Mechanical and heat transfer finite element modeling of transmission laser micro joining

Transmission laser joining of dissimilar materials is of importance for applications in packaging of micro-electro-mechanical systems (MEMS) and bio-implantable devices. The system to be joined by using transmission laser bonding technique consists of a transparent (polymer, glass) and absorbing (titanium foil, silicon) part at given wavelength of the laser. The laser energy passes through the transparent layer and is absorbed by the opaque substrate, inducing the heat directly at the interface, thus allowing the formation of chemical bonds between the materials. The advantages of the transmission laser joining technique over the conventional techniques for joining of dissimilar materials, such as soldering and adhesive bonding, are the excellent focusing in the micrometer range and controlled heat input in the joining region. Numerical Finite Element Model is an important tool for optimization of the laser process parameters such as laser travelling speed, laser power and laser beam diameter. The parameters for successful joining of titanium/polyimide and silicon/glass systems were investigated by using FE Heat Transfer analysis. The accuracy of this model was verified by comparing the temperature measurement data of the simulation and experiment. Also the simulation results of the actual bonded region are compared with the bond width measured experimentally for both titanium/polyimide and silicon/glass systems and good agreement has been found in between them. The importance of the mechanical finite element model is its ability to predict the residual stress from the evolution of the temperature profile during the joining process. Since materials to be joined have different thermal expansion co-efficient and mechanical property the evaluation of the residual stress can help us to predict the failure of the component. The finite element analysis provides insights in both the thermal and mechanical aspects of the transmission laser micro-joining process.;3D lagrangian mesh has been used to develop complete finite element model by using FE code ABAQUS(TM) which addresses both heat transfer phenomena during heating and cooling as well as the residual stress phenomena after cooling at room temperature in two sequential analyses. First For titanium/polyimide and silicon/glass the heat transfer analysis results were obtained for various parameters in order to find the laser parameters for fabrication of good laser bonds. It was found that slow laser speed and high laser power result in burning of the transparent material, while lower laser power and high laser speed result in formation of very weak bond. For titanium/polyimide in the residual stress analysis the numerical convergence was addressed as the most critical issue because of the highly non-linear nature of the analysis. The results from the residual stress analysis shows that in the polyimide side of the sample the residual stresses are concentrated on the edge of the bond area because of the melting of the polymer during the joining process, while the results obtained for the titanium side of the sample suggest that the stresses are concentrated in the middle of the bond area because the titanium foil remains solid throughout the entire joining process.