Study on thermally reworkable underfills for flip chip, BGA, and CSP applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between an integrated circuit (IC) chip and an organic printed wiring board (PWB), and encapsulates the solder interconnects. This underfill material dramatically enhances the reliability of the flip-chip devices as compared to the non-underfilled devices. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which places an obstacle in Flip-Chip on Board (FCOB) technology developments. Thermally reworkable underfill is not only the key material to address the non-reworkability of the FCOB packages, but it can also be used to enhance the board-level reliability of ball grid array (BGA) and chip scale package (CSP) devices without losing their good reworkability feature.; In this work, three approaches for developing thermally reworkable underfills including syntheses of thermally degradable epoxies, incorporation of special additives into standard epoxy matrix, and utilization of thermoplastic-thermoset copolymers/blends are studied. Based on these studies, reworkable underfills are then developed. Reworkable underfills developed in-house and from commercial sources are then evaluated for their reworkability and reliability to the encapsulation of FCOB, BGA, and CSP.; To begin with, various thermal degradable epoxies containing carbonate or carbamate linkdage are synthesized and characterized. Among them, two diepoxides that decompose around solder reflow temperature and have good overall properties are selected and formulated into reworkable underfills for conventional capillary-flow underfilling process.; A group of additives named chemical blowing agents are identified to have the capability to make the epoxy removable under certain conditions when they are incorporated into a standard epoxy matrix. Interactions between the additives and the typical epoxy formulation are thoroughly studied for the purpose of identifying good additives that provide reworkability to the epoxy without adversely affecting its overall good properties. One additive is then selected and incorporated into a commercial nonreworkable underfill to make it a reworkable formulation for the capillary-flow underfilling process.; Thermoplastic-thermoset blends are attractive material candidates for reworkable underfills as they have the potential of combining reworkability of thermoplastics with good reliability performance of thermosets. Thermoplastic-epoxy blends and diepoxide-monoepoxide copolymers are studied and reworkable underfills are developed based on the selected blends.; Reworkable underfills developed in-house and from commercial sources are then evaluated for their reworkability and reliability to the encapsulation of FCOB, BGA, and CSP. For comparison, commercial non-reworkable underfills are also included in the evaluation as baseline underfills. The evaluation process includes rework process development, rework test, and reliability assessment. Based on the evaluation, reworkable underfills with good reworkability and reliability are identified. It is found that some reworkable underfills have comparable reliability performance as to the baseline underfills. Particularly, the incorporation of the selected additive into a non-reworkable underfill provides the reworkability without adversely affecting its reliability performance.; Feasibility of a no-flow reworkable underfill is demonstrated through the development of reworkable underfills for no-flow underfilling process by modifying a no-flow underfill formulation using a thermally degradable epoxy and incorporation of a special additive. Feasibility of a package-level reworkable underfill is demonstrated through the development of a reworkable underfill by incorporating a high temperature additive and an activator into a commercial underfill.