Measurement and Prediction of Interface Crack Growth at the PCB-Epoxy Interfaces Under High-G Mechanical Shock

Abstract

Surface mount electronic components reinforced with underfills and epoxy-potting compounds have shown increase in the survivability expectations under extreme thermomechanical loading. Additional structural support and shock damping are provided by potting. Electronic components are also potted to protect sensitive equipment from environmental conditions (such as moisture), as well as to insulate electrical leads in the event at other components fail. Potting of electronics has become one of the most viable and cost-effective solutions to enhance electronic package survivability. Electronic components subjected to mechanical shock undergo tremendous strain, which in turn is responsible for solder joint failure in BGA components. Due to the bulk of material surrounding the PCB, potting and encapsulation resins are commonly two-part systems which when mixed together form a solid, fully cured material, with no by-products. The cured potting materials are prone to interfacial delamination under dynamic shock loading which in turn cause failures in the package interconnect. The study of interfacial fracture resistance in PCB/epoxy potting systems under dynamic shock loading is important in mitigating the risk of system failure in mission-critical applications. In this paper, we focus on the mechanics of the interface delamination of the epoxy potted PCB samples. Determination of the fracture parameters such as fracture toughness and strain energy release rate at steady state stress is important in selecting and the reliability study of the supplemental restraint systems. Sample specimens of Epoxy/PCB systems were prepared and subjected to quasi-static three-point bend loading to observe the fracture behavior of the bi-material samples and study the interface delamination mechanisms. The fracture toughness and crack initiation of the PCB/Epoxy bi-material system were compared with the cure schedule and temperature. A cohesive zone model was developed for mode-I delamination of PCB/Epoxy specimen under three-point bending. Damage is assumed to occur at interfaces modeled through cohesive zone elements in the material, while the bulk material is assumed linear elastic. The fracture parameters obtained from the experiment were used in the finite element fracture model to predict the damage accumulation and compared with the fracture characteristics of the cohesive zone constitutive law.