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Fluxing underfill eliminates process steps in the assembly of flip chip-on-laminate (FCOL) when compared to conventional capillary flow underfill processing. In the fluxing underfill process, the underfill is dispensed onto the board prior to die placement. During placement, the underfill flows in a "squeeze flow" process until the solder balls contact the pads on the board. The material properties, the dispense pattern and resulting shape, solder mask design pattern, placement force, placement speed, and hold time all impact the placement process and the potential for void formation. A design of experiments was used to optimize the placement process to minimize placement-induced voids. The major factor identified was board design, followed by placement acceleration.During the reflow cycle, the fluxing underfill provides the fluxing action required for good wetting and then cures by the end of the reflow cycle. With small, homogeneous circuit boards it is relatively easy to develop a reflow profile to achieve good solder wetting. However, with complex SMT assemblies involving components with significant thermal mass this is more challenging. To get the large thermal mass components to temperature, the small flip chip die will be at higher temperatures for longer periods of time. Use of predictive software tools to optimize the reflow profile and minimize temperature differences across the board is required. A series of experiments were performed using these tools to optimize the reflow profile of a complex FCOL/SMT assembly. The profile obtained was used to successfully assemble flip chip die with fluxing underfill.In liquid-to-liquid thermal shock testing ( 40 C to +125 C, 5 min hold times and 1 min transition), the characteristic life of the assembly was 1083 cycles and the first failure occurred at 992 cycles.

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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.