Evolution of Interfacial Properties under Long Term Isothermal Aging of PCB/Potting Compound Interfacial Samples under Pure Mode-I Loading

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Potting electronic components is one of the most effective methods of protecting electronics assembly in challenging harsh environments. Potting is the process of filling a complete electronic assembly with an Epoxy material. Potting protects the components against vibration, shock, and exposure to moisture and corrosive agents. Electronics in aerospace and military applications are potted to protect against extreme thermomechanical loads. Delamination at the Epoxy and PCB interface has been one of the major failure modes in potted electronics. Delamination propagates along with the interface, ultimately resulting in solder interconnect failure in these mission-critical applications. Interfacial delamination depends entirely on the fracture toughness of the Epoxy/PCB interface. The properties of the interface also depend on the temperature to which the system has been exposed over the period. Interfacial fracture resistance of Epoxy/PCB interfaces under thermal exposure and bending loads has not been studied widely yet. To understand the failure modes and change in fracture resistance at Epoxy/PCB interfaces, bi-material beam specimens of Epoxy/PCB were fabricated and subjected to bending loads. Five different potting materials are examined in this study. The samples are subjected to long-term isothermal aging at 100°C for 30 days, 60 days, 90 days, 120 days, and 180 days. The samples are under pure mode I, four-point bend dynamic loading. The interfacial crack initiation, propagation, and failure modes are recorded. The Strain-energy release rate and fracture toughness for the interfaces are calculated. The change in interface properties is examined with respect to the number of days of aging. To predict the interfacial delamination behavior of the Epoxy/PCB interface, a cohesive zone model was developed for pure mode-I delamination under four-point bend loading. The bulk material is assumed to be linear elastic during bend load. Similarly, the damage is assumed to occur at the interface and the cohesive zone is modeled at the interface. The damage accumulation is predicted using the fracture parameters obtained from the experiment for both pristine and aged test conditions.