Novel Thermally Reworkable Underfill Encapsulants For Flip-Chip Applications

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Title: Novel Thermally Reworkable Underfill Encapsulants For Flip-Chip Applications
Author: Wong, C. P. ; Wang, Lejun
Abstract: The flip-chip technique of integrated circuit (IC) chip interconnection is the emerging technology for high performance, high input/output (I/O) IC devices. Due to the coefficient of thermal expansion mismatch between the silicon IC (CTE = 2.5 ppm/℃) and the low cost organic substrate such as FR-4 printed wiring board (CTE = 18–22 ppm/℃), the flip-chip solder joints experience high shear stresses during temperature cycling. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder interconnects. Current underfill encapsulants are filled epoxybased materials that are normally not reworkable after curing. This places an obstacle in flip-chip on board (FCOB) technology development, where unknown bad dies (UBD) are still a concern. Approaches have been taken to develop the thermally reworkable underfill materials in order to address the nonreworkability problem of the commercial underfill encapsulants. These approaches include introduction of thermally cleavable blocks into epoxides and addition of additives to the epoxies. In the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with hardener and catalyst. Then the mixture properties of Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy resin ERL-4221E. These mixtures all decomposed at lower temperature than the standard formulation. Moreover, one mixture, Epoxy5, showed acceptable Tg, low viscosity, and fairly good adhesion. In the second approach, two additives were discovered that provide die removal capability to the epoxy formulation without interfering with the epoxy cure or properties of the cured epoxy system. Furthermore, the combination of the two approaches showed positive results.
Description: ©1999 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or distribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.
Type: Article
Citation: IEEE Transactions on Advanced Packaging, Vol. 22, no. 1, February 1999, 46-53
Date: 1999-02
Publisher: Georgia Institute of Technology
Institute of Electrical and Electronics Engineers, Inc., New York
Subject: Chip removal
Degradable epoxy
Flip-chip devices
Flip chip rework
Removable underfill
Repairable underfill
Reworkable underfill
Thermally reworkable

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