Purpose A measurement method has been developed to reveal the viscosity change of solder pastes during stencil printing. This paper aimed to investigate thixotropic behaviour, the viscosity change of a lead-free solder paste (Type 4). Design/methodology/approach The viscosity change of the solder paste during stencil printing cycles was characterised in such a way that the time-gap between the printing cycles was modelled with a rest period between every rheological measurement. This period was set as 15, 30 and 60 s during the research. The Cross model was fitted to the measurement results, and the η0 parameter was used to characterise the viscosity change. The number of printing cycles necessary for reaching a stationary state in viscosity was determined for various rest periods. Findings It was found that the decrease in zero-shear viscosity is significant (25 per cent) in the first cycles, and it starts to become stationary at the sixth-seventh cycles. This means a printing process can provide the appropriate deposits only after the 7th cycle with the investigated Type 4 solder paste. Originality/value Time-dependent rheological behaviour of solder pastes was studied in the literature, but only the viscosity change over continuous time at constant shear rates was examined. The time-gap between stencil printing cycles was not considered, and thixotropic behaviour of solder pastes was also neglected. Therefore, the authors developed a measurement set which is able to model the effect of time-gap between printing cycles on the viscosity change of solder pastes.

A solution for applying through hole (TH) components in mixed assembly is the pin-in-paste (PIP) technology, when solder paste is deposited onto the plated through holes and the components are placed into the holes through the paste. An important factor in defining the quality of TH soldering is the hole filling, which strongly relies on the capillary force. In our experiments, our aim was to define an optimal paste amount for rectangular and circular pins, for different diameter plated through holes, using two types of SAC solder paste, reflow soldered by vapour phase technology.Index Terms-Pin-in-paste, hole filling, vapour phase soldering, rectangular and circular pins