Determination of the Thermo-Oxidative Ageing Depth and its Influence on Mechanical Properties in (Highly) Filled Polymers

Realizing reliable electrical components and systems is one way to reduce electronic waste [1]. Therefore, knowledge about the properties of materials being used should be present during the development phase of such components and systems. Polymeric materials are being used in these applications increasingly for obvious reasons like low cost and high flexibility in form factor. These materials tend to undergo property changes, known as ageing, when exposed to conditions such as temperature, humidity, and UV light during their usage phase. Focus of this study is to determine the penetration depth of thermo-oxidative ageing effects and to study in what manner the mechanical properties are influenced due to this oxidation. Two different materials, a printed circuit board (PCB) and an epoxy mold compound (EMC), are studied. The ageing front is visualized and measured through fluorescence microscopy analysis of sample cross-sections aged for various durations. For the EMC a clear dependency of ageing depth over time is observed. The glass fiber fabric in the PCB material acts as a barrier to oxidation. Over ageing time, a noticeable discoloration of the outer surface is observed, but there is no increase in depth. To confirm the changes in mechanical properties, nanoindentation experiments are conducted. Due to the glass fibers in the PCB and filler particles in the EMC, both materials are on a microscopic scale not homogeneous. To avoid evaluating measurements influenced by the glass fiber and filler particles, a grid of nanoindents is applied to the differently aged samples. The statistical evaluation of these experiments shows that, due to ageing, the modulus increases. Additionally, in areas where no visual ageing effects (discoloration) are observed, the modulus values do not change significantly over time. In summary, the thermo-oxidative ageing effects vary based on the material's microstructure. The previously published simulation workflow by the authors can be applied unchanged to the PCB material. However, for the EMC, where the oxidation layer increases over ageing time, modifications to the workflow are required. This paper presents the foundational elements necessary for these modifications.