9522929 Wang Flip Chip of Board (FCOB) technology generally requires encapsulation of the solder joints to reduce the loads on such joints during thermal excursions. At present the encapsulation process constitutes the primary obstacle to widespeard acceptance and implementation of FCOB technology because of the long process times involved, and because of the subsequent lack of repairability. Underfill process times are long because current practice involves dispensing a filled epoxy resin along the edges of the chip and relying on capillary flow (i.e. a surface-tension-driven effect) to fill the space between chip and substrate. Because of the small driving force, this is unavoidably slow, requiring anywhere from one to ten minutes to underfill a typical chip. This is clearly unacceptable as an in-line process in mass-production. In this project an alternative encapsulation process with a significantly shorter duration on the order of seconds, rather than minutes will be explored. This will involve using positive pressure as a driving force in some kind of miniaturized transfer-molding process. The project will also include characterization of encapsulant properties (such as rheology and curing kinetics) as well as process modeling. Such property characterization and modeling can be used to optimize the process. Current underfill materials require cure times on the order of hours (this long cure time is due to the long filling time), thus requiring that the process be done in a batch mode, and do not allow for subsequent removal for repair if chip or board proves to be defective. However, the new process to be developed will allow for considerably more flexibility in materials selection. With the possibility of replacing current epoxy-based thermoset encapsulants with faster-curing thermosets or even with a thermoplastic polymer. The latter would allow repairability of the board, when required. Close collaboration with industry ensures that this research is not only relevant to the electronics manufacturing sector but also that if successful, it will have the necessary market pull for bringing the technology into commercialization. The flip-chip market is estimated at only 5% of the total semi conductor chip sector with the most applications in the automotive industry (for example sensors for air bags and sensors for electronic fuel injectors). Successful completion of this research has the potential to drive down the manufacturing cost and enable repairability option in the flip chip allowing for far wider applications into the computer and telecommunications industries.
