Abstract
Abstract
The quality of printing and reflow in surface mount technology depends very much on the selection of a solder paste with suitable tackiness and rheology. A series of studies has been conducted to find out the effects of exposure time of the solder paste on its tackiness and rheology. A qualitative method was designed for tackiness analysis. The results of the analysis show that tackiness decreases as exposure time increases and their relationship can be expressed by a power law. It is recommended that the values of the characteristic constants in the power law should be provided as part of the solder paste specification to facilitate calculation of the decay in tackiness over time by the user. The Malcom system was used to investigate rheology changes of the solder paste. Based on the solder paste used for the investigation, it was found that the viscosity increases with exposure time and becomes stable at about 2 h. On a log—log scale plot the viscosity and shear rate have a linear dependence on time of exposure for all solder pastes, and their thixotropy indices remain almost unchanged. However, the non-recovery rate, used for measuring the recovery ability of the solder paste, decreases rapidly in the first 40 min of exposure and then remains almost unchanged.