Mitigation of Assembly-Induced Cracking in Cyclo Olefin Polymer (COP) Optics through Annealing and Step-Torquing Technique

Light Emitting Diode (LEDs) are the main source of light in Aircraft lighting systems and are mounted on a Circuit Board Assembly (CBA). For some applications, Optics are mounted concentric to these LEDs so that light is focused in the required areas. Polymeric optical materials such as Cyclo Olefin Polymer (COP) are adopted as Optics due to their excellent optical clarity, dimensional stability, moldability and weight saving advantages over glass. However, their relatively high notch sensitivity and the presence of residual molding stresses make them prone to crack initiation during mechanical fastening. In this case study, Optic was fastened directly onto the CBA using self-threading fasteners. During its installation, crack formation was consistently observed around self-tapping screw interfaces, raising concerns over reliability, maintainability, and compliance with aerospace durability requirements. A structured Design of Experiments (DOE) was performed to identify root causes and evaluate potential mitigation methods. The investigation revealed that residual stresses in these Cyclo Olefin Polymers, combined with localized stress concentrations during screw tightening, were the primary drivers of crack initiation. Two complementary process improvements were developed and validated: (i) Annealing of the optics prior to installation, to relieve internal stresses, and (ii) Step-torquing of fasteners, to gradually distribute applied loads and reduce localized stress peaks. Post-assembly observation confirmed a significant reduction in crack initiation. Furthermore, assemblies were subjected to Highly Accelerated Life Testing (HALT) to validate long-term performance. The combined annealing and step-torquing approach demonstrated a substantial reduction in crack generation probability, providing a practical and repeatable process for enhancing the robustness of polymeric optics assemblies. This work contributes a generalizable methodology for mitigating assembly-induced failures in advanced polymer materials and supports broader adoption of lightweight, high-performance optics in aerospace applications.