Wetting Dynamics of Eutectic and Lead-Free Solders with Various Reflow Conditions and Surface Finish Metallizations

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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 a surface tension imbalance, viscous dissipation, molecular kinetic motion, chemical reactions and diffusion. In this paper, an experimental study on solder wetting dynamics will be presented along with an analytical predicting solder ball wetting. The effects of solder reflow process parameters and bonding materials is 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 are eutectic tin-lead solder and lead-free 95.5Sn-4.0Ag-0.5Cu solder. The wetting dynamics of the solder materials were investigated on Cu, Cu/OSP, 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. Moreover, this research develops an analytic methodology based on solder wetting dynamics, that can be used to predict solder interconnect formation during electronics assembly. The major benefit of such an advanced process model is that it enables process design and process parameter optimization through simulation. The model also reveals the cause of wetting problems that may occur during the assembly process and provides a solution. Low cost assembly process will be achieved via optimizing an assembly process time and reducing a interconnect failure rate. This work will lead to a fundamental basis for better understanding the complex phenomenon of solder wetting during electronics assembly