Use of Flow Simulation for Design and Process Optimisation for Flip Chip Underfill

A study on impact of geometry and process parameters on underfill flow has been conducted. Relevant material properties (time and temperature dependent viscosity, contact angles to relevant surfaces, surface tension) have been measured and fit into numerical models as input for CFD modelling tool. First, a simple flow test vehicle (glass slide) has been modelled and compared with flow time measurements. It can be stated that the simulation is in very good agreement with experiments. This was the prerequisite for the following modelling of a flip chip. A variation of geometry parameters (e.g. stand-off, bump pitch) as well as process parameters (e.g. dispense pattern, needle velocity) have been modelled. Additionally, the impact of these parameters has been studied experimentally. Again, modelling results are in very good agreement with experimental results. Thus, modelling of underfill flow has been established as tool for optimisation of underfill parameters and prediction of process robustness.