A study of normal, restoring, and fillet forces and solder bump geometry during reflow in concurrent underfill/reflow flip chip assembly

Simulation of flip chip solder joints in an underfill environment was performed to evaluate the effect of underfill volume and material properties on concurrent underfill and solder reflow manufacturing technique. Forces during solder reflow, fillet shape and collapsed solder ball geometry after reflow are reported. A multiple ball model was created based on single ball model and underfill fillet studies, to predict die stand-off in the presence of a pre-dispensed, fluxing underfill. The predictions agree with experimental results within 1.5 percent. Modeling allows the prediction of self-centering forces, gap height, and die floating as a function of underfill volume and properties in a no-flow, fluxing underfill assembly process.