Ultra-Low Warpage and Excellent Filling Ability Liquid MUF for Advanced Fan-Out Wafer Level Package

ABSTRACT In recent years, the demands for higher speed and smaller and thinner semiconductor devices are increasing, and the demand for fan-out wafer level package (FOWLP) technology is expected to grow in the future [1–3]. Resin formulation for FOWLP is very important because the properties of the epoxy molding compound (EMC) greatly affect the performance of the devices. To meet the demand for higher performance devices, EMC development will be required to have improve functions such as low loss characteristics and high thermal performance [4–6]. Meanwhile, the New package designs tend to have large I/O counts to support higher functionality, even while miniaturization of the interconnect pitch accelerates. Additionally, there will be a demand for fan-out Wafer Level System in Package (SiP) to integrate multiple ICs into one package [7]. Along with the evolution of packages, reduction of warpage and filling the narrow gap under IC chip and between IC chips without voids is becoming more difficult. Liquid capillary flow underfill (CUF) has traditionally been used to fill these narrow spaces. After the dispensing process, the CUF is cured and molded with EMC. However, this two-step method is less than optimal in terms of productivity [8]. We have developed liquid Molded Underfill (MUF) exhibiting ultra-low warpage and excellent filling ability for high density FOWLP by using the following approaches: 1) Optimizing the resin viscosity and filler particle size and distribution to improve filling ability, 2) Modifying the coefficient of thermal expansion, modulus and curing shrinkage of the material to improve warpage performance, 3) Evaluate the influence of curing conditions on resin shrinkage rate and warpage. This study confirmed that low package warpage was achieved and narrow gap filling (below 10 μm) is possible using the developmental liquid MUF. These materials were also evaluated on a CoW test vehicle to verify their viability.