Cohesive Zone Modeling and Damage Prediction of Interfacial Delamination in Potted Electronics Subjected to High-G Mechanical Shock

摘要

Surface mount electronic components reinforced with underfills and epoxy potting have shown to increase the survivability expectations under extreme mechanical 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. At extreme mechanical shock loads, the electronic components 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 potentially cause failures in the package interconnects. 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 assembly. A finite element model framework was developed for a circular PCB with fine pitch BGA packages which are encapsulated with potting material. The interface between the PCB and the potting compound was modeled using cohesive zone elements. Damage is assumed to occur at interfaces modeled in the material, while the bulk material is assumed to be linear elastic. The damage initiation and progression were defined by the traction-separation law. The test assembly model was subjected to high-g mechanical shock loads up to 25,000g. The board strains and displacement from the dynamic explicit model was correlated with the findings from the DIC results of the actual stress test. The damage accumulation versus the number of drops and survivability of the components as a function of damage progression has been reported and compared with the fracture characteristics of the cohesive zone constitutive law.