Integration of Thin Flip Chip in Liquid Crystal Polymer Based Flex

Abstract

Thin embedded active assemblies have been developed that combine a lower profile and high functional density (per volume) with a flexible appearance, meeting the demands for the miniaturization of electronic products. The assembly investigated in this study was a thin flip chip embedded in liquid crystal polymer (LCP) based flex. The focus of this work was on establishing a process route to enable the integration of thin integrated circuit (IC) assemblies. The flexible assembly consists of three primary elements: the thin IC, the flexible substrate and small interconnections between the die and the substrate. Bending of the delicate thin dies and warping of the LCP substrate generated many new challenges for the assembly process, which are not an issue in the assembly of conventional packages. The research work reported here was divided into three separate steps: die thinning, flexible substrate fabrication and assembly.

The preparation of the 50µm thick silicon dies included die thinning, thinned die transfer and gold stud bumping. Die thinning was accomplished by backside mechanical grinding. A quasi-waferscale thinning process, where shallow grooves were diced in the wafer before thinning, gives a high yield of quality thinned dies compared to the individual die thinning process which was employed in the early investigations. The die transfer was performed by mounting the delicate thin die on a thick flat handling die with adhesive, offering two benefits: easy handling and it maintained the thin die flat. The study of the transfer process was performed using a design of experiment approach to optimize the process. Gold stud bumps were directly applied to this transferred thin die by thermosonic bonding.

Liquid crystal polymer was used as the substrate material to provide high strength, light weight, flexibility, high melting temperature, thermal dimensional stability, and lower coefficient of thermal expansion (8 ppm/°C). A backside assembly design substrate was used for this research. The substrate fabrication was addressed in terms of three main processes: bottom copper circuit formation, via etching and copper surface finishing.

A thermal compression approach for the assembly of thin flip chip die onto the LCP flex was also investigated. The die was directly thermocompression bonded to the LCP substrate. This direct die attachment eliminates the need for underfill, further decreasing the overall height and improving the flexibility of the package.

A SOI CMOS based operational amplifier circuit was assembled to evaluate the electrical performance of this embedded membrane structure. The results demonstrated that this structure can withstand a large mechanical deformation with little impact on electrical performance.