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Materials Contributing to Warpage Mitigation & Stress Relief in Advanced Semiconductor Packages (2)
Following Part 7
, which focused on package substrates and solder resists, this article continues the discussion by introducing materials that contribute to warpage mitigation in advanced semiconductor packaging.
Overview of 3D Packaging and Materials Contributing to Warpage Reduction and Stress Relief 
Materials Contributing to Warpage Mitigation
Photosensitive Polyimide (Insulating Material for RDL)
RDL (Redistribution Layer) is formed on the surface of chips and interposers and plays a key role in optimizing the layout of fine interconnection terminals. In particular, in advanced semiconductor packages that employ fan-out technologies to extend wiring beyond the chip area or require the shortest possible inter-chip interconnections, RDL has become an indispensable technology. It resolves terminal layout challenges associated with higher pin counts, enabling device miniaturization and higher signal speeds.
However, RDL consists of a complex multilayer structure of heterogeneous materials. As a result, stress relaxation caused by differences in CTE (Coefficient of Thermal Expansion) between layers and control of substrate warpage are critical issues. RDL contains a multilayer structure of copper-based conductors and insulating materials that protect them. While conductors route electrical signals to their intended destinations, the insulating layers prevent short circuits between interconnects but also protect circuits from thermal and mechanical stress. Although phenolic and epoxy resins are also used as insulating materials, polyimide leads the advanced semiconductor packaging field. In addition to excellent heat resistance and mechanical strength, polyimide offers an optimal level of flexibility that relieves stress generated by thermal history, thereby maintaining interconnection reliability under harsh operating conditions.
HD MicroSystems*1, a group company of Resonac, is an industry leader offering a broad portfolio of high-heat-resistant coating materials centered on polyimide. Its negative-tone photosensitive polyimide developed specifically for RDL applications in advanced semiconductor packaging, can be cured at relatively low temperatures of approximately 200 °C. This low-temperature curing minimizes shrinkage mismatch between materials caused by thermal history that effectively suppresses package warpage.
The material exhibits excellent gap-filling and planarization performance that enables flat surfaces in multilayer structures. It achieves both high resolution, that allows the formation of 6 µm vias in 5 µm-thick films, and a high elongation of 50 %. HD MicroSystems contributes to long-term reliability and the realization of even higher-density advanced semiconductor packaging by preventing cracks and delamination, which are key concerns in high-density redistribution designs.
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Polyimide Solution Appearance
Epoxy Molding Compounds (EMC)
EMC, previously introduced as a material contributing to thermal management, not only protects chips from the external environment but also plays a vital role in suppressing package warpage by mitigating CTE mismatches between materials and relaxing internal stresses.
Resonac’s granular EMC CEL Series (CEL‑400ZHF40) is specifically developed for advanced packaging applications, combining a low coefficient of thermal expansion of 10 ppm/°C or less with a low elastic modulus of 20 GPa or less. This balance effectively suppresses package warpage caused by curing shrinkage and thermal stress.
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Epoxy Molding Compounds for Organic Substrates
Enhancing the reliability of power semiconductors! High Tg, Heat-Resistant Epoxy Molding Compounds
Granular EMC Product Appearance
For applications such as FOWLP (Fan-Out Wafer Level Package), where more precise thickness control is required, film-type EMCs in the EB Series are particularly suitable. Their film form ensures uniform encapsulation layer thickness, which eliminates localized distortion caused by thickness variation, while their low CTE minimizes mismatch with the chip. In addition, these materials can be cured at low temperatures, reducing process-induced stress and further contributing to overall package warpage suppression.
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Film-Type EMC Product Appearance
*1 HD MicroSystems: HD MicroSystems L.L.C., a group company of Resonac
Next Topic
In the next installment, we will introduce materials technologies that support stress relief in advanced semiconductor packages.
Published: May 20, 2026
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