate to the physics of solder wetting and ultimately the interconnect process yield and solder joint reliability. The experimental setup consists of a high-speed image acquisition system and a temperature chamber which were used to measure the time dependent behavior of molten solder spheres onto bond pads under an isothermal condition. The solder materials investigated were eutectic tin-lead solder and lead-free 95.5Sn-4.0Ag-0.5Cu solder. The wetting dynamics of the solder materials were investigated on pure Cu bond pads and Cu/Ni/Au bond pads, with several different flux systems, at different environmental temperatures and with various solder sphere sizes. The experimental observations indicate that the wetting dynamics clearly depend on temperature, solder materials, and substrate metallization but do not depend significantly on the flux system or the solder sphere size.