As electronic devices continue to advance toward greater integration, miniaturization and thinner designs, solder resist (SR) — a key protective material for IC substrates — faces increasingly demanding performance requirements. Serving as the "protective barrier" for circuit substrates, it must provide reliable thermal stability, mechanical toughness, chemical resistance and electrical insulation throughout fabrication and long-term operation. However, conventional epoxy-based SR has has long faced an inherent trade-off: rigid modified epoxy resins deliver favorable thermal stability and adhesion but suffer from brittleness and high curing shrinkage, while flexible epoxy acrylate resins fall short in thermal and chemical durability. This dilemma has become a critical bottleneck limiting the reliability of advanced electronic packaging.
In a study published in Progress in Organic Coatings, a team led by SUN Rong from the Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, developed a novel polyurethane–imide oligomer to achieve a balanced rigid–flexible structure in SR for advanced packaging substrates.
Rather than relying solely on single-component resin modification strategies, researchers adopted a precise molecular engineering strategy. They synthesized a multifunctional polyurethane-polyimide (PI-PU) oligomer integrating C=C double bonds, flexible urethane segments and rigid imide units into a single molecular backbone, paired with a custom alkali-soluble epoxy acrylate resin (PE-1) with tailored acid value. The two components form a multi-scale synergistic crosslinked network via sequential photo-curing and thermal-curing: flexible urethane segments relieve internal stress to boost toughness and copper adhesion, while rigid imide structures and aromatic backbones in PE-1 enhance the thermal resistance and mechanical strength.
Researchers revealed that the SR achieves a balanced overall performance at the optimal 20 wt% PI-PU loading: 94.3 MPa tensile strength, 5.6% elongation at break, a 176 °C glass transition temperature, a T5% of 356 °C thermal stability. It also shows improved dielectric properties, enhanced hydrophobicity, stable acid-alkali resistance, and maintains 60 μm photolithography resolution, satisfying the fabrication demands of high-density circuit substrates.
Through the molecular design and synthesis of SR matrix resins, this study achieves the coordinated enhancement of thermal stability, mechanical strength and toughness in SR materials. The optimized formulation effectively improves the reliability of the material, including thermal shock resistance and cracking resistance, during the processing, assembly and service of packaging substrates. This work reveals the promising application potential of next-generation high-performance solder resist films, and lays a solid foundation for the development of advanced electronic manufacturing and integrated circuit substrate materials.
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