Modelling and computer simulation of the behaviour of solder paste in stencil printing for surface mount assembly
One of the main challenges facing the electronics manufacturing industry in solder paste printing for ultra-fine pitch surface mount and flip-chip assembly is the difficulty in achieving consistent paste deposit volumes from pad-to-pad. At the very small aperture geometries required for ultra-fine pitch and flip chip assembly, flow properties of the paste becomes one of the dominant factors in the printing process. It is widely accepted that over 60% of assembly defects originate from the solder paste printing stage, and hence the urgent need for a better understanding of solder paste rheology, its behaviour during printing, and its impact on defect generation. This understanding is essential for achieving proper control of the printing process. This thesis presents the result of work on the modelling and computer simulation of solder paste behaviour during printing, and consists of three main parts. The first part concerns the modelling of paste behaviour in stencil printing using a vibrating squeegee. The performance of the vibrating squeegee is analysed and process models developed for predicting the ideal printing conditions. In the second part, the random packing of solder powder and the microstructure of solder paste are numerically simulated by applying Monte Carlo method. The effect particle size distributions on the paste microstructure is studied in this part. Based on the simulation results of the second part, the third part concerns the study of the effect of particle size distribution on the paste viscosity and the hydrodynamic interaction between adjacent particles during paste flow. A theoretical enhanced model for predicting the viscosity of dense suspensions such as solder pastes has been developed. This correlates relative viscosity with particle size distribution and with solid volume fraction of dense suspensions. The results of the work have wide applicability: firstly for solder paste manufacturers in optimising paste printing performance at the development stage and for stencil printing equipment manufacturers in specifying the ideal conditions for defect free printing. The simulation algorithm and the viscosity model are also applicable for a wide range of industrial processing applications; in particular metal or ceramic powder compaction, material surface coating, chemical or food material transportation.