Study On Mechanical Properties Of The Epoxy Packaging Structure And Its Effect On Performance Of The μDMFC

The packaging techniques of polymer Micro Direct Methanol Fuel Cell (μDMFC) is developed on the basis of polymer-packaged microelectronics and MEMS devices, which has made an impressive progress in reducing the cell volume and improving the cell energy density. However, the inner stress distribution of the packaging structure tends to experience changes under the thermal-mechanical cycling load, which would have a significant influence on the performance and life of the μDMFC. Consequently, relevant mechanical experiments for the packaging material was carried out, and epoxy-packaged μDMFC with different structures was designed and analysised in this thesis. Meanwhile, the crack propagation on the μDMFC interface under the thermal cycling load was preliminarily researched.Firstly, the mechanical properties of the epoxy packaging structure were determined by means of a standard material experiment. This experiment got the relevant parameters of the creep theory model by describing the creep characteristics of the epoxy resin using the time hardening creep theory. The modulus of elasticity, Poisson’s ratio, thermal expansion coefficient and creep curve of the epoxy structure were obtained in the standard material tensile test, which provided the relevant material parameters needed in the subsequent simulation of the μDMFC packaging. After comparing the simulation results and experimental results, it shows that the creep characteristics of the epoxy structure calculated by using the obtain material parameters is in good agreement with the experimental results.Secondly, the normal stress distribution on the polar plate-membrane electrode interface in μDMFC with different separator structure and geometric parameter was simulated. The contact resistance between the polar plate and electrode interfaces under the cycling load was solved using a half experienced, parallel connect model of micro resistance units array. It indicates that the creep effect of the packaging structure has a significant impact on the stress distribution while has a minimal impact on the contact resistance. It also shows that the stress distribution of the μDMFC with packaging structure of point-like convex array is uniform, and the comprehensive performance is the best.Thirdly, the μDMFCs with different structures were packaged by using a heat-curing technique of the epoxy resin. Then, the variation of the contact resistance and the overall resistance of the μDMFC, with point-like convex array and asymmetric triangle under the thermal-mechanical cycling load, was compared and analysised. It indicates that due to the uniform stress distribution, μDMFC with point-like convex array would retain stable after its resistance soared to some level. However, the non-uniform stress distribution in μDMFC with asymmetric triangle would lead to constant resistance increase and eventually lead to the packaging failure.Finally, to investigate the influences of the thermal-cycling load on the propagation of the interfacial creak tip in an epoxy-packaged MEMS fuel cell, finite element based simulation of the package under three different thermal cycling loads was carried out. Result shows that the J-integral of creak tip changes significantly with the thermal cyclic loading process. Under a thermal cycle from-25to+55℃with a period of3660s, the maximum J-integral change is49.65%after18thermal cycles. It also shows that period of the thermal cycling only has slight effects on the J-integral, while the cycling numbers, as well as the temperature range of the thermal load, have significant effect on the value of J-integral. On the other hand, creep properties of the packaging materials can restrain the growth of crack at high temperature, but this restrain effect will be weakened at low temperature.