New Methods to Model and Simulate Both Air Exchange and Particle Contamination of Portable Devices

Telecommunication equipments are exposed to different kind of stresses during their life cycle. Stresses can be divided into mechanical, chemical, thermal, electrical and radiation stresses. One part of this stress is related to transport of potentially harmful chemical substances into small mechanical structures of devices. The transport of these chemical substances, partly or totally bound to fine particles, happens by molecular diffusion and advection by airflow. Both direct measurements and simulation of flow velocities in electronic devices are complicated tasks due to detailed and heterogeneous structure of those devices. In addition to complex structure, the effect of heat producing components has to be taken into account since they may alter both the flow velocity and the structure of the device. Our approach has included several steps: 1. development of model for transport on air both inside and outside the equipment 2. development of model for contamination processes - most often small particles in our cases 3. combining and simplification of this knowledge to a contamination model For example, study of air exchange can be carried out with separate steps: First, gas flows inside the mobile phone were investigated by helium based leakage method, which method was first time applied to electronic devices, and observed that the results had no real meaning without extensive modeling. Therefore, these results were fitted by analytical models - incapable for any prediction calculations - to approximate diffusivity and permeability inside the phone. Second, a much more complicated numerical model was developed. A commercial tool, FEMLABr, was applied to both apply realistic geometric structures and couple several physical phenomena: transport of mass, heat and momentum. Finally, the numerical model was modified to cases where temperature differences (due to the heat producing components) are driving the gas transport. Based on these fours steps, we were able calculate air exchange coefficients values (3 - 10 1/h) for a specific mobile phone model, for situation, where the mobile is in use, in vertical position and in warm conditions. This last model was then applied to estimate ventilation coefficients, which may be used in models estimating the long term contamination of electronic devices. The deposition of harmful aerosol particles into a mobile phone was approached with similar multi step approach: Deposition of particles of different sizes was estimated by using a combination of analytical and numerical approach. For this purpose, a coupled particle convection/diffusion and flow velocity model was developed to estimate deposition coefficients. First, the equations for the flow velocity calculations and aerosol particle transport were formed. Second, the equations were implemented in a commercial numerical partial differential equation solver, FEMLABr together with Chemical Engineering Module. Finally, the model was applied in calculation important parameter values needed in the simplified contamination model. Data from these two steps was combined with experimental field data, phone user characteristics and an indoor air model modified suitable for the conditions inside a phone. During the modeling work we observed, that it was important to include all three basic transport phenomena (mass, heat and momentum) and their couplings (via transport parameters) in model calculations. As a first result, we were able to calculate aerosol particle deposition rates for a portable electronic device. Second, it was clear that very small and big particles do not easily penetrate deep into the structure. Medium size particle (diameter varies between 50 and 500 nm) on the contrary penetrate into the inner structures and may even pass through whole apparatus. The basic principle in our work has been the development of a model hierarchy starting from small details and ending up with a model useful for industrial applications. In the future, even smaller detailed level modeling going down to molecular dynamics and system of few atoms, should be applied. A chain of models from nano- to macrospic level, from scientifically well founded models to models of practical interest and high utilization potential, will be in near future very common in industrial applications like contamination studies of telecommunication and other electronic equipments.