RHEOLOGICAL CHARACTERISATION AND COMPUTATIONAL SIMULATIONS OF ELECTRICALLY CONDUCTIVE ADHESIVE FLOWS FOR ADVANCED STENCIL MANUFACTURING TECHNOLOGY

摘要

Despite the challenges of achieving consistent defect free prints at sub 100-micron pitch, stencil printing is still considered as a viable process for depositing electrically conductive adhesives in flip-chip applications due to the cost benefits and high throughput possible in large scale production. To overcome the difficulties, advanced microsystems manufacturing technology is being applied to produce stencils with excellent shape definition and smooth side-walls at ultra-fine pitch. Data from rheological studies of electrically conductive adhesives coupled with multi-scale computational simulation methods can provide an insight into paste flow characteristics during printing and help understand the mechanisms that lead to process difficulties. The paste investigated in this work is an isotropically conductive adhesive, consisting of approximately 80% silver particles by weight in an epoxy resin. Computational simulations have previously been used to analyse the macroscopic bulk flow of solder pastes rolling over the stencil and the motion of the spherical solder particles at the microscopic aperture scale. These methods are adapted in this work to account for the effects of the highly irregular shaped silver particles, which result in different rheological characteristics and flow behaviour in comparison to solder pastes. Macroscopic simulations of the paste roll are conducted that assume the adhesive to be a homogeneous continuum. The motions of individual silver particles are ignored. Their effect is modelled instead by experimentally determined macroscopic properties such as non-Newtonian shear rate/viscosity relationships. Pressure, shear rate and viscosity distributions are predicted within the paste roll at the stencil surface. The results are coupled with microscopic scale simulations within the aperture, using computational methods that simultaneously model the flow of the epoxy resin, the motions of the individual silver particles and their interactions with each other and the adjacent solid stencil walls. Conclusions are drawn on the benefits of computational simulations in determining the issues and parameters that effect the conditions which are critical to successful paste entry and release from the stencil apertures.