Critical aspect of curing epoxy adhesive: fiber pistoning of LC connector

We investigated the root cause of fiber pistoning using thermal analyses on a commercially available epoxy adhesive. The adhesive studied consists of bisphenol and imidazole as a curing agent and is extensively used in the fiber assembly, connectors, and passive components. Differential scanning calorimetry (DSC) was used to elucidate cure kinetics, and cure parameters were determined. To understand the effect of outgassing on void formation, thermogravimetric analysis (TGA) and a glass slide spun-coated with epoxy were used. Physical properties of the cured epoxy, i.e., glass transition temperature (T_g), storage modulus, and the coefficient of thermal expansion (CTE), were measured by a dynamic mechanical analyzer (DMA) and a thermomechanical analyzer (TMA). Results were utilized to formulate a processing guideline for product applications. The effectiveness of the processing guideline was evaluated by using the magnitude of fiber pistoning in LC connectors. Experiments have shown that any process deviations linked to improper curing, results in compromising the final properties of the cured epoxy. In particular, temperature gradients in the cure oven must be minimized to maintain a uniform T_g of the epoxy. The results indicate that the higher the T_g, the smaller the degree of fiber pistoning. An optimal processing (cure) schedule is proposed based on these results.