Simulation Of Mechanical Behavior Of Plastic Electronic Package Under Temperature And Humidity Environment

Virtual mechanical prototyping with the consideration of thermal and moistureinfluence on plastic electronic package is the most promising method for investigationof the root cause of mechanical failure, impact trend and level of different factors, andmaterial and structure selecting. The mechanical properties of the packaging materialsand interface between different materials are necessary for the research.In this paper, the process of heat transfer was simulated for plastic packageduring process of reflow. The moisture absorption characterization of the moldingcompound was determined by employing the theory of mass transfer, and it was foundthat the phenomena of moisture diffusion did not observe the Fick's second law, whichwas attributed to the occurrence of the second phase. The governing equation ofdiffusion was corrected to agree with the moisture diffusion phenomena at differenttemperature, humidity and moisture concentration of molding compound. It was alsofound that the saturated moisture content of molding compound was proportional tothe relative humidity at certain temperature. The moisture diffusion was simulated byemploying the thermal analysis module of commercial finite element code, ANSYS,through comparison of the governing equation of heat transfer and mass diffusion.The vapor pressure during the reflow process was obtained with the result of moisturediffusion simulation and solubility-temperature curve according to Arrhenius equation,and it was found that the vapor pressure in initial defect during reflow was much lessthan the saturated value at the same temperature. The coefficient of moistureexpansion of the molding compound was determined. Finite element models with theimpact of thermal expansion and moisture expansion were built, and the couple ofimpact of thermal expansion and moisture expansion on structure field was realized.The impact factors, such as, thermal expansion, moisture expansion, vapor pressureand initial length of the crack, on crack propagation during reflow process of solderwere analyzed based on interface fracture mechanics and mixed mode fracture. It wasfound that thermal expansion played the key role while crack was small, but theimpact of the moisture expansion and vapor pressure cannot be neglected becausehigh proportion of the mode I of crack. The vapor pressure becomes more and moreimportant during the propagation of the crack. A set of methods for macroscopicmechanical research of the plastic electronic package under temperature andhumuidity condiction was established upon the progresses mentioned above.