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Materials Contributing to Warpage Mitigation and Stress Relief in Advanced Semiconductor Packages (3)
Materials Contributing to Warpage Mitigation
Capillary Underfill (CUF)
Capillary underfill was introduced in Part 6
as a material contributing to thermal management and plays a critical role in stress relief in semiconductor packages.
CUF is filled into the gaps between components with large differences in the coefficient of thermal expansion (CTE) in chips, interposers, and package substrates. When warpage occurs in a semiconductor package, stress tends to concentrate at fine interconnect structures such as micro-bumps. After curing, the stiffness of the CUF protects these interconnects and distributes stress over the entire interface helping to prevent delamination and electrical disconnection.
In advanced semiconductor packages employing interposers, CUF can be broadly classified into two types depending on the application location. CUF for Top, which fills the gap between the chip and the interposer, must penetrate increasingly narrow gaps as bump pitches continue to shrink. As a result, the key requirements are high flowability prior to curing and the use of fine-particle fillers that prevent clogging in narrow spaces. In contrast, CUF for Bottom, which fills the area between the interposer and the package substrate, has a much larger filling area and is directly affected by the relatively large thermal expansion of the substrate. For next-generation products to further reduce CTE, we are pursuing development from two approaches by increasing the elastic modulus to enhance protection strength and lowering the modulus to provide flexibility that conforms to package warpage. Through these efforts, we are advancing the establishment of a system capable of providing CUF solutions that meet diverse needs.
To meet these stringent and layer-specific requirements, Resonac leverages its extensive experience in the development of liquid encapsulation materials to provide CUF solutions optimized for each application layer. This contributes to improving long-term reliability of advanced semiconductor packages.
Learn more:
Liquid Encapsulant for Flip Chip Package Underfill CEL-C Series
CUF Product Appearance
Conclusion and Preview of the Next Series
Across Parts 7 through 9, we have introduced materials that contribute to warpage mitigation and stress relief in semiconductor packaging. Resonac provides these materials as a comprehensive portfolio, enabling optimal material combinations tailored to individual challenges.
In line with the technology roadmap, we will continue to accelerate the development of materials that enhance package reliability, including low-temperature curing, low curing shrinkage, and low-CTE properties. More details will be presented in the next series.
Resonac Technology Roadmap for Large Body Package Materials 
Published: June 9, 2026
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