Formulation and Cure of Light-Curable Adhesives Across 2D, 2.5D and 3D Assemblies: Optical Access, Bondline Kinetics and Dark-Zone Control

A light-curable adhesive is not qualified merely because an exposed bead becomes hard. In an assembly, the cure-driving spectrum must reach every location whose state matters, while interfacial chemistry, attenuation, oxygen, reaction after irradiation and geometrical shadowing can make the bondline non-uniform. ER-186 provides a bounded engineering framework for connecting formulation, optical access and time-dependent cure-state verification. It separates chemical cure evidence from structural-joint qualification and treats inaccessible or weakly illuminated regions as explicit process states rather than assumed extensions of the visible surface.