Simulation of no-flow underfill process for flip-chip assembly

The no-flow underfill process and materials were investigated for flip-chip interconnects in active components for the automotive market. The desired mechanical reliability of these components requires an underfill process in order to reinforce the flip-chip interconnects. The conventional underfill process uses epoxy resins that are dispensed after solder bump reflow. Capillary effects force the resin to flow underneath the chip and to fill the gap between chip and chip carrier. The resin is then cured in a heating operation. No-flow underfill materials do not require subsequent reflow, dispense and cure processes. They are applied prior to chip attachment, act as fluxing agent for the solder reflow and form a solid during reflow cycle that reinforces the interconnects. This process provides significant cost savings as it reduces the number of process steps and increases the throughput. However, it requires a well-optimised set of process parameters. The curing procedure of the resin is very different from that of the conventional capillary flow applications. A good understanding of the reaction kinetics of the resin is required in order to get a high assembly yield and reliable interconnects. Motorola recently completed an extensive study of the no-flow underfill process and materials. Results of this study are presented, such as differential scanning calorimeter (DSC) tests, thermal simulations of the cure process under different conditions, assembly experiments and accelerated fatigue tests of flip-chip interconnects.