Solder paste reflow modeling for flip chip assembly

Solder paste printing and reflow can provide low cost techniques for producing the solder bumps on flip chips. Solder paste consists of a dense suspension of solder particles in a fluid medium (vehicle) that acts as an oxide reducing agent (flux) during reflow, cleaning the metal surfaces of oxides. This paper reports on optical observations of paste behaviour at the small length scales associated with flip chip solder joints, and attempts to model the process using computational fluid dynamics (CFD). Comparison of optical observations and CFD modelling show that the behaviour of the solder cannot be described simply by surface tension and viscous flow effects and it is deduced that oxides are still present on the solder surfaces during the early stages of reflow. The implications for the paste heating method and solder volume are discussed, and a preliminary CFD model (based on FIDAP) incorporating the effect of the oxide layers is presented.