Key Takeaways & Executive Findings
- •• • Achieved CIE L* index of 21.39 and cut-off wavelength ≈ 555 nm, demonstrating deep visible-light blocking essential for preventing photoaging and ensuring circuit security in high-power-density packaging. • • Demonstrated lithographic sensitivity with fine copper line patterning at 40/80 μm line width/spacing, enabling high-density interconnects beyond the ~70 μm limit of subtractive processing. • • Achieved robust Cu/B-PSPI interfacial adhesion of 16.6 MPa, ensuring mechanical reliability under repeated deformation for flexible electronics. • • Utilized a main-/side-chain spatial decoupling strategy with precise monomer stoichiometric ratio, resolving the trade-off between UV photosensitivity and visible-light absorption, a critical bottleneck in B-PSPI development.
Abstract
The escalating power density of electronic devices necessitates effective visible-light shielding in advanced packaging to ensure circuit security and long-term reliability. Photosensitive polyimides (PSPI) serve dual roles as photodefinable dielectrics and structural layers, but intrinsically black PSPI (B-PSPI) suffer from competitive ultraviolet (UV) absorption between chromophores and photosensitive moieties, limiting co-optimization of deep visible-light blocking and lithographic resolution. Here, we report a main-/side-chain spatial decoupling strategy to synthesize a novel B-PSPI. By polymerizing pyromellitic dianhydride with a main-chain coloring monomer (4,4'-diaminodiphenylamine) and a side-chain photosensitive monomer (1,4-dihydropyridine-functionalized diamine), the monomer stoichiometric ratio is precisely engineered. This design spatially isolates functional groups and enhances charge transfer, yielding exceptional visible-light shielding (CIE L* index of 21.39, cut-off wavelength ≈ 555 nm) with good lithographic sensitivity. UV exposure triggers in situ generation of coordination sites from photosensitive groups, anchoring active metal species for electroless copper plating. This enables direct additive fabrication of fine copper lines (40/80 μm line width/spacing) with robust Cu/B-PSPI interfacial adhesion of 16.6 MPa. This work provides a robust molecular design paradigm for B-PSPI, integrating superior optical shielding and surface metallization for high-density interconnect applications.
1. Introduction
Conventional subtractive manufacturing of copper traces on flexible printed circuits faces critical limitations as line widths shrink below ~70 μm. Lateral etching and undercut distort trace geometry, undermining dimensional control and reliability. Photosensitive polyimides (PSPI) offer a shortened additive workflow, but traditional yellow PSPI exhibit high visible-light transmittance, risking photoaging and design safety in advanced packaging. Intrinsically black PSPI (B-PSPI) are needed for effective light shielding, yet they suffer from competitive UV absorption between chromophores and photosensitive moieties, limiting lithographic resolution.
This work introduces a main-/side-chain spatial decoupling strategy to synthesize a B-PSPI that overcomes this trade-off. By polymerizing pyromellitic dianhydride with a main-chain coloring monomer and a side-chain photosensitive monomer, the design spatially isolates functional groups, enhancing charge transfer for deep visible-light absorption while preserving UV sensitivity. The resulting material achieves a CIE L* of 21.39 and a cut-off wavelength of 555 nm, with demonstrated fine-pitch copper patterning via electroless plating, offering a practical route to light-shielding, additive-metallization-compatible B-PSPI laminates.
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Hao Zhong, Dezhi Zhan, Yongjian Zhu, Jing Li, Wanying Zhang, Yiyong Chen, Xin Liu, Chengbin Zou, Jinru Li, Zhenxuan Huang, Min Cao, Rui Chen, Shilong Zhong, Cheng Wang, Xudong Chen (2026). Balancing photosensitivity and visible-light absorption in intrinsically black photosensitive polyimide: synthesis and metal patterning. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4274-x
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Frequently Asked Questions
What is the mechanism by which UV exposure generates coordination sites for electroless copper plating, and how does this affect the adhesion strength?
UV exposure triggers in situ generation of coordination sites from the side-chain 1,4-dihydropyridine photosensitizer groups. These sites anchor palladium (Pd) catalyst during seeding, enabling selective electroless copper deposition. The resulting Cu/B-PSPI interfacial adhesion is 16.6 MPa, indicating strong chemical bonding between the deposited copper and the polymer surface.
How does the main-/side-chain spatial decoupling strategy resolve the trade-off between visible-light absorption and UV photosensitivity?
By placing the coloring monomer (4,4'-diaminodiphenylamine) in the main chain and the photosensitive monomer (1,4-dihydropyridine-functionalized diamine) in the side chain, the functional groups are spatially isolated. This prevents competitive UV absorption, allowing the charge-transfer complex to enhance visible-light absorption (CIE L* 21.39, cut-off 555 nm) while maintaining sufficient UV sensitivity for photopatterning.
What are the scalability limitations of this B-PSPI for industrial production of fine-pitch copper traces?
The current process achieves 40/80 μm line/space. Further scaling to finer pitches may be limited by the density and efficiency of photoactivated sites, as well as lateral overgrowth during Pd seeding and copper deposition. Increasing the density of photosensitive groups could improve resolution but may compromise blackness or mechanical properties.
How does the performance of this B-PSPI compare to traditional yellow PSPI in terms of optical and electrical reliability?
Traditional yellow PSPI have high visible-light transmittance, leading to photoaging and potential circuit malfunction. This B-PSPI provides deep visible-light blocking (cut-off 555 nm), preventing light-induced degradation. The copper interconnects exhibit stable electrical performance under repeated deformation, indicating robust reliability for flexible applications.
What is the cost implication of using this B-PSPI compared to conventional subtractive processes?
The fully additive process eliminates multiple steps (e.g., photoresist coating, etching, stripping) and reduces material waste, potentially lowering manufacturing costs. However, the synthesis of the specialized monomers and the electroless plating process may involve higher material costs. A detailed cost analysis is not provided, but the process simplification and improved yield could offset initial expenses.
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