Creep modeling of precision adhesive joints in opto-electronic devices

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Opto-electronic devices consist of optical elements attached to a substrate; the most common method of fixing these components to the substrate is by means of adhesives. Adhesives are susceptible to creep and stress relaxation under the influence of internal and external stresses, which may lead to subtle perturbation of the position of optical components, thereby affecting performance over time. Opto-mechanical design needs to account for this end of life (EOL) behavior of precision adhesive bonds. There is a lack of predictive model for designing against these mechanisms, leading to rule-of-thumb design criteria and material selection processes. Adhesives may be subjected to residual stresses during the curing process, e.g. due to mismatch in coefficient of thermal expansion (CTE) between the optical elements, adhesive, and the substrate. External stresses arise from forces such as gravity and vibration. In optical devices, the external stresses are typically a fraction of the ultimate material strength and normally creep rupture is not a concern. However, the devices are sensitive to very small changes in position, and therefore even low stress can affect device performance. Finite element analysis (FEA) is a powerful tool in opto-mechanical design, and ideal for modeling stress state due to the above loading for complex product geometry. There are several standard creep models incorporated in commercial FEA software such as ANSYS; however, it is not clear they are applicable to the materials of interest, or to low stress creep behavior. The focus of the proposed collaborative research between Professor Spelt and JDSU Ottawa is to measure the creep properties of candidate adhesives and then develop and evaluate adhesive creep models to be used in FEA software for the prediction of creep in opto-electronic adhesive joints.