The two most important trends in electronic industry are ?miniaturisation? and ?increased functionality?. As these trends are set to continue, there is an increasing demand for better understanding of soldering technology, particularly in the area of solder pastes used in the reflow soldering of surface mount devices. This means that the paste flow and deformation behaviour is very important in defining the printing performance of any solder paste. The work reported in this book is focused on experimetal investigation and empirical modelling of time-dependent rheological behaviours of solder paste and flux mediums. The printing trials of four different lead-free solder paste samples were also carried out to investigate the effect of post-print rest period on slumping behaviour of solder paste. The paste manufacturers and formulators can use the techniques developed to predict and quantify the slumping behaviour of solder paste.The end-users, for example the electronics assemblers may use the techniques to optimize their assembly process by minimising/preventing slumping of solder paste.